; Choosing which disk the registers mean.
;
; SEVERAL DISKS ARE ONE CONTROLLER AND NOT SEVERAL DEVICES, and the instruction set is the
; reason. A port is an immediate byte inside the OUT that names it, so a program cannot
; compute one - "the disk on port 0x20 plus drive times four" is not a thing this machine can
; say. Two disks as two devices would mean a branch per access in every place the filesystem
; names a disk port.
;
; So the drive is a register, the way a floppy controller has always done it.
;
; Written by Anachronaut

#Program

start:
  ; How many are plugged in. A fact about the machine, so it is read only.
  INA 0x25
  INIB 0d48
  CCF
  ADD
  OUTQ 0x00

  INIA 0d1
  OUTA 0x24
  INA 0x24
  CCF
  ADD                       ; B is still 48, from the count above.
  OUTQ 0x00                 ; It took.

  ; ---- A drive that is not there is refused, not wrapped ----
  ;
  ; Wrapping to drive 0 would mean a program asking for a drive this machine does not have
  ; quietly reading the one it does. That is the same shape of fault as taking a bank number
  ; somebody else is using: it succeeds, and the wrong disk answers.
  INIA 0d9
  OUTA 0x24
  INA 0x24
  CCF
  ADD
  OUTQ 0x00                 ; Still 1.

  ; ---- And reading follows the selection ----
  ;
  ; Drive 1 has no disk in it here, which is not the same as not existing: the controller has
  ; four drives whether or not there are disks in them, so this selects, reads, and fails.
  INIA 0d1
  OUTA 0x24
  RSTA
  OUTA 0x20
  OUTA 0x21
  INIA 0x01
  OUTA 0x22
waitDisk:
  INA 0x23
  INIB 0x01
  AND
  BNQ waitDisk
  INA 0x23
  INIB 0x02
  AND
  BRQ readWorked
  INIA 0d78                 ; 'N', which is what an empty drive should give.
  OUTA 0x00
  BRI done
readWorked:
  INIA 0d89
  OUTA 0x00
done:
  INIA 0d10
  OUTA 0x00
  HALT

#Vectors

  Boot start