SplitBit-Emulator/Source/Emulator/io.c

// io.c
// I/O for the SplitBit CPU Emulator
// Written by Anachronaut
// 10/16/2024

#include "io.h"
#include "../Assembler/assembly.h" // For the fault vector numbers.
#include "controller.h"
#include "video.h"
#include "pad.h"
#include "sound.h"
#include "font.h"
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>
#include <errno.h>
#include <termios.h>
#include <unistd.h>
#include <poll.h>

// ---- The console ----
//
// The console owns its own reading rather than going through getchar. stdio keeps a
// buffer, and the status port asks the operating system what is waiting; those two
// disagree the moment stdio has read ahead, and the status port would then swear nothing
// was there while a read returned instantly. One byte of pushback here is enough, because
// nothing needs to look further ahead than the byte it is about to take.

static int consoleKeyMode = 0;

static int cursorColumn = 0;
static int cursorRow = 0;
// What every cell the console draws is given. Zero is the pair the machine wakes up in,
// grey on black.
static uint8_t consoleAttribute = 0;
static int consoleCursorShown = 0;

// The device draws the cursor, so it has to be told where the console put it. Called
// wherever the cursor moves, which is every one of the few places that move it.
static void consoleCursorMoved(void) {
    videoSetCursor(cursorRow, cursorColumn, consoleCursorShown);
}

static int consoleEnded = 0;
static int consolePushback = -1;          // A byte already taken from the host, or -1.
static int consoleInterrupts = 0;         // Whether an arriving byte puts the line up.
static struct termios consoleSavedTerminal;
static int consoleTerminalSaved = 0;      // There is a copy of how the terminal was found.
static int consoleTerminalRaw = 0;        // The terminal is currently in this machine's mode.
static int consoleGuardsInstalled = 0;    // The handlers below are in place.

// Hands the terminal back exactly as it was found, WITHOUT forgetting what the program
// asked for. Separate from consoleRestore because the two are wanted in different places:
// a machine that is stopping wants both, and a machine that is being suspended wants only
// this, since it is going to carry on wanting keys when it is resumed.
static void consoleReleaseTerminal(void) {
    if (consoleTerminalRaw) {
        tcsetattr(STDIN_FILENO, TCSANOW, &consoleSavedTerminal);
        consoleTerminalRaw = 0;
    }
}

static void consoleTakeTerminal(void) {
    if (consoleTerminalRaw || !consoleTerminalSaved) {
        return;
    }
    struct termios raw = consoleSavedTerminal;
    raw.c_lflag &= (tcflag_t)~(ICANON | ECHO);
    raw.c_cc[VMIN] = 1;
    raw.c_cc[VTIME] = 0;
    if (tcsetattr(STDIN_FILENO, TCSANOW, &raw) == 0) {
        consoleTerminalRaw = 1;
    }
}

void consoleRestore(void) {
    consoleReleaseTerminal();
    consoleKeyMode = 0;
    // Whatever the console was in the middle of asking for is withdrawn along with the
    // mode. A line left standing here would be answered by whatever ran next, which had
    // nothing to do with it and never asked to be interrupted.
    consoleInterrupts = 0;
    clearInterrupt(PORT_CONSOLE);
}

// ---- Giving the terminal back whatever happens ----
//
// A machine that stops in key mode and does not undo it leaves the shell that started it
// with no echo and no line editing, which is a far worse failure than anything the program
// was doing, and one the user has no obvious way to connect to this program.
//
// atexit covers stopping on purpose and nothing else. It does NOT run when a process is
// killed by a signal, so every way of dying that matters has to be caught and undone by
// hand. The list below is every signal whose default action ends the process and which can
// be caught at all - SIGKILL and SIGSTOP cannot, and nothing can be done about those.
//
// SIGHUP is on the list for a specific reason, learned the hard way: it is what arrives
// when the terminal or the session that started this machine goes away, which is exactly
// what happens when whatever launched it crashes. Handling INT and TERM and stopping there
// covers the polite endings and misses the one that actually leaves a broken terminal
// behind.

// Puts the terminal back and then dies the way it would have died anyway, so that whatever
// is waiting sees the signal it expected rather than a machine that exited quietly.
static void consoleFatalSignal(int signalNumber) {
    consoleReleaseTerminal();
    signal(signalNumber, SIG_DFL);
    raise(signalNumber);
}

static void consoleContinueSignal(int signalNumber);

// Suspending is not dying, so the terminal goes back but the mode is remembered. Whoever
// gets the terminal next is entitled to find it as they left it, and this machine is
// entitled to have its keys again when it is resumed.
static void consoleStopSignal(int signalNumber) {
    consoleReleaseTerminal();
    signal(SIGCONT, consoleContinueSignal);
    signal(signalNumber, SIG_DFL);
    raise(signalNumber);
}

static void consoleContinueSignal(int signalNumber) {
    (void)signalNumber;
    signal(SIGTSTP, consoleStopSignal);
    signal(SIGCONT, consoleContinueSignal);
    if (consoleKeyMode) {
        consoleTakeTerminal();
    }
}

static void consoleInstallGuards(void) {
    if (consoleGuardsInstalled) {
        return;
    }
    consoleGuardsInstalled = 1;
    // Installed on the first use of key mode rather than at startup, so a run that never
    // asks for it installs nothing at all.
    atexit(consoleRestore);

    static const int fatal[] = {
        SIGHUP, SIGINT, SIGQUIT, SIGILL, SIGABRT, SIGFPE,
        SIGBUS, SIGSEGV, SIGPIPE, SIGALRM, SIGTERM
    };
    for (size_t i = 0; i < sizeof(fatal) / sizeof(fatal[0]); i++) {
        signal(fatal[i], consoleFatalSignal);
    }
    signal(SIGTSTP, consoleStopSignal);
    signal(SIGCONT, consoleContinueSignal);
}

static void consoleSetMode(int wantKeys) {
    if (wantKeys == consoleKeyMode) {
        return;
    }
    if (!wantKeys) {
        consoleRestore();
        return;
    }
    // Nothing to configure when input is not a terminal, but the mode is still recorded:
    // a program asking the status port what mode it is in should be told what it asked
    // for, whether or not there was a terminal to carry it out on.
    consoleKeyMode = 1;
    if (!isatty(STDIN_FILENO)) {
        return;
    }
    if (!consoleTerminalSaved) {
        if (tcgetattr(STDIN_FILENO, &consoleSavedTerminal) != 0) {
            return;
        }
        consoleTerminalSaved = 1;
    }
    consoleInstallGuards();
    consoleTakeTerminal();
}

// Puts the line up if the console has something to say and has been asked to say it.
// Called wherever news arrives and wherever a program declares it wants to hear news, so
// that enabling interrupts while a byte is already waiting is not a way to miss it.
static void consoleAnnounce(void) {
    if (consoleInterrupts && (consolePushback >= 0 || consoleEnded)) {
        raiseInterrupt(PORT_CONSOLE);
    }
}

// The whole control port in one write. The two bits are independent, so both are read out
// of the byte and applied, and neither is inferred from the other.
static void consoleSetControl(uint8_t control) {
    // The mode goes first because turning key mode off restores the terminal, and that
    // withdraws any standing request along with it. Setting the interrupt bit afterwards
    // means one write can ask for line mode and interrupts together, which is an ordinary
    // thing to want and would otherwise be undone in the same breath as it was asked for.
    consoleSetMode((control & CONSOLE_MODE_KEY) != 0);
    consoleCursorShown = (control & CONSOLE_CONTROL_CURSOR) != 0;
    consoleCursorMoved();

    int wantInterrupts = (control & CONSOLE_CONTROL_INTERRUPT) != 0;
    if (!wantInterrupts) {
        // Asking to stop being interrupted takes down whatever was already asked for. A
        // request that outlived the setting that made it would arrive at a program that
        // had just said it did not want it.
        clearInterrupt(PORT_CONSOLE);
    }
    consoleInterrupts = wantInterrupts;
    consoleAnnounce();
}

// Everything already written is put where it can be seen before the machine asks the host
// anything. Standard output is line buffered on a terminal, so a prompt with no newline
// after it - "> " is exactly that, and exactly why this matters - would sit in the buffer
// while the machine waited for an answer to a question nobody had been shown.
//
// getchar used to do this by accident, because reading through stdio flushes the line
// buffered streams first. Reading with read() does not, so what was a side effect of the
// old way is done deliberately here.
static void consoleShowWhatIsWritten(void) {
    fflush(stdout);
}

