SplitBit-Emulator/Source/Emulator/video.c

// video.c
// The Voyager's video device.
// Written by Anachronaut

#include "video.h"
#include <stdio.h>
#include <string.h>

// The bank the device brings. Registered by whoever enumerates the hardware, reached only
// through the memory controller, and never by the CPU directly - the same arrangement the
// disk's buffer has always had.
static uint8_t videoRAM[VIDEO_MEMORY_BYTES];

static uint8_t mode;
// Which map row is drawn at the top. THE MAP IS A RING: rendering row r reads map row
// (scroll + r) wrapped, so scrolling a screen moves this byte and moves no memory at all.
//
// That is worth more than it looks. Blitting a 40 by 25 screen up one line is 1,920 bytes
// inside one bank, which is 1,920 cycles even with the controller widened - twelve percent
// of a frame, every line. A program printing one page would spend six frames shuffling
// memory. Here it costs one port write, and the rows that scrolled off are still there,
// which is where the console gets scrollback it never had.
static uint8_t scroll;

static uint8_t pixels[VIDEO_MAX_WIDTH * VIDEO_MAX_HEIGHT * 3];
static int renderedWidth = 0;
static int renderedHeight = 0;

static int columnsFor(uint8_t m) { return m == VIDEO_MODE_80x50 ? 80 : 40; }
static int rowsFor(uint8_t m)    { return m == VIDEO_MODE_80x50 ? 50 : 25; }

void videoReset(void) {
    memset(videoRAM, 0, sizeof(videoRAM));
    mode = VIDEO_MODE_40x25;
    scroll = 0;
    renderedWidth = 0;
    renderedHeight = 0;
}

uint8_t *videoMemory(uint32_t *capacity) {
    *capacity = VIDEO_MEMORY_BYTES;
    return videoRAM;
}

uint8_t videoWrite(uint8_t value, uint8_t port) {
    switch (port) {
        case VIDEO_MODE:
            // A mode that does not exist is not taken. Refusing outright would be the other
            // choice, but a screen is not the place to stop the machine: a program that
            // asked for something impossible still has the screen it had.
            if (value < VIDEO_MODE_COUNT) {
                mode = value;
            }
        break;
        case VIDEO_SCROLL:
            // Wrapped rather than clipped, because the map is a ring and every byte names a
            // row that exists.
            scroll = (uint8_t)(value % VIDEO_MAP_ROWS);
        break;
        default:
            // Everything else is read only or not there yet. Writing does nothing rather
            // than refusing: a port block reserved for later should be quiet, not fatal.
        break;
    }
    return 0;
}

uint8_t videoRead(uint8_t port) {
    switch (port) {
        // Reserved for the frame interrupt, which is the next rung. Zero until then.
        case VIDEO_STATUS:  return 0;
        case VIDEO_MODE:    return mode;
        // Asked rather than assumed. A program that wants to know how wide the screen is
        // should be able to find out, the same way it asks the console what mode it is in.
        case VIDEO_COLUMNS: return (uint8_t)columnsFor(mode);
        case VIDEO_ROWS:    return (uint8_t)rowsFor(mode);
        case VIDEO_SCROLL:  return scroll;
        default:            return 0;
    }
}

void videoRender(void) {
    const int columns = columnsFor(mode);
    const int rows = rowsFor(mode);
    const int width = columns * VIDEO_CELL_PIXELS;

    for (int row = 0; row < rows; row++) {
        // The ring. Rows that scrolled off the top are still in the map, which is what
        // makes scrollback free rather than something the console has to keep itself.
        const int mapRow = (scroll + row) % VIDEO_MAP_ROWS;
        const uint8_t *cells = videoRAM + VIDEO_MAP_BASE + mapRow * VIDEO_MAP_STRIDE;
        for (int column = 0; column < columns; column++) {
            const uint8_t tile = cells[column * VIDEO_CELL_BYTES];
            const uint8_t attribute = cells[column * VIDEO_CELL_BYTES + 1];
            // ---- The additive nibble ----
            //
            // The low nibble of the attribute is added to every palette index in the tile,
            // sixteen at a time. A tile drawn in indices 0 to 15 therefore appears in any
            // of sixteen colour schemes without a second copy of it in tile memory, and a
            // tile that wants all 256 colours simply leaves the nibble at zero and gets
            // them. One adder in hardware, and neither use costs the other anything.
            const uint8_t bank = (uint8_t)((attribute & 0x0F) << 4);
            const uint8_t *art = videoRAM + VIDEO_TILE_BASE + tile * VIDEO_TILE_BYTES;
            for (int y = 0; y < VIDEO_CELL_PIXELS; y++) {
                uint8_t *out = pixels + ((row * VIDEO_CELL_PIXELS + y) * width
                                         + column * VIDEO_CELL_PIXELS) * 3;
                for (int x = 0; x < VIDEO_CELL_PIXELS; x++) {
                    // Wrapping, because a byte plus a byte is a byte. A tile using the
                    // high end of the palette with a nibble set comes round the bottom,
                    // which is what an adder does and what the manual says it does.
                    const uint8_t index = (uint8_t)(art[y * VIDEO_CELL_PIXELS + x] + bank);
                    const uint8_t *entry = videoRAM + VIDEO_PALETTE_BASE
                                         + index * VIDEO_PALETTE_BYTES;
                    *out++ = entry[0];
                    *out++ = entry[1];
                    *out++ = entry[2];
                }
            }
        }
    }
    renderedWidth = width;
    renderedHeight = rows * VIDEO_CELL_PIXELS;
}

const uint8_t *videoPixels(int *width, int *height) {
    *width = renderedWidth;
    *height = renderedHeight;
    return pixels;
}

// A binary PPM, because it is the smallest format that needs no library to write and no
// library to read - which matters when the thing reading it is a test script.
int videoWriteImage(const char *path) {
    videoRender();
    FILE *file = fopen(path, "wb");
    if (file == NULL) {
        fprintf(stderr, "Error: Couldn't write the screen to: %s\n", path);
        return 1;
    }
    fprintf(file, "P6\n%d %d\n255\n", renderedWidth, renderedHeight);
    size_t bytes = (size_t)renderedWidth * (size_t)renderedHeight * 3;
    size_t written = fwrite(pixels, 1, bytes, file);
    fclose(file);
    if (written != bytes) {
        fprintf(stderr, "Error: The screen was not written whole to: %s\n", path);
        return 1;
    }
    return 0;
}