Give the disk four drives, behind one controller

SEVERAL DISKS ARE ONE CONTROLLER AND NOT SEVERAL DEVICES, and the
instruction set decided that rather than taste. A port is an immediate byte
inside the OUT that names it - portOut takes it from Program Memory - so a
program cannot compute one. "The disk on port 0x20 plus drive times four"
is not something this machine can say, and two disks as two devices would
mean a branch on the drive number in all eleven places sbfs.asm names a
disk port. A drive register is what a floppy controller has always been.

  0x24  Drive, which the block, command and status registers refer to
  0x25  Drives, read only: how many are plugged in

--disk given more than once fills them in order. What is per drive is the
image, its size and its write protection; the block register, the status
and the one buffer belong to the controller, which is the same division
real hardware makes.

A drive that is not there is refused rather than wrapped, because wrapping
means a program asking for a drive this machine has not got quietly reading
the one it has - the same shape of fault as taking a bank number somebody
else was using. An EMPTY drive is a different thing and is selectable: a
controller has its drives whether or not there are disks in them, and
reading one fails with the error bit the way an empty drive should.

Changing drives finishes whatever the one being left was in the middle of.
A transfer waits for the clock, so one may be owed at any moment, and
running it against the disk that is arriving would be a fault with no
owner.

Also stops parseOptions setting its defaults field by field. It was nine
assignments beside a struct, and a list beside a thing drifts from the
thing: adding two fields left them holding whatever was on the stack, so a
machine given one disk was told it already had four drives. It is one
zeroing now, and a default that is not nought can be written under it where
it reads as the exception. That struct growing a field once before left
Voyager linked against an object that disagreed about its size.

Nothing in CosmOS uses any of this yet. The mount record is next.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW

Source/Emulator/utility.h

@@ -8,6 +8,7 @@
 #define UTILITY_H
 
 #include <stdint.h>
+#include "io.h"
 #include "cpu.h"
 
 // Results of reading the command line.
@@ -20,7 +21,14 @@ typedef struct {
     uint8_t fast;          // Ignore the cycle rate and run as fast as the host allows.
     unsigned long cycles;  // Stop after this many cycles. Zero means run until the program halts.
     unsigned long diskCycles; // How long a block move takes. Zero is instant, and the default.
+    // ---- The drives, in the order they were named ----
+    //
+    // --disk given more than once fills them in turn, so the first is drive 0 and the machine
+    // has as many as were asked for. One name is the ordinary case and reads exactly as it
+    // did when there could only be one.
     const char *disk;      // Disk image to attach, or NULL for a machine with no disk.
+    const char *disks[DISK_DRIVE_COUNT];
+    int diskCount;
     uint8_t writeProtect;  // Attach the disk read only, the way a tab on a floppy would.
     const char *screen;    // Where to save a picture of the screen when the machine stops.
     const char *keyboard;  // Feed the console from this file as a keyboard, not a terminal.