rdos/fat.asm

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4.1 KiB
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; Memory layout:
%define segment 0x00100
%define self (0x7C00-(segment<<4)) ; 1 sector
%define fattab (self+0x200) ; variable size
%define rootdir 0x00100 ; variable size
%define prog 0x0F000 ; 4K at the end for OS
org self
jmp short init
cluster_offset:
dw 0
times (0x0B - ($-$$)) db 0
fdc:
.ss:
dw 0x200 ; sector size
.sc:
db 2 ; sectors per cluster
.rsc:
dw 1 ; reserved sector count
.fn:
db 2 ; number of file allocation tables
.rde:
dw 0x70 ; number of root directory entries
.ts:
dw 720 ; total number of sectors
.mi: ; medium identifier
db 0xFD ; 5.25-inch Double sided, 40 tracks per side, 9 sectors per track (360 KB)
.sf: ; sectors per fat
dw 2
.spt:
dw 9 ; sectors per track
.nos:
dw 2 ; number of sides (heads)
.po:
dd 0 ; partition offset (in LBA blocks)
.lrgts:
dd 0
.drv:
db 0 ; drive number
db 0
db 0x29 ; efdc signature
.vid:
dd 0 ; volume id
.vlabel:
db "2B"
times (54 - ($-$$)) db " "
.fstype:
db "FAT12"
times (62 - ($-$$)) db " "
; mformat keeps writing until here
; if we place init earlier, code gets overwritten
times (62 - ($-$$)) nop
init:
cli
jmp segment:main
main:
; Stack grows down from 64k
mov ax, cs
mov ss, ax
mov sp, prog
mov ds, ax
mov es, ax
mov [fdc.drv], dl ; save drive number in fd
sti
; load fat table into memory
mov ax, [fdc.rsc]
mov cx, [fdc.sf]
xor dx, dx
mov bx, fattab
call load_sectors
; calculate length of rootdir
mov ax, [fdc.rde]
mov cl, 4
shr ax, cl ; 32 bytes per entry
mov cx, ax
; load root dir
xor dx, dx
mov ax, [fdc.sf]
mul byte [fdc.fn]
add ax, [fdc.rsc]
mov bx, rootdir
call load_sectors
; remember where we left off
; clusters start after rootdir
mov [cluster_offset], ax
; Load kernel
mov bx, prog
mov ax, kernel_name
call load_file
mov bp, 0x3332
jc error
mov dl, [fdc.drv]
; jump into kernel
jmp segment:prog
; Load a file into memory
; IN AX pointer to 8.3 filename
; ES:BX pointer to target area
; OUT CF flag set if error
; DI file size in bytes (<64K)
load_file:
mov si, rootdir
mov cx, [fdc.rde]
.search:
push cx
push si
mov di, ax
mov cx, 11
repe cmpsb
pop si
pop cx
je .read
add si, 0x20
loop .search
xor di, di
stc
ret
.read:
mov ax, [si+0x1A]
mov di, [si+0x1C]
jmp read_clusters
; Read the file given by cluster number
; into the target program area
; in ax cluster number
read_clusters:
; read cluster into area for target file
push ax
sub ax, 2
mul BYTE [fdc.sc]
add ax, [cluster_offset]
xor dx, dx
mov cx, [fdc.sc]
xor ch, ch
call load_sectors
pop ax
; calculate index in FAT
mov si, ax
shr si, 1
add si, ax
add si, fattab
; load entry from FAT, truncate to 12 bit
mov dx, [si]
test ax, 1
jz .noshift
mov cl, 4
shr dx, 4
.noshift:
mov ax, dx
and ax, 0x0FFF
cmp ax, 0x0FF8
jc read_clusters
ret
; Read sectors from disk
; Does not return on error
; ax and bx will be incremented, cx decremented
; in dx:ax sector number
; es:bx buffer
; cx number of sectors to read
; out dx:ax next sector to read
; es:bx next free buffer
; cx zero
load_sectors:
; fail instantly if reading sectors > 16 bit
test dx, dx
mov bp, 0x3330
jnz error
.loop:
push ax
push cx
push dx
; 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
mov bp, 0x3331
jc error
pop dx
pop cx
pop ax
; count up for next sector
add bx, 0x0200
inc ax
loop .loop
ret
error:
mov ax, bp
mov ah, 0x0e
mov bx, 7
int 0x10
mov al, 0x21
int 0x10
xor ax, ax
int 0x16
int 0x19
kernel_name:
db "BDOS BIN"
times (0x1FE - ($-$$)) db 0
dw 0xAA55