Reboot, and the machine device that makes it possible

Until now the only way to restart was to stop the emulator and run it
again, which meant the one thing the machine could not do was the thing
Once was written for. The loop now closes without leaving it:

  > Once /System/Boot/bare.bin
  next start: /System/Boot/bare.bin, once
  > Reboot
  starting again
  stage two
  just this once: /System/Boot/bare.bin
  bare metal: no system, just this

Writing 1 to port 0x13 asks the machine to start over. A PORT RATHER THAN A
SERVICE, because a reset has to work when the system does not: something
only askable through SWI would be unavailable in exactly the case that
wants it most, and a program that owns the whole machine has no system to
ask. It is device class 0x04, in the range kept for the machine rather than
among the peripherals, because it is not one - it is not attached to
anything and cannot be unplugged.

WHAT A RESET REPEATS IS HOW THE MACHINE STARTED. Named an image, the
emulator places it again; named none, the ROM is shadowed again and reads
the disk. Anything else would mean a reset changed what the machine IS,
which is the one thing a reset must not do. Both are tested.

Taken 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 behind consistently.

The disk stays attached and keeps everything written to it - that is what
warm means. The vector table is cleared, which is the one deliberate
departure from leaving memory alone: a vector points into whatever
installed it, and after a reset that program is not running, so a handler
left behind would aim an interrupt at an address belonging to something
gone. It is the argument CosmOS already makes at exit, applied to the
machine.

Reboot is 45 bytes, most of them the word it prints.

Source/Emulator/emulator.c

@@ -7,6 +7,7 @@
 
 #include "rom.h"
 #include "bootstrap.h"
+#include "../Assembler/assembly.h"
 #include <stdio.h>
 #include <stdint.h>
 #include <stdlib.h>
@@ -177,6 +178,38 @@ int main (int argc, char *argv[]) {
             cycleCount += took;
             // Time has passed, so anything waiting on it may be finished.
             deviceTick(cycleCount);
+
+            // ---- Starting over ----
+            //
+            // Between instructions, which is the only place it can happen: a device cannot
+            // restart the machine from inside the instruction that asked for it.
+            //
+            // WHAT A RESET REPEATS IS HOW THIS MACHINE STARTED. Named an image, it is
+            // placed again; named none, the ROM is shadowed again and reads the disk for
+            // the rest. Anything else would mean a reset changed what the machine is,
+            // which is the one thing a reset must not do.
+            //
+            // The disk is not unplugged and its image keeps everything written to it. That
+            // is what warm means: the machine starts again, the world it starts into does
+            // not.
+            if (takeResetRequest()) {
+                // The vector table goes, and that is a deliberate departure from leaving
+                // memory alone. A vector points into whatever installed it, and after this
+                // that program is not running - so a handler left behind would aim an
+                // interrupt at an address belonging to something gone. It is the argument
+                // CosmOS already makes when it takes a program's vectors back at exit.
+                memset(Program + SOFTWARE_VECTOR_BASE, 0,
+                       (size_t)(0x10000 - SOFTWARE_VECTOR_BASE));
+                uint8_t failed = (programFile != NULL)
+                               ? loadFile(programFile, Program, Data)
+                               : loadROM(bootROM, bootROMBytes, Program, Data);
+                if (failed) {
+                    fprintf(stderr, "Error: The machine could not be started again.\n");
+                    return 1;
+                }
+                initializeCPU(&cpu, Program, Data);
+                break;      // Out of this batch; the loop above carries on with a new CPU.
+            }
             if (cpu.Status & STATUS_HALT) {
                 // We've halted.
                 break;