rdos/boot/fat.asm

268 lines
4.4 KiB
NASM
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; Memory layout:
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%define self 0x00600 ; 1 sector
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%define fattab 0x01000 ; variable size
%define rootdir 0x06000 ; variable size
%define psp (prog - 0x100)
%define prog 0x07C00
%define arguments (psp + 0x81)
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org self
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jmp short init
cluster_offset:
dw 0
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
; Stack grows down from PSP + 64k
mov ax, (psp >> 4)
mov ss, ax
xor sp, sp
push sp
; Relocate from [prog] to [self]
xor ax, ax
mov ds, ax
mov es, ax
mov si, prog
mov di, self
mov cx, 0x100
rep movsw
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jmp 0:main
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main:
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
call load_file
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mov bp, 0x3332
jc error
; setup int 19h call if prog returns
mov WORD [psp], 0x19CD
; clean registers
xor ax, ax
xor cx, cx
xor dx, dx
xor bx, bx
; restore boot drive number
mov dl, [fdc.drv]
; setup segment registers
push ss
pop ds
push ss
pop es
; jump into target
jmp 0x07B0:0x0100
; 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
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mov bp, 0x3330
jnz error
.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
inc ax
loop .loop
ret
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; Load the file in [filename]
; or exit with carry set
load_file:
mov si, rootdir
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mov cx, [fdc.rde]
.loop:
call file_match
je read_clusters
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add si, 0x20
loop .loop
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stc
ret
; Compares the FAT dirent against [filename]
; in ds:si filename
; out ZF zero flag set if equal
file_match:
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push cx
push si
mov di, filename
mov cx, 11
repe cmpsb
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pop si
pop cx
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ret
; Read the file given by cluster number
; into the target program area
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; in ax cluster number
read_clusters:
add si, 0x1A
lodsw
mov bx, prog
.loop:
; 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 .loop
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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
filename:
db "KERNEL COM"
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times (0x1FE - ($-$$)) db 0
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dw 0xAA55