// ---- The console draws as well as it speaks ----
//
// THE VOYAGER'S CONSOLE IS A DISPLAY CONTROLLER: it takes a byte stream and puts glyphs on
// a screen, keeps a cursor, and scrolls. That is an ordinary kind of chip - it is what a
// video terminal's character generator did - and it is why CosmOS needs no changes at all
// to run in a window. It already writes bytes to the console.
//
// It also keeps writing to standard output, and that is deliberate rather than an
// oversight. A machine with a screen AND a serial line is completely ordinary, the emulator's
// standard output is that serial line, and having both is what lets the whole test suite
// hold Voyager to the same recorded results as SplitBit. It is also what makes --screen
// work on the plain machine: there is one console, and it drives everything it has.

void consoleHome(void) {
    cursorColumn = 0;
    cursorRow = 0;
    consoleAttribute = 0;
    consoleCursorShown = 0;
    consoleCursorMoved();
}

static void consoleNewLine(void) {
    cursorColumn = 0;
    if (cursorRow + 1 < videoRows()) {
        cursorRow++;
    } else {
        // At the bottom, the screen moves under the cursor rather than the cursor moving
        // off the screen. One port write in the device, and no memory moves at all.
        videoScrollUp();
    }
}

// Clears the screen, or from the cursor to the end of it. Reached through the console's
// Command port rather than through an escape sequence: this machine talks to its devices in
// registers, and a screen it can address directly needs no protocol to reach it.
static void consoleClearScreen(int fromCursor) {
    const int rows = videoRows();
    const int columns = videoColumns();
    for (int row = fromCursor ? cursorRow : 0; row < rows; row++) {
        const int from = (fromCursor && row == cursorRow) ? cursorColumn : 0;
        for (int column = from; column < columns; column++) {
            videoPutCell(row, column, 0, 0);
        }
    }
}

// ---- Driving a terminal on the other end of the serial line ----
//
// The machine speaks registers. A HOST TERMINAL SPEAKS ANSI, and bridging to the host is the
// emulator's job - the same job it does reading standard input. So the escapes are GENERATED
// here, outbound, for the set this device chooses, rather than parsed inbound as though the
// machine were a terminal itself.
//
// That is the whole difference in shape. Parsing means accepting an open protocol somebody
// else defines and putting a state machine in the hardware. Generating means one device
// knowing how to talk to one kind of host, in one direction, for exactly the things it can
// be asked to do - and the set cannot grow behind our backs, because we are the ones saying
// it.
static void consoleTellTerminal(const char *sequence) {
    for (const char *at = sequence; *at != '\0'; at++) {
        putchar(*at);
    }
}

// ---- Told once, and only when it matters ----
//
// A terminal cares where the cursor is at the moment something is about to be drawn there,
// not at the moment a register was written. Announcing on every register write sent two
// sequences for one move, because setting a row and a column is two writes. So a write only
// marks it, and the next character sends it.
static int cursorTold = 1;

static void consoleSayCursor(void) {
    if (cursorTold) {
        return;
    }
    cursorTold = 1;
    // Room for the widest this can be, and then some. The compiler cannot see that a cursor
    // is bounded by the screen, and a warning about a buffer is not worth being clever over.
    char sequence[32];
    snprintf(sequence, sizeof(sequence), "\033[%d;%dH", cursorRow + 1, cursorColumn + 1);
    consoleTellTerminal(sequence);
}

static void consoleDraw(uint8_t byte) {
    // ---- Nowhere to put a glyph ----
    //
    // A console is a display controller, and a display controller draws characters on a
    // character screen. In bitmap mode there is not one - the memory it would write into is
    // somebody's picture - so it draws nothing and says everything down the serial line
    // instead, which is where it was always going as well.
    //
    // The alternative is what a machine with shared video memory really does, which is
    // scribble. That is honest and useless: nobody can read the marks and they ruin the
    // picture, and a program that has taken the screen has not stopped wanting to print.
    if (videoTextRows() == 0) {
        return;
    }
    switch (byte) {
        case '\n':
            consoleNewLine();
            consoleCursorMoved();
        return;
        case '\r':
            cursorColumn = 0;
            consoleCursorMoved();
        return;
        case 0x08:      // Backspace, which CosmOS sends when a line is being edited.
            if (cursorColumn > 0) {
                cursorColumn--;
                videoPutCell(cursorRow, cursorColumn, 0, consoleAttribute);
                consoleCursorMoved();
            }
        return;
        case '\t': {
            // ---- A tab moves the cursor and marks nothing ----
            //
            // The byte reaches standard output either way, so anything reading the serial
            // line has always seen tabs and a host terminal has always laid them out. The
            // screen dropped them, which is why a tab separated file - an assembler symbol
            // table, say - came out of Type or More as its fields run together.
            //
            // MOVING RATHER THAN WRITING SPACES is what a terminal does, and the difference
            // shows the moment anything is already on the line: a tab that wrote blanks
            // would rub out what it passed over.
            //
            // Eight columns is the conventional stop and the one everything that prints a
            // tab assumes. Reaching or passing the right edge wraps, which is what an
            // ordinary character does at the edge - the alternative, stopping in the last
            // column, invents a rule that only tabs obey.
            int stop = (cursorColumn / CONSOLE_TAB_WIDTH + 1) * CONSOLE_TAB_WIDTH;
            if (stop >= videoColumns()) {
                consoleNewLine();
            } else {
                cursorColumn = (uint8_t)stop;
            }
            consoleCursorMoved();
        }
        return;
        default:
        break;
    }
    // Anything below the font's first character has no glyph and no agreed meaning here.
    // Drawing a box for it would put marks on the screen that nothing asked for.
    if (byte < CONSOLE_FONT_FIRST) {
        return;
    }
    videoPutCell(cursorRow, cursorColumn, (uint8_t)(byte - CONSOLE_FONT_FIRST),
                 consoleAttribute);
    if (++cursorColumn >= videoColumns()) {
        consoleNewLine();
    }
    consoleCursorMoved();
}

// ---- Waiting for a key when there is no terminal to wait on ----
//
// A window has no standard input, and a machine that blocked on it inside a frame would
// stop drawing and stop answering. So a front end with a window installs a hook: called
// while the console has nothing, it gets to keep the window alive and hands back a byte
// when one is typed, or -1 to say the window has gone.
static int (*inputHook)(int mayWait) = NULL;

// About one frame, which is how long a hook that presents takes to come back. It does not
// have to be exact - nothing is being measured, and the only thing downstream of it is a
// cursor blinking at somebody who is thinking about what to type.
#define CONSOLE_WAIT_CYCLES 16667

static unsigned long idleCycles = 0;

// The machine's clock as the devices last heard it. Kept here rather than passed about,
// because a device that has to know how long it has been waiting has to know what time it
// is now.
static unsigned long deviceNow = 0;

// A frame has gone by with nobody typing. TWO THINGS FOLLOW FROM THAT, and only doing the
// first is what left the cursor frozen: the CPU is told afterwards that it was stopped for a
// while, which is what idle cycles are - and the DEVICES are told now, because they are
// still running. A display controller blinking a cursor does not stop because the processor
// is waiting on a key, and neither does a disk finishing a read.
static void consoleWaited(void) {
    idleCycles += CONSOLE_WAIT_CYCLES;
    deviceTick(deviceNow + CONSOLE_WAIT_CYCLES);
}

unsigned long takeIdleCycles(void) {
    const unsigned long taken = idleCycles;
    idleCycles = 0;
    return taken;
}

// ---- Line editing, which the terminal used to do ----
//
// A TERMINAL IN LINE MODE DOES NOT HAND A PROGRAM EVERY KEYSTROKE. It collects a line,
// rubs out a backspace, and delivers the finished thing when Return is pressed. CosmOS has
// always relied on that, and behind a window there is no terminal to do it - so the raw
// backspace reached the shell, which put 0x08 in its command buffer and then could not find
// a command by that name. Correcting a typo made the line unrecognisable while looking
// perfectly right on screen.
//
// So the console does it, because behind a window the console IS the terminal. In key mode
// it does not: a program in key mode asked for every keystroke as it happens, which is the
// whole point of key mode.
#define CONSOLE_LINE_BYTES 256
static unsigned char consoleLine[CONSOLE_LINE_BYTES];
static int consoleLineLength = 0;
static int consoleLineAt = 0;

