79 lines
3.1 KiB
Plaintext
79 lines
3.1 KiB
Plaintext
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.long 0, start
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start:
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move.w #fakestack, a7
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move.b #0x1, d0 ; Find the ROM card
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bra.b find_first_card
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romfindret:
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move.l a0, a6 ; Save the ROM card IO base in a6 for later
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lea (0xEE, a6), a7 ; Set up the stack at the end of the ROM's IO space RAM
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bsr.b find_largest_ram ; Find the largest RAM card and put the IO base in a5
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move.l a0, a5
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move.l d0, d7
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move.b #0x4, d0 ; Find a storage card and put the IO base in a4
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bsr.b find_first_card
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move.l a0, a4
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; Transfer the bootsector load code to the ROM's
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; built in RAM at the start of it's IO space
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move.w #(ramcode_end - ramcode), d0 ; Put the length of the ramcode in d0
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move.w #ramcode, a0 ; Put the address of the ramcode in a0
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move.l a6, a1 ; Put the start of the ROM's IO space RAM in a0
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ramcode_loop:
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move.b (a0)+, (a1)+ ; Transfer a byte of ramcode to the ROM's IO space RAM
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dbra d0, ramcode_loop ; Loop back if there is more to transfer
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jmp (a6) ; Jump to the ramcode
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stop #0x2700
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ramcode:
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move.b #0x0, (0xFE, a6) ; Disable the ROM
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move.l #0x1, (a5) ; Enable the RAM at base 0x0
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move.l d7, a7
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; Load sector 0 to 0x0
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move.l #0x0, (a4) ; Set the sector number to 0
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; Transfer 0x100 (256) bytes from the storage card's data register to 0x0
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move.w #0x0, a1 ; Load a1, the destination address register, with 0x0
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move.w #0xFF, d0 ; Load d0 with the sector size - 1.
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sector_loop:
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move.b (4, a4), (a1)+ ; Transfer a byte of sector data to the destination
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dbra d0, sector_loop ; Loop back if there is more to transfer
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jmp (0x0).W ; Jump to the loaded sector
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stop #0x2700
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ramcode_end:
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; Finds the first card with the type in d0.b, and returns it's IO base address in a0, or 0 if not found
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; Clobbers d1
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find_first_card:
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move.l #0xff0000, a0 ; a0 holds the address of the current card
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ffc_loop:
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lea (0x100,a0), a0 ; adda.l #$100,a0 ; Move to the next card
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move.b (0xff, a0), d1 ; Load the type of the card into d1
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beq.b ffc_done ; If the type is 0 (empty slot), we have scanned all cards, so exit the loop
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cmp.b d0, d1 ; If the card is the type we want, return with the address in a0
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beq.b ffc_done
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bra.b ffc_loop ; Loop back and check the next card
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ffc_done:
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rts
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; Finds the largest RAM card, and returns it's IO base address in a0 and size in d0, or 0 if not found
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; Clobbers d1, a1
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find_largest_ram:
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move.l #0x0, d0 ; d0 holds the size of the largest RAM card found
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move.w #0x0, a0 ; a0 holds the address of the largest RAM card found
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move.l #0xff0000, a1 ; a1 holds the address of the current card
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flr_loop:
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lea (0x100,a1), a1 ; adda.l #$100,a0 ; Move to the next card
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move.b (0xff, a1), d1 ; Load the type of the card into d1
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beq.b flr_done ; If the type is 0 (empty slot), we have scanned all cards, so exit the loop
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cmp.b #0x2, d1 ; If the card isn't a RAM card, skip it
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bne.b flr_loop
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move.l (0x4, a1), d1 ; Load the card's size into d1
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cmp.l d0, d1 ; If the current size is less than the largest size found, go back to the start of the loop
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ble.b flr_loop
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move.l d1, d0 ; Store the sise and address of the new latgest card in s0 and a0
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move.l a1, a0
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bra.b flr_loop ; Loop back and check the next card
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flr_done:
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rts
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fakestack:
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.long romfindret
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