rdos/boot/floppy.asm

267 lines
4.4 KiB
NASM

; Memory layout:
%define mbr 0x00600 ; 1 sector
%define self 0x00800 ; 1 sector
%define fattab 0x01000 ; variable size
%define rootdir 0x06000 ; variable size
%define psp 0x07B00 ; 0x100 bytes
%define prog 0x07C00 ; 64k - psp
org self
fdc:
.jmp:
jmp short main
times (0x0B - ($-$$)) db 0
.ss:
dw 0x200 ; sector size
.sc:
db 2 ; sectors per cluster
.rsc:
dw 1 ; reserved sector count
.fn:
db 2 ; number of file allocation tables
.rde:
dw 0x70 ; number of root directory entries
.ts:
dw 720 ; total number of sectors
.mi: ; medium identifier
db 0xFD ; 5.25-inch Double sided, 40 tracks per side, 9 sectors per track (360 KB)
.sf: ; sectors per fat
dw 2
.spt:
dw 9 ; sectors per track
.nos:
dw 2 ; number of sides (heads)
.po:
dd 0 ; partition offset (in LBA blocks)
.lrgts:
dd 0
.drv:
db 0 ; drive number
db 0
db 0x29 ; efdc signature
.vid:
dd 0 ; volume id
.vlabel:
db "2B"
times (54 - ($-$$)) db " "
.fstype:
db "FAT12"
times (62 - ($-$$)) db " "
main:
cli
; Stack grows down from PSP + 64k
mov ax, (psp >> 4)
mov ss, ax
xor sp, 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
; Jump into relocated code
jmp 0:.relocated
.relocated:
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
mov bp, 0x0032
jc error
; get length of arguments
mov cx, 0x007F
mov di, arguments
xor ax, ax
repne scasb
dec di
sub di, arguments
mov cx, di
; setup arguments field
mov [psp+0x080], cl
mov di, 0x081
mov si, arguments
rep movsb
mov BYTE [di], 0x0D
; setup int 19h call at 0000, and push its address to stack
mov WORD [psp], 0x19CD
push sp
; clear out registers and jump into target
xor ax, ax
xor cx, cx
xor dx, dx
xor bx, bx
mov dl, [fdc.drv]
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
mov bp, 0x0030
jnz error
.loop:
push ax
push cx
push dx
; 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, 0x0031
jc error
pop dx
pop cx
pop ax
; count up for next sector
add bx, 0x0200
inc ax
loop .loop
ret
; Load the file in [filename]
; or exit with carry set
load_file:
mov si, rootdir
mov cx, [fdc.rde]
.loop:
call file_match
je read_clusters
add si, 0x20
loop .loop
ret
; Compares the FAT dirent against [filename]
; in ds:si filename
; out ZF zero flag set if equal
file_match:
push cx
push si
mov di, filename
mov cx, 11
repe cmpsb
pop si
pop cx
ret
; Read the file given by cluster number
; into the target program area
; 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
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
cluster_offset:
dw 0
filename:
db "KERNEL COM"
arguments:
db "", 0
times (0x1FE - ($-$$)) db 0
dw 0xAA55