// Collects until Return, echoing as it goes, and leaves the line to be handed out a byte at
// a time. Returns 0 if the window closed while it was waiting.
static int consoleGatherLine(void) {
    consoleLineAt = 0;
    consoleLineLength = 0;
    for (;;) {
        const int got = inputHook(1);
        if (got == CONSOLE_GONE) {
            return 0;
        }
        if (got < 0) {
            // Nobody has typed yet, and the hook spent a frame keeping the window alive.
            consoleWaited();
            continue;
        }
        if (got == 0x08) {
            // Nothing to rub out at the start of a line, and rubbing out past it would eat
            // the prompt, which belongs to whoever printed it.
            if (consoleLineLength > 0) {
                consoleLineLength--;
                consoleDraw(0x08);
            }
            continue;
        }
        if (got == '\n' || got == '\r') {
            consoleLine[consoleLineLength++] = '\n';
            consoleDraw('\n');
            return 1;
        }
        // No room, or nothing a line is made of. A control byte that means something to a
        // terminal means nothing here yet, and putting it in the line would only hand a
        // program something it cannot use.
        //
        // THE CONSOLE'S OWN KEYS GO THE SAME WAY, from above rather than below: this is the
        // gatherer, which is what line mode IS behind a window, and line mode delivers
        // characters. An arrow key pressed while something else is collecting the line
        // arrived too late to move anything.
        if (got < 0x20 || (got >= CONSOLE_KEY_FIRST && got <= CONSOLE_KEY_LAST) ||
            consoleLineLength >= CONSOLE_LINE_BYTES - 1) {
            continue;
        }
        consoleLine[consoleLineLength++] = (unsigned char)got;
        consoleDraw((uint8_t)got);
    }
}

void consoleSetInputHook(int (*hook)(int mayWait)) {
    inputHook = hook;
}

// ---- Reading standard input, in one place ----
//
// The blocking read and the poll behind the status port each reached for read() themselves,
// which was fine while one byte from the host was one byte for the program. It stopped
// being fine the moment a key could arrive as several: a sequence half taken by one path
// and half by the other is not a key, it is two pieces of rubbish. So there is one way in
// now, and the translation below sits on top of it.
//
// Answers a byte, CONSOLE_NOTHING_YET when nothing is waiting and it was told not to wait,
// or CONSOLE_GONE at the end of input. Deliberately the same three answers a front end's
// hook gives, so that everything above this treats a terminal and a window alike.
static int consoleFromInput(int mayWait) {
    if (!mayWait) {
        struct pollfd waiting = { .fd = STDIN_FILENO, .events = POLLIN, .revents = 0 };
        if (poll(&waiting, 1, 0) <= 0 || (waiting.revents & (POLLIN | POLLHUP)) == 0) {
            return CONSOLE_NOTHING_YET;
        }
    }
    unsigned char byte;
    for (;;) {
        const ssize_t got = read(STDIN_FILENO, &byte, 1);
        if (got == 1) {
            return byte;
        }
        if (got == 0) {
            return CONSOLE_GONE;
        }
        if (errno != EINTR) {
            return CONSOLE_GONE;
        }
        // Interrupted before anything arrived, so ask again.
    }
}

// ---- What a terminal sends for a key that is not a character ----
//
// ESC [ A and its neighbours. A window hands over the key somebody pressed; a terminal
// hands over the sequence it was taught to send decades ago, and something has to turn one
// into the other. It happens here because the console is already the thing that turns a
// window's Enter key into a newline, and because doing this once in C is a great deal
// better than doing it in every program that wants an arrow key.
//
// ONLY WHEN THERE IS A TERMINAL. Nothing else sends these: a file or a pipe holds exactly
// the bytes somebody put in it, and translating there would mean an escape byte followed by
// a bracket could never be read back as what it is. It also keeps the timing problem below
// out of every test this machine has, which is the larger gain - a test writes the key
// values themselves and no terminal is involved in reading them.
//
// THE TIMING PROBLEM, which only the terminal case has: pressing Escape and pressing Up
// both begin with 0x1B, and the difference is that Up is followed immediately by more. So
// after an escape the console waits a moment to see whether anything is. A keyboard
// delivers a whole sequence in one go, so the wait is only ever spent when somebody really
// did press Escape, and it is far shorter than the gap between two keystrokes.
#define CONSOLE_ESCAPE_WAIT_MS 30

static int consoleTerminalKnown = 0;   // isatty has been asked.
static int consoleHasTerminal = 0;     // and this is what it said.

static int consoleTranslatingKeys(void) {
    if (!consoleTerminalKnown) {
        consoleHasTerminal = isatty(STDIN_FILENO);
        consoleTerminalKnown = 1;
    }
    return consoleHasTerminal;
}

// A byte taken while deciding what an escape was, which turned out not to belong to it.
// One is enough: it is only ever the byte immediately after an escape that can be both
// part of a sequence and an ordinary character.
static int consoleHeldByte = -1;

// The next byte of a sequence, or below zero if the terminal has stopped talking. The wait
// is what tells a sequence from a keypress, so this is the one read in the console that
// times out rather than blocking.
static int consoleSequenceByte(void) {
    struct pollfd waiting = { .fd = STDIN_FILENO, .events = POLLIN, .revents = 0 };
    if (poll(&waiting, 1, CONSOLE_ESCAPE_WAIT_MS) <= 0 || (waiting.revents & POLLIN) == 0) {
        return -1;
    }
    return consoleFromInput(1);
}

// An escape has just been read. Answers the key it began, 0x1B if it was the Escape key
// itself, or below zero for a sequence this console has no key for.
static int consoleKeyFromEscape(void) {
    const int intro = consoleSequenceByte();
    if (intro < 0) {
        // Nothing followed it, so somebody pressed Escape.
        return 0x1B;
    }
    if (intro != '[' && intro != 'O') {
        // An escape and then something else, close enough together to look like one thing.
        // It was two: the escape is delivered now and the other byte waits its turn rather
        // than being dropped, because it is an ordinary character somebody typed.
        consoleHeldByte = intro;
        return 0x1B;
    }
    // ESC [ 3 ~ carries a number and ESC [ A does not, and both end in a byte that says
    // which kind it was. So the digits are collected and the ending decides.
    int number = 0;
    int final = consoleSequenceByte();
    while (final >= '0' && final <= '9') {
        number = number * 10 + (final - '0');
        final = consoleSequenceByte();
    }
    switch (final) {
        case 'A': return CONSOLE_KEY_UP;
        case 'B': return CONSOLE_KEY_DOWN;
        case 'C': return CONSOLE_KEY_RIGHT;
        case 'D': return CONSOLE_KEY_LEFT;
        // Terminals disagree about Home and End more than about anything else here, so both
        // spellings of each are taken: the lettered one, and the numbered one that the
        // terminals which do not use letters send instead.
        case 'H': return CONSOLE_KEY_HOME;
        case 'F': return CONSOLE_KEY_END;
        case '~':
            switch (number) {
                case 1: case 7: return CONSOLE_KEY_HOME;
                case 4: case 8: return CONSOLE_KEY_END;
                case 3: return CONSOLE_KEY_DELETE;
                default: break;
            }
        break;
        default: break;
    }
    // A sequence this console has no key for. THE WHOLE OF IT GOES rather than the bytes
    // being handed on, because it is a control sequence and not text: a program given the
    // tail of one would put a bracket and a letter into whatever it was reading, which is
    // the exact fault this whole translation exists to end.
    return -1;
}

// One key from standard input: a byte as it arrived, or one of the console's own key values
// where a terminal sent a sequence meaning one.
// ---- The terminal's erase key is not the one this machine knows ----
//
// SplitBit's Backspace is 0x08. That is what Voyager's keyboard sends and what CosmOS's line
// editor looks for. A POSIX terminal sends whatever ITS erase character is, and on most of
// them that is 0x7F.
//
// It went unnoticed for as long as the terminal was doing the editing: in canonical mode the
// tty eats its own erase character and hands over a finished line. Key mode turns ICANON off,
// which is the point of it, and from then on the byte arrives raw and means nothing to the
// editor. So Backspace worked in Voyager and stopped working in the console-only emulator on
// the day the shell started editing the line itself.
//
// VERASE IS ASKED RATHER THAN 0x7F ASSUMED, because the terminal is what knows: it was saved
// on the way into raw mode, some terminals really are set to 0x08, and a person who has moved
// their erase key somewhere else has said where it is.
//
// ONLY WHEN STANDARD INPUT IS A TERMINAL. A file or a pipe holding 0x7F holds a byte that
// somebody wrote, not a key somebody pressed, and rewriting it would corrupt input that has
// nothing to do with terminals. consoleTerminalSaved is exactly that question: it is only
// ever set after an isatty succeeded.
static int consoleEraseNormalized(int got) {
    if (!consoleTerminalSaved || got < 0) {
        return got;
    }
    const cc_t erase = consoleSavedTerminal.c_cc[VERASE];
    // A terminal with no erase key at all, which says so this way, has nothing to translate.
    if (erase == _POSIX_VDISABLE) {
        return got;
    }
    return got == (int)erase ? 0x08 : got;
}

