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uefix.s: add CHS mode #13

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uefix.s: add CHS mode #13

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7a3a2f0
uefix.s: add CHS mode
sirocyl Jul 2, 2023
19df402
off by one error
sirocyl Jul 2, 2023
5cf0527
uefix.s: fix comment on drive numbers
sirocyl Jul 5, 2023
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61 changes: 49 additions & 12 deletions uefix.s
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Original file line number Diff line number Diff line change
Expand Up @@ -16,21 +16,46 @@
rep movsb
jmp 0:start
start:
mov ah, 0x42
mov si, packet
; Both CHS and LBA code is present in the loader, as we have the free space
; and switching between the two manually is a single-byte edit.
%ifdef FLOPPY
jmp load.chs
%else
jmp load.lba
%endif
Comment on lines +21 to +25
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Perhaps check whether the drive number provided by BIOS is less than 0x80, instead of hardcoding this at compile time?

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It was quick and dirty, but honestly that's probably a better approach. I'll hop on this and the 8 = HDD comment when I get back from breakfast.

Register DL contains the disk number on boot, so test 0x80, dl and jnz load.lba seems correct here, right?


load:
.chs:
mov ah, 0x02 ; function, chs load sector
mov al, 1 ; count sectors

mov dl, 0 ; drive (0 = floppy A:, 1 = floppy B:, 80 = HDD C:)
mov dh, 0 ; head
mov cl, 2 ; sector
mov ch, 0 ; cylinder
mov bx, 0x7c00 ; buffer
int 0x13
jc error
jc error.chs
jmp 0x7c00
.lba:
mov ah, 0x42 ; function, lba extended load
mov si, lbapacket ; location of int13h/AH=42h arguments packet
int 0x13
jc error.lba
jmp 0x7c00
packet:
db 0x10
db 0
dw 1 ; count
dw 0x7c00, 0 ; buffer
dq 1 ; LBA

error:
mov si, errmsg
.lba:
mov si, errmsg.lba
jmp error.loop
.chs:
; TODO: some FDDs may be slow to start, and need to reset and reread
; the sector, while the disk spins up. Implement code here to do so
; three or four times, then go ahead and print the error message.
mov si, errmsg.chs
jmp error.loop
.loop:
; TODO: Read error numbers from BIOS return regs, and print them with errmsg.
lodsb
or al, al
jz .done
Expand All @@ -43,13 +68,25 @@ error:
jmp .done

errmsg:
db "Disk error :(", 0
.lba:
db "Forth: BIOS returns an LBA loader error.", 0
.chs:
db "Forth: BIOS returns a CHS loader error.", 0

lbapacket:
db 0x10
db 0
dw 1 ; count
dw 0x7c00, 0 ; buffer
dq 1 ; LBA

blank:
times 446 - ($ - $$) db 0

mbrinfo:
db 0 ; not active
db 0, 1, 0 ; start CHS
db 0x42 ; type
db 0x7f ; partition type (0x7f: experimental)
db 0, 1, 0 ; end CHS
dd 1 ; start LBA
dd 1 ; length
Expand Down