emu/rom.68k

  .long 0, start
start:
  move.w #fakestack, a7 
  move.b #0x1, d0 | Find the ROM card
  bra.b find_first_card
romfindret:
  move.l a0, a6 | Save the ROM card IO base in a6 for later
  lea (0xEE, a6), a7 | Set up the stack at the end of the ROM's IO space RAM
  bsr.b find_largest_ram | Find the largest RAM card and put the IO base in a5
  move.l a0, a5
  move.l d0, d7
  move.b #0x4, d0 | Find a storage card and put the IO base in a4
  bsr.b find_first_card
  move.l a0, a4
  | Transfer the bootsector load code to the ROM's
  | built in RAM at the start of it's IO space
  move.w #(ramcode_end - ramcode), d0 | Put the length of the ramcode in d0
  move.w #ramcode, a0 | Put the address of the ramcode in a0
  move.l a6, a1 | Put the start of the ROM's IO space RAM in a0
  ramcode_loop:
  move.b (a0)+, (a1)+ | Transfer a byte of ramcode to the ROM's IO space RAM
  dbra d0, ramcode_loop | Loop back if there is more to transfer
  jmp (a6) | Jump to the ramcode
  stop #0x2700

ramcode:
  move.b #0x0, (0xFE, a6) | Disable the ROM
  move.l #0x1, (a5) | Enable the RAM at base 0x0
  move.l d7, a7
  | Load sector 0 to 0x0
  move.l #0x0, (a4) | Set the sector number to 0
  | Transfer 0x100 (256) bytes from the storage card's data register to 0x0
  move.w #0x0, a1 | Load a1, the destination address register, with 0x0
  move.w #0xFF, d0 | Load d0 with the sector size - 1.
  sector_loop:
  move.b (4, a4), (a1)+ | Transfer a byte of sector data to the destination
  dbra d0, sector_loop | Loop back if there is more to transfer
  jmp (0x0).W | Jump to the loaded sector

  stop #0x2700 
ramcode_end:

| Finds the first card with the type in d0.b, and returns it's IO base address in a0, or 0 if not found
| Clobbers d1
find_first_card:
  move.l #0xff0000, a0 | a0 holds the address of the current card
ffc_loop:
  lea (0x100,a0), a0 | adda.l #$100,a0 ; Move to the next card
  move.b (0xff, a0), d1 | Load the type of the card into d1
  beq.b ffc_done | If the type is 0 (empty slot), we have scanned all cards, so exit the loop
  cmp.b d0, d1 | If the card is the type we want, return with the address in a0
  beq.b ffc_done
  bra.b ffc_loop | Loop back and check the next card
ffc_done:
  rts

| Finds the largest RAM card, and returns it's IO base address in a0 and size in d0, or 0 if not found
| Clobbers d1, a1
find_largest_ram:
  move.l #0x0, d0 | d0 holds the size of the largest RAM card found
  move.w #0x0, a0 | a0 holds the address of the largest RAM card found
  move.l #0xff0000, a1 | a1 holds the address of the current card
flr_loop:
  lea (0x100,a1), a1 | adda.l #$100,a0 ; Move to the next card
  move.b (0xff, a1), d1 | Load the type of the card into d1
  beq.b flr_done | If the type is 0 (empty slot), we have scanned all cards, so exit the loop
  cmp.b #0x2, d1 | If the card isn't a RAM card, skip it
  bne.b flr_loop
  move.l (0x4, a1), d1 | Load the card's size into d1
  cmp.l d0, d1 | If the current size is less than the largest size found, go back to the start of the loop
  ble.b flr_loop
  move.l d1, d0 | Store the sise and address of the new latgest card in s0 and a0
  move.l a1, a0
  bra.b flr_loop | Loop back and check the next card
flr_done:
  rts
fakestack: 
  .long romfindret