static int consoleKeyFromInput(int mayWait) {
    for (;;) {
        int got;
        if (consoleHeldByte >= 0) {
            // Already normalized on the way in, and doing it twice would be wrong the day
            // 0x08 is somebody's erase character: it is its own answer.
            got = consoleHeldByte;
            consoleHeldByte = -1;
        } else {
            got = consoleEraseNormalized(consoleFromInput(mayWait));
        }
        if (got < 0) {
            return got;
        }
        if (got == 0x1B && consoleTranslatingKeys()) {
            got = consoleKeyFromEscape();
        }
        // Nothing to hand over: either that sequence meant nothing here, or it meant a key
        // and line mode does not deliver keys. Both are the same answer to a caller - there
        // is still no byte - so a blocking read asks again and a poll says so and leaves.
        const int undeliverable =
            !consoleKeyMode && got >= CONSOLE_KEY_FIRST && got <= CONSOLE_KEY_LAST;
        if (got < 0 || undeliverable) {
            if (!mayWait) {
                // ---- A LOOK MUST NOT CONSUME WHAT IT CANNOT REPORT ----
                //
                // This is the status port asking, and either way it has no byte to report.
                // But a key that line mode will not deliver is not the same as a key that is
                // gone: THE MODE CAN CHANGE. A program that polls and then asks for key mode
                // - which is exactly what the shell does before it reads a line - would find
                // that the first key it was reaching for had been swallowed by the looking.
                //
                // So it is held rather than dropped, and delivered as soon as something is
                // willing to take it. The blocking read below drops it instead, and must:
                // that read IS the delivery, line mode genuinely has no use for the key, and
                // a byte held there would be met again forever.
                if (undeliverable) {
                    consoleHeldByte = got;
                }
                return CONSOLE_NOTHING_YET;
            }
            continue;
        }
        return got;
    }
}

uint8_t consoleReadByte(void) {
    // Taking the byte answers whatever the console was asking about, so the line comes
    // down here as well as when the CPU acknowledges it. Otherwise a program that reads
    // the data port with the Interrupt Flag down would be interrupted afterwards on
    // behalf of a byte it already has, and find nothing waiting when it looked.
    clearInterrupt(PORT_CONSOLE);
    if (consolePushback >= 0) {
        uint8_t byte = (uint8_t)consolePushback;
        consolePushback = -1;
        return byte;
    }
    consoleShowWhatIsWritten();
    if (inputHook != NULL) {
        if (!consoleKeyMode) {
            // A line already gathered is handed out a byte at a time, which is what the
            // program is asking for. Only when it runs out is another one collected.
            if (consoleLineAt >= consoleLineLength && !consoleGatherLine()) {
                consoleEnded = 1;
                return 0xFF;
            }
            return consoleLine[consoleLineAt++];
        }
        for (;;) {
            const int got = inputHook(1);
            if (got >= 0) {
                // Nothing echoes in key mode: a program that asked for every keystroke as it
                // happens is drawing its own screen, and marks it did not make would be in
                // the way.
                return (uint8_t)got;
            }
            if (got == CONSOLE_GONE) {
                // The window has closed, which is this machine's end of input the way a
                // closed pipe is the other one's.
                consoleEnded = 1;
                return 0xFF;
            }
            // Nothing typed yet. The hook kept the window alive, which took a frame, and a
            // frame of waiting is a frame of time passing.
            consoleWaited();
        }
    }
    const int got = consoleKeyFromInput(1);
    if (got >= 0) {
        return (uint8_t)got;
    }
    // End of input. Still 0xFF, which is what getchar's EOF became when this was the only
    // answer available, so nothing written against the old behaviour changes. The ENDED bit
    // is the new way to know it was not a real byte.
    consoleEnded = 1;
    return 0xFF;
}

// Asking the host whether anything is waiting, and TAKING IT IF THERE IS. The byte goes
// into the pushback and the next read of the data port hands it over, so nothing is lost
// and no program can tell that it was fetched early.
//
// Fetching it early is what makes the answer worth having. The operating system will say a
// pipe is readable when what is waiting is the end of it, so asking without reading can
// only report that SOMETHING is there. Reading settles which: a byte, or the end. Without
// this, ENDED could not go up until a program had already read the 0xFF that stands for
// it, and every program would have to swallow one imaginary byte to find out there were
// none.
static void consoleFetch(void) {
    if (consolePushback >= 0 || consoleEnded) {
        return;
    }
    // Flushed here too. A program that draws something and then polls rather than reads is
    // just as entitled to have the drawing appear, and it never reaches the read that
    // would otherwise have flushed for it.
    consoleShowWhatIsWritten();
    // ---- Where a byte comes from when there is no standard input ----
    //
    // A window's keys arrive through the hook, and THE STATUS PORT HAS TO ASK IT TOO. It did
    // not, so a program polling READY in a window was asking a standard input nobody was
    // typing at: Snake saw no keys at all, and would suddenly see one if the terminal behind
    // the window happened to be focused. Asked without waiting, because a poll is a poll -
    // the front end presents a frame when the console genuinely blocks, not when it looks.
    if (inputHook != NULL) {
        if (!consoleKeyMode) {
            // READY means there is a byte to be had, and in line mode there is one only
            // while a gathered line is still being handed out. A poll must not take a key
            // from under the gatherer, and half a line is not a line.
            if (consoleLineAt < consoleLineLength) {
                consolePushback = consoleLine[consoleLineAt++];
                consoleAnnounce();
            }
            return;
        }
        const int got = inputHook(0);
        if (got >= 0) {
            consolePushback = got;
            consoleAnnounce();
        } else if (got == CONSOLE_GONE) {
            consoleEnded = 1;
        }
        return;
    }
    const int got = consoleKeyFromInput(0);
    if (got >= 0) {
        consolePushback = got;
    } else if (got == CONSOLE_GONE) {
        consoleEnded = 1;
    } else {
        // Nothing waiting, which is the ordinary answer and not news.
        return;
    }
    // A read that failed for any other reason is left alone: the next attempt asks again,
    // and an interrupted poll is not news.
    //
    // Anything that was news puts the line up. This is the only place a byte arrives from
    // the outside world, so it is the only place that has to, and it raises AT MOST ONCE
    // PER BYTE for free: the pushback holds one, and while it is full there is nothing to
    // fetch and so nothing to announce. A handler that does not read what it was called
    // about is simply not called again, the way a receive register with one byte in it
    // stops asking. The end of input announces itself once for the same reason - it is
    // discovered once, and every later look leaves before it gets here.
    consoleAnnounce();
}

// How many instructions the machine runs between glances at the console. Nothing here
// happens alongside the CPU, so noticing a keystroke costs a system call, and asking on
// every instruction costs more than executing one: a poll is about 150ns against roughly
// 9ns for an instruction at full tilt, so it would slow the machine by nearly twenty
// times. At the emulated clock this stride is a quarter of a millisecond between glances,
// which no one typing has ever been able to tell from immediately.
#define CONSOLE_SERVICE_STRIDE 256

void serviceDevices(void) {
    // The common case is a machine nobody is interrupting, and it costs one test.
    if (!consoleInterrupts) {
        return;
    }
    static unsigned int untilNextGlance = 0;
    if (untilNextGlance > 0) {
        untilNextGlance--;
        return;
    }
    untilNextGlance = CONSOLE_SERVICE_STRIDE - 1;
    consoleFetch();
}

static uint8_t consoleStatus(void) {
    uint8_t status = 0;
    if (consoleKeyMode) {
        status |= CONSOLE_STATUS_KEYMODE;
    }
    if (consoleInterrupts) {
        status |= CONSOLE_STATUS_INTERRUPT;
    }
    if (consoleCursorShown) {
        status |= CONSOLE_STATUS_CURSOR;
    }
    consoleFetch();
    if (consoleEnded) {
        // READY IS NOT SET HERE, although a read would answer immediately. The bit means
        // "there is a byte to be had", and at the end of input there is not; what a read
        // returns then is 0xFF standing in for nothing. A program looping while READY
        // stops on its own at the end, which is the behaviour worth having, and one that
        // wants to know why asks ENDED.
        return status | CONSOLE_STATUS_ENDED;
    }
    if (consolePushback >= 0) {
        status |= CONSOLE_STATUS_READY;
    }
    return status;
}

// The machine's own lines. A peripheral core's device keeps its own set, which is the
// entire reason these are a struct rather than an array sitting here.
static InterruptLines machineLines;

// And its own controller, for the same reason. Everything on this bus that moves memory means
// this one.
static Controller theMachinesController;

