rdos/boot/vbr.asm

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2.6 KiB
NASM
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org 0x7C00
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fdc: ; FDC Descriptor as per ECMA-107
jmp _startup
times (0x03 - ($-$$)) nop
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.oem:
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db "2B" ; creating system identifier
times (0x0B - ($-$$)) db " " ; space-padded
.ss:
dw 0x200 ; sector size
.sc:
db 0 ; sectors per cluster
.rsc:
dw 2 ; reserved sector count
.fn:
db 0 ; number of file allocation tables
.rde:
dw 0 ; number of root directory entries
.ts_old:
dw 0 ; legacy: total number of sectors
.mi: ; medium identifier
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db 0xFD ; 5.25-inch Double sided, 40 tracks per side, 9 sectors per track (360 KB)
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.sf: ; sectors per fat
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dw 0
.spt:
dw 9 ; sectors per track
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.nos:
dw 2 ; number of sides (heads)
.po:
dd 0 ; partition offset (in LBA blocks)
.ts:
dw 720
.drv:
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db 0 ; drive number
db 0
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db 0x29 ; efdc signature
.vid:
dd 0 ; volume id
.vlabel:
db "2B"
times (54 - ($-$$)) db " "
.fstype:
db "FAT12"
times (62 - ($-$$)) db " "
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_startup:
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mov [fdc.drv], dl ; backup driver number
mov ah, 0x08
int 0x13 ; required, QEMU detects 360kB as 1200kB with CHS 80 2 15
jc .noadjust ; skip if function does not exist
inc dh
mov [fdc.nos], dh
mov ax, cx
and ax, 0x003F
mov [fdc.spt], ax ; no adjustment because sectors are 1-indexed
mov ax, cx
xchg al, ah
mov cl,6
shr ah,cl
inc ax ; convert from maximum number (0-based) to total number (1-based) of cylinders
mul word [fdc.nos] ; number of tracks = number of cylinders * heads
mul word [fdc.spt] ; number of sectors = number of tracks * sectors per track
mov [fdc.ts], ax
.noadjust:
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mov ax, 0x0500
mov es, ax
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mov bx, 0x0000
mov ax, 1
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call loadblk
call dump
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mov al, [fdc.sc]
call print8
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.end:
hlt
jmp .end
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; Load a cluster from cluster offset
loadcluster:
ret
; Load a single cluster into memory
; in ax sector number
; es:bx buffer
; out al error code,
; carry set if error
loadblk:
xor dx, dx
div word [fdc.spt] ; ax:temp = (lba / spt)
inc dx ; dx:sector = (lba % spt) + 1
mov cl, dl ; sector number
xor dx, dx
div word [fdc.nos] ; ax:cylinder = (tmp / heads)
; dx:head = (tmp % heads)
mov ch, al ; cylinder number
mov dh, dl ; head number
mov dl, [fdc.drv] ; driver number
mov ax, 0x0201 ; ah=0x02 al=0x01
int 0x13
ret
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dump:
mov si, 0x0000
mov cx, 0x10
.nextline:
mov ax, si
call print16
mov al, 0x3A
call printc
mov al, 0x20
call printc
push cx
mov cx, 0x0020
.words:
es lodsb
call print8
loop .words
pop cx
mov al, 0x0A
call printc
mov al, 0x0D
call printc
loop .nextline
ret
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%include "print.asm"
times (0x1FE - ($-$$)) nop
dw 0xAA55