rom/rom.68k

  section .text,text
  dc.l fakestack, _start
  public _start
_start:
  move.w #$1, d0 ; Find the ROM card
  bra.b find_first_card
romfindret:
  move.l a0, a2 ; Save the ROM card IO base in a6 for later
  move.l #$10000, a7 ; Set up the stack at the end of the ROM card's RAM
  bsr.w find_largest_ram ; Find the largest RAM card and put the IO base in a3
  move.l a0, a3
  move.l d0, d2
  move.l #$8100, a1
  bsr.w find_all_ram_cards
  move.l #$8100, a0
  bsr.w sort_ram_cards
  move.w #$4, 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 a2, 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 (a2) ; Jump to the ramcode

ramcode:
  move.b #$0, ($F3,a2) ; Disable the ROM
  move.l #$1, (a3) ; Enable the RAM at base $0
  cmpi.l #$1000000, d2
  bcs.b sp_ok
  move.l #$ff0000, d2
  sp_ok:
  move.l d2, a7
  ; Load sector 0 to $0
  move.l #$0, ($0,a4) ; Set the sector number to 0
  move.l #$1, ($4,a4) ; Set the sector count to 1
  move.l #$0, ($C,a4) ; Set the destination address to $0
  move.w #$1, ($8,a4) ; Send a DMA read command
  jmp ($0).W ; Jump to the loaded sector
ramcode_end:

nop ; Padding to make sure ramcode_end and find_first_card are different

; Finds the first card with the type in d0.w, and returns it's IO base address in a0, or 0 if not found
find_first_card:
  move.l #$ff0000, a0 ; a0 holds the address of the current card
ffc_loop:
  lea ($100,a0), a0 ; adda.l #$100,a0 ; Move to the next card
  move.w ($fe,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.w 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
find_largest_ram:
  move.l #$0, d0 ; d0 holds the size of the largest RAM card found
  move.w #$0, a0 ; a0 holds the address of the largest RAM card found
  move.l #$ff0000, a1 ; a1 holds the address of the current card
flr_loop:
  lea ($100,a1), a1 ; adda.l #$100,a0 ; Move to the next card
  move.w ($fe,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.w #$2, d1 ; If the card isn't a RAM card, skip it
  bne.b flr_loop
  move.l ($4,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
  bls.b flr_loop
  move.l d1, d0 ; Store the size and address of the new largest card in d0 and a0
  move.l a1, a0
  bra.b flr_loop ; Loop back and check the next card
flr_done:
  rts
fakestack: 
  dc.l romfindret

; 1KB buffer in a1
; Returns list length in d0
find_all_ram_cards:
  move.b #0, d0 ; d0 holds the number of RAM cards found.
  move.l #$ff0000, a0 ; a0 holds the address of the current card.
farc_loop:
  lea ($100,a0), a0 ; adda.l #$100,a0 ; Move to the next card
  move.w ($fe,a0), d1 ; Load the type of the card into d1
  beq.b farc_done ; If the type is 0 (empty slot), we have scanned all cards, so exit the loop
  cmp.w #$2, d1 ; If the card isn't a RAM card, skip it
  bne.b farc_loop 
  move.l a0, (a1)+ ; Write the IO base address into the buffer and advance the buffer pointer
  addq.b #1, d0 ; Increment the count of found RAM cards
  bra.b farc_loop ; Loop back and check the next card
farc_done:
  move.l #0, (a1) ; Write a null terminator on the end of the list
  rts

; optimized bubble sort to sort RAM cards by size
; Buffer in a0
; Length in d0
sort_ram_cards:
  movem.l d2-d4/a2, -(a7) ; Save calle-preserved registers
src_outer_loop:
  move.b #0, d1 ; d1 = newlen
  move.b #1, d2 ; d2 = i
  move.l a0, a1 ; a1 holds the pointer to the current pair
src_inner_loop:
  cmp.b d0, d2 ; if i >= length, exit the inner loop
  bge.b src_inner_loop_done
  move.l (a1), a2 ; Read the first card's size
  move.l ($4,a2), d3
  move.l (4,a1), a2 ; Read the second card's size
  move.l ($4,a2), d4
  cmp.l d3, d4 ; if d4 <= d3, branch to src_pair_sorted
  bls.b src_pair_sorted
  move.l (a1), d3 ; Read the first value
  move.l ($4,a1), d4 ; Read the second value
  move.l d3, ($4,a1) ; Write the first value where the second value was
  move.l d4, (a1)+ ; Write the second value where the first value was and increment a1 for the next pair
  move.b d2, d1 ; i = newlem
src_pair_sorted:
  addq.b #1, d2 ; i++
  bra.b src_inner_loop
src_inner_loop_done:
  move.b d1, d0 ; len = newlen
  cmp #1, d0 ; if n > 1, branch to outer loop
  bgt.b src_outer_loop
  movem.l (a7)+, d2-d4/a2 ; Restore calle-preserved registers
  rts