Controller *machineController(void) {
    return &theMachinesController;
}

static uint8_t machineControllerWrite(uint8_t value, uint8_t port) {
    return controllerWrite(&theMachinesController, value, port);
}

static uint8_t machineControllerRead(uint8_t port) {
    return controllerRead(&theMachinesController, port);
}

static unsigned long machineControllerCycles(void) {
    return controllerTakeCycles(&theMachinesController);
}

// Whether somebody has asked the machine to start over, and taking that request away.
static int resetWanted = 0;

void requestReset(void) {
    resetWanted = 1;
}

void consoleResetInput(void) {
    consolePushback = -1;
    consoleEnded = 0;
    consoleLineLength = 0;
    consoleLineAt = 0;
    clearInterrupt(PORT_CONSOLE);
}

int resetIsPending(void) {
    return resetWanted;
}

int takeResetRequest(void) {
    int wanted = resetWanted;
    resetWanted = 0;
    return wanted;
}

// ---- The bus this machine's processor is on ----
//
// Everything a CPU asks of the world outside it, for the world this file is. A peripheral
// core is handed a different one of these by whatever device contains it, which is the whole
// of what a private bus is: not a number to be checked, a different set of answers.
static const Bus theMachinesBus = {
    OutputHandler,
    InputHandler,
    machineControllerCycles,
    takeIdleCycles,
    nextPendingInterrupt,
    clearInterrupt,
};

const Bus *machineBus(void) {
    return &theMachinesBus;
}

void linesRaise(InterruptLines *lines, uint8_t port) {
    lines->bits[port >> 3] |= (uint8_t)(1u << (port & 7));
}

void linesClear(InterruptLines *lines, uint8_t port) {
    lines->bits[port >> 3] &= (uint8_t)~(1u << (port & 7));
}

// The machine's own, which is what every device in this file means when it asks for
// attention. Wrappers rather than a change at every call site, because every one of those
// devices really is on this bus and saying so twenty times would not make it truer.
void raiseInterrupt(uint8_t port) {
    linesRaise(&machineLines, port);
}

void clearInterrupt(uint8_t port) {
    linesClear(&machineLines, port);
}

void clearAllInterrupts(void) {
    memset(&machineLines, 0, sizeof(machineLines));
}

int linesNext(const InterruptLines *lines) {
    // Lowest numbered port wins. This is a scan rather than a priority encoder, which
    // means there is no arbitration to explain and a programmer can work out what
    // happens next by reading the port numbers.
    for (int group = 0; group < INTERRUPT_LINE_BYTES; group++) {
        if (lines->bits[group] == 0) {
            continue;
        }
        for (int bit = 0; bit < 8; bit++) {
            if (lines->bits[group] & (1u << bit)) {
                return group * 8 + bit;
            }
        }
    }
    return -1;
}

int nextPendingInterrupt(void) {
    return linesNext(&machineLines);
}

// ---- Refusing ----
//
// Set when a device will not do what it was asked, and read by the CPU immediately
// after the instruction that asked. It is not a queue: an instruction does one thing to
// one port, so there is only ever one refusal outstanding.

static uint8_t refusedVector = 0;
static uint8_t refusedPort = 0;

void refuseAccess(uint8_t faultVector) {
    refusedVector = faultVector;
}

uint8_t takeRefusal(void) {
    uint8_t vector = refusedVector;
    refusedVector = 0;
    return vector;
}

uint8_t refusingPort(void) {
    return refusedPort;
}

// ---- The disk ----
//
// A block device and nothing more. It knows numbered blocks and has never heard of a
// file, which is the whole point: a filesystem is software this machine will run, not
// something the host does on its behalf. A disk that understood filenames would be the
// emulator doing the work and the machine pretending it had.

// ---- What belongs to a drive, and what belongs to the controller ----
//
// A disk is write protected and has a size; a controller has a block register, a status and
// one buffer. So these three are per drive and everything below is not - which is the same
// division a real controller makes, and the reason the buffer holding whichever drive was
// last read is correct rather than a shortcut.
static FILE *diskImage[DISK_DRIVE_COUNT];
static uint32_t diskBlockCount[DISK_DRIVE_COUNT];
static uint8_t diskProtected[DISK_DRIVE_COUNT];

// A drive whose blocks are memory. Everything else about it is a drive: it selects, it reads
// and writes, it has a size, and a filesystem on it is a filesystem. What it does not have is
// a file behind it, so it comes up as zeroes and goes away when the machine does.
static uint8_t *diskMemory[DISK_DRIVE_COUNT];

// Which one the registers refer to, and how many are plugged in at all.
static uint8_t diskDrive = 0;
static uint8_t diskDrives = 0;

static uint8_t diskBuffer[DISK_BLOCK_BYTES];

// ---- A disk that takes time ----
//
// The command is checked at once, because a refusal is not work: asking for a block that
// is not there, or writing to a protected disk, fails before any head moves. What takes
// time is the transfer, so that is remembered here and done when the machine has run far
// enough - and until then the buffer holds the block BEFORE this one, which is exactly
// what a program that ignores the busy bit deserves to read.
// The machine's clock as devices see it, which the emulator advances as the CPU spends
// cycles. A device says when it will be finished in these, and is believed.
static void diskTransfer(uint8_t command);
static void diskSettle(void);

static unsigned long diskLatency = 0;
static unsigned long diskReadyAt = 0;
static uint8_t diskPending = 0;
static uint16_t diskBlock = 0;
static uint8_t diskStatus = 0;

// Attaching gives the next free drive number, so the order they are named on the command
// line is the order the machine has them in.
uint8_t attachDisk(const char *path, uint8_t writeProtect) {
    if (diskDrives >= DISK_DRIVE_COUNT) {
        fprintf(stderr, "Error: This machine has %d drives.\n", DISK_DRIVE_COUNT);
        return 1;
    }
    const uint8_t at = diskDrives;
    diskProtected[at] = writeProtect ? 1 : 0;
    diskImage[at] = fopen(path, "r+b");
    if (diskImage[at] == NULL) {
        // It may be there and simply not writable, which is a read only disk rather than
        // a missing one. Try that before deciding to make a new one.
        diskImage[at] = fopen(path, "rb");
        if (diskImage[at] != NULL) {
            diskProtected[at] = 1;
        }
    }
    if (diskImage[at] == NULL) {
        // Nothing there, so make one. A fresh image is zeroes, which is what an unwritten
        // block should read as.
        diskImage[at] = fopen(path, "w+b");
        if (diskImage[at] == NULL) {
            fprintf(stderr, "Error: Couldn't open or create the disk image: %s\n", path);
            return 1;
        }
        static const uint8_t empty[DISK_BLOCK_BYTES] = {0};
        for (uint32_t i = 0; i < DISK_DEFAULT_BLOCKS; i++) {
            if (fwrite(empty, 1, DISK_BLOCK_BYTES, diskImage[at]) != DISK_BLOCK_BYTES) {
                fprintf(stderr, "Error: Couldn't write the disk image: %s\n", path);
                fclose(diskImage[at]);
                diskImage[at] = NULL;
                return 1;
            }
        }
    }
    if (fseek(diskImage[at], 0, SEEK_END) != 0) {
        fprintf(stderr, "Error: Couldn't measure the disk image: %s\n", path);
        fclose(diskImage[at]);
        diskImage[at] = NULL;
        return 1;
    }
    long size = ftell(diskImage[at]);
    // A part written block at the end is not a block, so it is not counted.
    diskBlockCount[at] = (size > 0) ? (uint32_t)(size / DISK_BLOCK_BYTES) : 0;
    // The protect bit is a standing property, so it reads true before anything has been
    // asked of the disk rather than only after a write has been turned away.
    // The protect bit is a standing property of the drive now selected, so it reads true
    // before anything has been asked of it rather than only after a write is turned away.
    diskDrives++;
    diskStatus = diskProtected[diskDrive] ? DISK_STATUS_PROTECTED : 0;
    return 0;
}

uint8_t attachRamDisk(uint32_t blocks) {
    if (diskDrives >= DISK_DRIVE_COUNT) {
        fprintf(stderr, "Error: This machine has %d drives.\n", DISK_DRIVE_COUNT);
        return 1;
    }
    if (blocks == 0) {
        fprintf(stderr, "Error: A disk of no blocks is not a disk.\n");
        return 1;
    }
    const uint8_t at = diskDrives;
    diskMemory[at] = calloc(blocks, DISK_BLOCK_BYTES);
    if (diskMemory[at] == NULL) {
        fprintf(stderr, "Error: Couldn't make a %u block disk in memory.\n", blocks);
        return 1;
    }
    diskBlockCount[at] = blocks;
    diskProtected[at] = 0;
    diskDrives++;
    return 0;
}

void detachDisk(void) {
    for (int at = 0; at < DISK_DRIVE_COUNT; at++) {
        if (diskImage[at] != NULL) {
            fclose(diskImage[at]);
            diskImage[at] = NULL;
        }
        free(diskMemory[at]);
        diskMemory[at] = NULL;
    }
    diskDrives = 0;
    diskDrive = 0;
}

