rdos/boot/mbr.asm

137 lines
1.6 KiB
NASM

; Memory layout:
%define self 0x00600 ; 1 sector
%define prog 0x07C00 ; 1 sector
; FDC fields in VBR
%define spt (prog + 0x18)
%define nos (prog + 0x1A)
%define po (prog + 0x1C)
org self
cpu 8086
init:
cli
; Stack grows down from PSP + 64k
xor ax, ax
mov ss, ax
mov sp, ax
push dx
; Relocate from [prog] to [self]
mov ds, ax
mov es, ax
mov si, prog
mov di, self
mov cx, 0x100
rep movsw
jmp 0:main
main:
mov bp, 0x3335
mov si, part1
mov cx, 4
.loop:
test BYTE [si], 0x80
jnz loadpart
add si, 0x10
loop .loop
jmp error
loadpart:
; transfer starting address into DAP
push si
add si, 0x08
mov di, dap.blocknum
movsw
movsw
pop si
; load sector
push si
mov si, dap
mov bp, 0x3336
mov ah, 0x42
stc
int 0x13
jc error
pop si
cmp BYTE [si+4], 0x01
jne jump
adjust:
push dx
mov bp, 0x3337
mov ah, 0x08
stc
int 0x13
jc error
; update sectors per track
xor ax, ax
mov al, cl
mov [spt], ax
; update number of sides
xor ax, ax
mov al, dh
mov [nos], ax
; update partition offset
push si
add si, 0x08
mov di, po
movsw
movsw
pop si
pop dx
jump:
jmp 0:prog
error:
mov ax, bp
mov ah, 0x0e
mov bx, 7
int 0x10
mov al, 0x21
int 0x10
xor ax, ax
int 0x16
int 0x19
dap:
.size:
db 0
db 0
.count:
dw 1
.buffer:
dw prog
dw 0
.blocknum:
dq 0
times (0x1BE - ($-$$)) db 0
part1:
db 0x80
.chs_start:
db 0xFF, 0xFF, 0xFF
.type:
db 0x01
.chs_end:
db 0xFF, 0xFF, 0xFF
.begin:
dd 1
.end:
dd (FLOPPY * 2)
times (0x1FE - ($-$$)) db 0
dw 0xAA55