rdos/boot/fat.asm

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2.1 KiB
NASM
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%define segment 0x00100
%define self (0x7C00-(segment<<4)) ; 1 sector
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cpu 8086
org self
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jmp short init
times (0x0B - ($-$$)) db 0
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fdc:
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.ss:
dw 0x200 ; sector size
.sc:
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db 2 ; sectors per cluster
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.rsc:
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dw 1 ; reserved sector count
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.fn:
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db 2 ; number of file allocation tables
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.rde:
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dw 0x70 ; number of root directory entries
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.ts:
dw 720 ; total number of sectors
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.mi: ; medium identifier
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db 0xFD ; 5.25-inch Double sided, 40 tracks per side, 9 sectors per track (360 KB)
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.sf: ; sectors per fat
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dw 2
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.spt:
dw 9 ; sectors per track
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.nos:
dw 2 ; number of sides (heads)
.po:
dd 0 ; partition offset (in LBA blocks)
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.lrgts:
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dd 0
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.drv:
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db 0 ; drive number
db 0
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db 0x29 ; efdc signature
.vid:
dd 0 ; volume id
.vlabel:
db "2B"
times (54 - ($-$$)) db " "
.fstype:
db "FAT12"
times (62 - ($-$$)) db " "
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; mformat keeps writing until here
; if we place init earlier, code gets overwritten
times (62 - ($-$$)) nop
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init:
cli
jmp segment:main
main:
; Stack grows down from 64k
mov ax, cs
mov ss, ax
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mov sp, BDOS
mov ds, ax
mov es, ax
mov [fdc.drv], dl ; save drive number in fd
mov ax, 1
mov cx, 8
mov dx, 0
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mov bx, BDOS
.loop:
push ax
push cx
push dx
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; add partition offset (required for HDD)
add ax, [fdc.po]
adc dx, [fdc.po+2]
; calculate CHS data
div word [cs:fdc.spt] ; ax:temp = (lba / spt)
inc dx ; dx:sector = (lba % spt) + 1
mov cl, dl ; sector number
xor dx, dx
div word [cs:fdc.nos] ; ax:cylinder = (tmp / heads)
; dx:head = (tmp % heads)
mov ch, al ; cylinder number
mov dh, dl ; head number
mov dl, [cs:fdc.drv] ; driver number
mov ax, 0x0201 ; ah=0x02 al=0x01
int 0x13
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mov bp, 0x3331
jc error
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pop dx
pop cx
pop ax
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; count up for next sector
add bx, 0x0200
add ax, 1
adc dx, 0
loop .loop
mov dl, [fdc.drv]
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; jump into bios cold boot entry point
jmp segment:BIOS
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error:
mov ax, bp
mov ah, 0x0e
mov bx, 7
int 0x10
mov al, 0x21
int 0x10
xor ax, ax
int 0x16
int 0x19
times (0x1FE - ($-$$)) db 0
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dw 0xAA55