// Reads or writes the block the block registers name. The line goes up either way: the
// operation finished, and whether it worked is what Status is for.
static void diskCommand(uint8_t command) {
    // The protect bit describes the disk rather than the operation, so it survives.
    diskStatus = diskProtected[diskDrive] ? DISK_STATUS_PROTECTED : 0;
    if (command == DISK_COMMAND_WRITE && diskProtected[diskDrive]) {
        diskStatus |= DISK_STATUS_ERROR;
        raiseInterrupt(PORT_DISK);
        return;
    }
    if (diskMemory[diskDrive] != NULL) {
        // A drive made of memory. The same block, the same 256 bytes, and no seek: what
        // makes this worth having is that a program cannot tell except by how fast it was.
        if (diskBlock >= diskBlockCount[diskDrive]) {
            diskStatus |= DISK_STATUS_ERROR;
            raiseInterrupt(PORT_DISK);
            return;
        }
        uint8_t *at = diskMemory[diskDrive] + (size_t)diskBlock * DISK_BLOCK_BYTES;
        if (command == DISK_COMMAND_WRITE) {
            memcpy(at, diskBuffer, DISK_BLOCK_BYTES);
        } else {
            memcpy(diskBuffer, at, DISK_BLOCK_BYTES);
        }
        raiseInterrupt(PORT_DISK);
        return;
    }
    if (diskImage[diskDrive] == NULL || diskBlock >= diskBlockCount[diskDrive]) {
        diskStatus |= DISK_STATUS_ERROR;
        raiseInterrupt(PORT_DISK);
        return;
    }
    long offset = (long)diskBlock * DISK_BLOCK_BYTES;
    if (fseek(diskImage[diskDrive], offset, SEEK_SET) != 0) {
        diskStatus |= DISK_STATUS_ERROR;
        raiseInterrupt(PORT_DISK);
        return;
    }
    if (command != DISK_COMMAND_READ && command != DISK_COMMAND_WRITE) {
        diskStatus |= DISK_STATUS_ERROR;
        raiseInterrupt(PORT_DISK);
        return;
    }
    if (diskLatency == 0) {
        diskTransfer(command);
        return;
    }
    // It is going to take a while. Say so, and remember what to do when it is over.
    diskStatus |= DISK_STATUS_BUSY;
    diskPending = command;
    diskReadyAt = deviceNow + diskLatency;
}

// The transfer itself, whenever it happens to happen. The seek is done here rather than at
// the command, because nothing else may touch the image in between and doing it twice is
// the same answer.
// ---- Finishing what a drive was in the middle of ----
//
// A transfer waits for the clock, so at any moment one may be owed. Changing drives with one
// outstanding would run it against the disk that is arriving instead of the one that asked,
// so the drive register calls this first and the transfer happens now.
//
// The waiting is what is given up, not the work. A program that changes drives without
// looking at the status bit has not lost anything it had asked for.
static void diskSettle(void) {
    if (diskPending) {
        const uint8_t command = diskPending;
        diskPending = 0;
        diskTransfer(command);
    }
}

static void diskTransfer(uint8_t command) {
    size_t moved = 0;
    long offset = (long)diskBlock * DISK_BLOCK_BYTES;
    if (fseek(diskImage[diskDrive], offset, SEEK_SET) != 0) {
        diskStatus |= DISK_STATUS_ERROR;
    } else if (command == DISK_COMMAND_READ) {
        moved = fread(diskBuffer, 1, DISK_BLOCK_BYTES, diskImage[diskDrive]);
    } else {
        moved = fwrite(diskBuffer, 1, DISK_BLOCK_BYTES, diskImage[diskDrive]);
        fflush(diskImage[diskDrive]);
    }
    if (moved != DISK_BLOCK_BYTES) {
        diskStatus |= DISK_STATUS_ERROR;
    }
    diskStatus &= (uint8_t)~DISK_STATUS_BUSY;
    raiseInterrupt(PORT_DISK);
}

void setDiskLatency(unsigned long cycles) {
    diskLatency = cycles;
}

void deviceTick(unsigned long now) {
    deviceNow = now;
    // The screen blinks its cursor on the machine's own clock rather than the host's, so the
    // picture is the same at the same cycle count however fast anything ran.
    videoTick(now);
    // And the sound, which makes whatever samples are due by now. On the machine's clock,
    // so the same program makes the same sound in the same cycles.
    soundTick(now);
    // And the timer, which is the only beat a program can choose for itself.
    timerTick(now);
    // And the pads, whose recordings step on a frame so that a level read twice in one is
    // the same level both times.
    padTick(now);
    if (diskPending && now >= diskReadyAt) {
        diskSettle();
    }
}

// ---- A beat a program sets for itself ----
//
// COUNTED IN CYCLES, which is this machine's unit of time everywhere else: it is what the
// cost model counts and what a frame is measured in. A timer counting anything else would be
// a second thing to remember, and a prescaler would buy range that twenty four bits already
// covers - one cycle at one end and sixteen point seven seconds at the other, with 120 beats
// a minute sitting at 500,000 in the middle of it.
static uint32_t timerPeriod = 0;
static uint32_t timerLeft = 0;
static uint8_t timerControl = 0;
static uint8_t timerTicked = 0;
static unsigned long timerLast = 0;

void timerReset(void) {
    timerPeriod = 0;
    timerLeft = 0;
    timerControl = 0;
    timerTicked = 0;
    timerLast = 0;
    clearInterrupt(PORT_TIMER);
}

// ---- Caught up rather than counted ----
//
// The same shape as the screen's frame: the machine runs in batches, so more than one period
// can pass between two looks. What is owed is worked out from how far the clock moved rather
// than by being told once per cycle, and several periods at once still mean one tick - a
// missed one is missed, which is what missing one is.
void timerTick(unsigned long now) {
    const unsigned long moved = now - timerLast;
    timerLast = now;
    if (!(timerControl & TIMER_CONTROL_RUN) || timerPeriod == 0) {
        return;
    }
    if ((unsigned long)timerLeft > moved) {
        timerLeft -= (uint32_t)moved;
        return;
    }

    timerTicked = 1;
    if (timerControl & TIMER_CONTROL_INTERRUPT) {
        raiseInterrupt(PORT_TIMER);
    }
    if (timerControl & TIMER_CONTROL_REPEAT) {
        // What is left over carries into the next period, so a timer asked for 1,000 cycles
        // gets a tick every 1,000 and not every 1,000 plus however late anybody looked.
        const unsigned long over = moved - timerLeft;
        timerLeft = timerPeriod - (uint32_t)(over % timerPeriod);
    } else {
        timerControl = (uint8_t)(timerControl & ~TIMER_CONTROL_RUN);
        timerLeft = 0;
    }
}

static uint8_t timerWrite(uint8_t value, uint8_t port) {
    switch (port) {
        case TIMER_CONTROL: {
            const uint8_t wasRunning = timerControl & TIMER_CONTROL_RUN;
            timerControl = value;
            if ((value & TIMER_CONTROL_RUN) && !wasRunning) {
                // Starting loads the period. Asking it to run while it already is does not,
                // so a program that sets the interrupt bit half way through a period does not
                // silently move the beat it was keeping.
                timerLeft = timerPeriod;
            }
            if (!(value & TIMER_CONTROL_INTERRUPT)) {
                // Asking to stop being interrupted takes down whatever was already asked for,
                // the same as the screen and for the same reason.
                clearInterrupt(PORT_TIMER);
            }
        }
        break;
        case TIMER_PERIOD_HIGH:
            timerPeriod = (timerPeriod & 0x0000FFFFu) | ((uint32_t)value << 16);
        break;
        case TIMER_PERIOD_MID:
            timerPeriod = (timerPeriod & 0x00FF00FFu) | ((uint32_t)value << 8);
        break;
        case TIMER_PERIOD_LOW:
            timerPeriod = (timerPeriod & 0x00FFFF00u) | value;
        break;
        default: break;
    }
    return 0;
}

static uint8_t timerRead(uint8_t port) {
    switch (port) {
        case TIMER_STATUS: {
            uint8_t status = 0;
            if (timerTicked)                            status |= TIMER_STATUS_TICKED;
            if (timerControl & TIMER_CONTROL_RUN)       status |= TIMER_STATUS_RUNNING;
            if (timerControl & TIMER_CONTROL_INTERRUPT) status |= TIMER_STATUS_INTERRUPT;
            // Looking is what answers it, the same as every other status port here: a beat
            // that has been noticed is not still waiting to be, and the line goes down with
            // the flag because a program that polls is not one that will answer a handler.
            timerTicked = 0;
            clearInterrupt(PORT_TIMER);
            return status;
        }
        case TIMER_CONTROL:       return timerControl;
        case TIMER_PERIOD_HIGH:   return (uint8_t)(timerPeriod >> 16);
        case TIMER_PERIOD_MID:    return (uint8_t)(timerPeriod >> 8);
        case TIMER_PERIOD_LOW:    return (uint8_t)timerPeriod;
        default:                  return 0;
    }
}

// ---- A device that brings memory ----
//
// The simplest thing that owns a bank. Writing to its port fills its memory with the
// byte written, which stands in for a disk controller reading a sector: the CPU asks for
// something and the memory it owns then holds the answer. The waiting is taken out so a
// test runs the same way every time.

#define DEVICE_MEMORY_BYTES 256

static uint8_t deviceMemoryBlock[DEVICE_MEMORY_BYTES];

uint8_t *deviceMemory(uint8_t port, uint32_t *capacity) {
    if (port == PORT_MEMORY) {
        *capacity = DEVICE_MEMORY_BYTES;
        return deviceMemoryBlock;
    }
    if (port == PORT_DISK) {
        // The disk's buffer is one block. Reading fills it and writing takes what is in
        // it, and the only way to reach it is to register it as a bank and go through the
        // controller.
        *capacity = DISK_BLOCK_BYTES;
        return diskBuffer;
    }
    if (port == PORT_VIDEO || port == VIDEO_SCREEN0 || port == VIDEO_SCREEN1) {
        // Two banks: the atlas of tiles and colours on the base port, and the map or the
        // bitmap on its own. A program blits the part that changed and the rest stays as it
        // was, which is the whole reason the screen is memory rather than a window onto a
        // port - and having two means a picture costs the map and not the font.
        return videoMemory(port, capacity);
    }
    return NULL;
}

// ---- The bus registry ----
//
// What is plugged into this machine. The table is fixed when the machine is built: a
// program cannot write to it, because writing would only let a program lie to itself
// about what hardware exists. Which routine handles a device is a different question,
// and the vector table already answers it.
//
// Nothing here touches the device being asked about. That matters more than it looks:
// reading a port is a real operation, and asking the console what it is by reading it
// would take a character off standard input and block waiting for one.

typedef struct {
    uint8_t port;
    uint8_t deviceClass;
    uint8_t flags;
} DeviceRecord;

static const DeviceRecord deviceTable[] = {
    { PORT_CONSOLE,  DEVICE_CONSOLE,  0 },
    { PORT_TEST,     DEVICE_TEST,     0 },
    { PORT_REFUSE,   DEVICE_REFUSE,   0 },
    { PORT_MACHINE,  DEVICE_MACHINE,  0 },
    { PORT_MEMORY,   DEVICE_MEMORY,   DEVICE_FLAG_HAS_MEMORY },
    { PORT_DISK,     DEVICE_DISK,     DEVICE_FLAG_HAS_MEMORY },
    { PORT_VIDEO,    DEVICE_VIDEO,    DEVICE_FLAG_HAS_MEMORY },
    { PORT_SOUND,    DEVICE_SOUND,    0 },
    { PORT_TIMER,    DEVICE_TIMER,    0 },
    { PORT_PAD,      DEVICE_PAD,      0 },
    { PORT_REGISTRY, DEVICE_REGISTRY, 0 },
};
static const int deviceCount = (int)(sizeof(deviceTable) / sizeof(deviceTable[0]));

// Which port the registry is currently being asked about, and how far through that
// port's record it has been read. Selecting a port starts the record again.
static uint8_t registrySelected = 0;
static uint8_t registryCursor = 0;

static const DeviceRecord controllerRecord = { CONTROLLER_PORT_BASE, DEVICE_CONTROLLER, 0 };

static const DeviceRecord *deviceOnPort(uint8_t port) {
    // The controller answers on a block of ports rather than one, so every port in the
    // block reports it. Its own memory is bank 2, which is already registered, so it
    // does not set the flag that means "this brings memory somebody has to register".
    if (port >= CONTROLLER_PORT_BASE && port <= CONTROLLER_PORT_TOP) {
        return &controllerRecord;
    }
    if (port > PORT_CONSOLE && port <= PORT_CONSOLE_TOP) {
        // The status and control ports are the same device as the data port, which is the
        // one in the table and the one the console raises its line on.
        return deviceOnPort(PORT_CONSOLE);
    }
    if (port > PORT_DISK && port <= PORT_DISK_TOP) {
        // The base port is in the table proper, since that is the one that owns the
        // memory and raises the line. The rest of the block reports the same device.
        return deviceOnPort(PORT_DISK);
    }
    if (port > PORT_VIDEO && port <= PORT_VIDEO_TOP) {
        // Sixteen ports, one device, and the same rule again - with one difference, because
        // this device owns TWO banks. Forwarding the whole block to the base record used to
        // say that all sixteen ports brought memory, which was harmless only while nobody
        // believed it: a program that enumerated the block and registered everything
        // claiming memory would have faulted on the fourteen that have none.
        //
        // So the block answers honestly. The screen port says it brings memory because it
        // does, and the rest of the block says it does not.
        static const DeviceRecord videoScreen0Record =
            { VIDEO_SCREEN0, DEVICE_VIDEO, DEVICE_FLAG_HAS_MEMORY };
        static const DeviceRecord videoScreen1Record =
            { VIDEO_SCREEN1, DEVICE_VIDEO, DEVICE_FLAG_HAS_MEMORY };
        static const DeviceRecord videoPlainRecord = { PORT_VIDEO, DEVICE_VIDEO, 0 };
        if (port == VIDEO_SCREEN0) { return &videoScreen0Record; }
        if (port == VIDEO_SCREEN1) { return &videoScreen1Record; }
        return &videoPlainRecord;
    }
    if (port > PORT_SOUND && port <= PORT_SOUND_TOP) {
        return deviceOnPort(PORT_SOUND);
    }
    if (port > PORT_TIMER && port <= PORT_TIMER_TOP) {
        return deviceOnPort(PORT_TIMER);
    }
    // Sixteen ports, one device: four pads, a port saying which are there, and eleven kept
    // for the analogue axes that are not built.
    if (port > PORT_PAD && port <= PORT_PAD_TOP) {
        return deviceOnPort(PORT_PAD);
    }
    for (int i = 0; i < deviceCount; i++) {
        if (deviceTable[i].port == port) {
            return &deviceTable[i];
        }
    }
    return NULL;
}

// One byte of the selected port's record. Everything about a port that is not there
// reads as zero, which is the same answer an absent registry would give.
static uint8_t readRegistry(void) {
    const DeviceRecord *device = deviceOnPort(registrySelected);
    uint8_t answer = 0;
    if (device != NULL && registryCursor < DEVICE_RECORD_BYTES) {
        answer = (registryCursor == 0) ? device->deviceClass : device->flags;
    }
    if (registryCursor < DEVICE_RECORD_BYTES) {
        registryCursor++;
    }
    return answer;
}

uint8_t OutputHandler(uint8_t DataByte, uint8_t Address) {
    // Whichever port is being talked to is the one that would be doing any refusing.
    refusedPort = Address;
    // The controller answers on a block of ports, which is a range rather than a list.
    if (Address >= CONTROLLER_PORT_BASE && Address <= CONTROLLER_PORT_TOP) {
        return machineControllerWrite(DataByte, Address);
    }
    // And so does the screen.
    if (Address >= PORT_VIDEO && Address <= PORT_VIDEO_TOP) {
        return videoWrite(DataByte, Address);
    }
    if (Address >= PORT_SOUND && Address <= PORT_SOUND_TOP) {
        return soundWrite(DataByte, Address);
    }
    if (Address >= PORT_TIMER && Address <= PORT_TIMER_TOP) {
        return timerWrite(DataByte, Address);
    }
    // This function sends the DataByte to the appropriate place based on the Port Address.
    switch(Address) {
        case CONSOLE_DATA:
            // Both, always. The screen because this machine has one, and standard output
            // because the serial line is how everything that is not a person reads it.
            //
            // The terminal is told where the cursor went first, if it has not been told
            // since it was moved. Here rather than at the move, so setting a row and a
            // column costs one sequence rather than two.
            consoleSayCursor();
            consoleDraw(DataByte);
            putchar(DataByte);
        break;
        case CONSOLE_CONTROL: consoleSetControl(DataByte); break;
        case CONSOLE_CURSOR_ROW:
            // Clamped rather than refused. A cursor asked to go off the screen has an
            // obvious place to be, and stopping the machine over one is a poor trade.
            cursorRow = DataByte < videoRows() ? DataByte : videoRows() - 1;
            cursorTold = 0;
            consoleCursorMoved();
        break;
        case CONSOLE_CURSOR_COLUMN:
            cursorColumn = DataByte < videoColumns() ? DataByte : videoColumns() - 1;
            cursorTold = 0;
            consoleCursorMoved();
        break;
        case CONSOLE_COMMAND:
            if (DataByte == CONSOLE_COMMAND_CLEAR) {
                consoleClearScreen(0);
                // ---- And the cursor goes home ----
                //
                // A screen with nothing on it and a cursor half way down it is not a cleared
                // screen: the next thing written lands where the last thing happened to
                // leave off, which is a position that no longer means anything because
                // whatever gave it meaning has just been erased.
                //
                // The attribute is deliberately NOT reset. Clearing is about what is on the
                // screen and not about how the next thing will be drawn - a program that
                // chose a colour and then cleared still wants that colour, the same as on a
                // terminal, where 2J does not touch the graphics state either.
                cursorRow = 0;
                cursorColumn = 0;
                consoleCursorMoved();
                // 2J empties it and H is what puts the cursor at the top. A terminal given
                // only the first does exactly what this device did before this comment.
                consoleTellTerminal("\033[2J\033[H");
            }
            // Anything else does nothing. A command block reserved for later should be
            // quiet rather than fatal, the same as the screen's spare registers.
        break;
        case CONSOLE_ATTRIBUTE: consoleAttribute = DataByte; break;
        case CONSOLE_STATUS:
            // Read only. A device saying how it is does not take instructions through the
            // same hole, so a write here is ignored rather than meaning something.
        break;
        case DISK_BLOCK_HIGH: diskBlock = (uint16_t)(DataByte << 8) | (diskBlock & 0x00FF); break;
        case DISK_BLOCK_LOW:  diskBlock = (diskBlock & 0xFF00) | DataByte; break;
        case DISK_COMMAND:    diskCommand(DataByte); break;
        case DISK_DRIVE:
            // ---- Choosing which disk the registers mean ----
            //
            // Whatever the drive was doing is collected first. A controller told to change
            // drives in the middle of a transfer has no good answer, and the transfer it was
            // part way through belongs to the drive being left.
            //
            // A number past the end selects nothing rather than wrapping to drive 0. Wrapping
            // would mean a program asking for a drive that is not there quietly reading the
            // one that is, which is the same shape of fault as the bank number Grid took: it
            // succeeds, and the wrong disk answers. So the selection stands and every read of
            // it says so.
            diskSettle();
            if (DataByte < DISK_DRIVE_COUNT) {
                diskDrive = DataByte;
                diskStatus = diskProtected[diskDrive] ? DISK_STATUS_PROTECTED : 0;
            }
        break;
        case DISK_DRIVES:
        case DISK_FLAGS:
        case DISK_SIZE_HIGH:
        case DISK_SIZE_LOW:
            // Read only: how many drives there are, and what kind each one is, are facts
            // about the machine rather than instructions to it.
        break;
        case PORT_MACHINE:
            // Asked for here and acted on between instructions, because a device cannot
            // restart the machine from inside the instruction that asked: the CPU is part
            // way through a step and its state is not yet anything a reset could leave
            // consistently behind.
            if (DataByte == MACHINE_RESET) {
                resetWanted = 1;
            }
        break;
        case PORT_MEMORY:
            // Fills the memory this device owns with the byte written. Nothing is
            // reachable from here: to get at it, register it as a bank and go through
            // the controller, which is the only thing that can reach a device's memory.
            memset(deviceMemoryBlock, DataByte, DEVICE_MEMORY_BYTES);
        break;
        case PORT_REFUSE:
            // A device that refuses everything. It exists so that a device's ability to
            // stop the CPU can be tested before anything depends on it, and so that the
            // path stays tested once the memory controller is the only real user.
            refuseAccess(VECTOR_GUARD_VIOLATION);
        break;
        case PORT_REGISTRY:
            // Names the port the registry is being asked about. This is the only thing
            // that can be written to the registry, and it changes nothing about the
            // machine: it selects a question, it does not give an answer.
            registrySelected = DataByte;
            registryCursor = 0;
        break;
        case PORT_TEST:
            // A test device, and about the simplest one that can exist: writing to it
            // puts its own line up. It stands in for the shape a real device has, where
            // the CPU asks for something and is interrupted once the answer is ready,
            // with the waiting taken out so that a test runs the same way every time.
            // The byte written is ignored; only the asking matters.
            raiseInterrupt(PORT_TEST);
        break;
        default:
            // Writes to unused Output Ports are ignored.
            return 1;
        break;
    }
    return 0;
}

uint8_t InputHandler(uint8_t Address) {
    refusedPort = Address;
    if (Address >= CONTROLLER_PORT_BASE && Address <= CONTROLLER_PORT_TOP) {
        return machineControllerRead(Address);
    }
    if (Address >= PORT_VIDEO && Address <= PORT_VIDEO_TOP) {
        return videoRead(Address);
    }
    if (Address >= PORT_SOUND && Address <= PORT_SOUND_TOP) {
        return soundRead(Address);
    }
    if (Address >= PORT_PAD && Address <= PORT_PAD_TOP) {
        return padRead(Address);
    }
    if (Address >= PORT_TIMER && Address <= PORT_TIMER_TOP) {
        return timerRead(Address);
    }
    switch(Address) {
        case CONSOLE_DATA:
            // If data is sent here, it should be read from STDIN.
            return consoleReadByte();
        break;
        case CONSOLE_STATUS: return consoleStatus();
        case CONSOLE_CURSOR_ROW:    return (uint8_t)cursorRow;
        case CONSOLE_CURSOR_COLUMN: return (uint8_t)cursorColumn;
        case CONSOLE_ATTRIBUTE:     return consoleAttribute;
        case CONSOLE_COMMAND:
            // Write only. What it did is visible in the cursor and on the screen.
            return 0;
        break;
        case CONSOLE_CONTROL:
            // Write only. Reading it gives zero rather than what was last written, because
            // everything it sets is reported by the status port and one fact wants one
            // place to live.
            return 0;
        break;
        case DISK_BLOCK_HIGH: return (uint8_t)(diskBlock >> 8);
        case DISK_BLOCK_LOW:  return (uint8_t)(diskBlock & 0xFF);
        case DISK_DRIVE:      return diskDrive;
        case DISK_DRIVES:     return diskDrives;
        case DISK_FLAGS:
            // What the selected drive IS. Volatile means its contents do not survive the
            // machine stopping, which is the one thing a system cannot find out by looking.
            return (uint8_t)(diskMemory[diskDrive] != NULL ? DISK_FLAG_VOLATILE : 0);
        case DISK_SIZE_HIGH:  return (uint8_t)(diskBlockCount[diskDrive] >> 8);
        case DISK_SIZE_LOW:   return (uint8_t)(diskBlockCount[diskDrive] & 0xFF);
        case DISK_STATUS:
            // ---- Looking is what answers it ----
            //
            // The console takes its line down when the byte is read, because taking the byte
            // is what answers the console. The disk's answer is this port: the operation
            // finished, and whether it worked is what Status is for. So reading it takes the
            // line down, the same way.
            //
            // WITHOUT THIS THE ORDINARY IDIOM LEAVES A LINE STANDING. The documented shape of
            // waiting for a device reads the status, branches out if the device is already
            // done, and only WAITs otherwise - so on a disk fast enough to finish before the
            // first look, which is every disk here, the WAIT that would have taken the line
            // down is never reached. Nothing else was going to answer it either: the program
            // is masked and has no handler. The line then stands for the rest of the
            // machine's life, and the next program to set the Interrupt Flag is interrupted
            // on behalf of a read that finished before it was loaded.
            clearInterrupt(PORT_DISK);
            return diskStatus;
        case PORT_REFUSE:
            // Refuses reads as well, so both directions are covered.
            refuseAccess(VECTOR_GUARD_VIOLATION);
            return 0;
        break;
        case PORT_REGISTRY:
            // One byte of the selected port's record, then the next, and zero once the
            // record has run out.
            return readRegistry();
        break;
        default:
            // Reading from an unused port is ignored.
            return 0;
        break;
    }
}