rdos/boot/floppy.asm
2019-05-19 20:07:08 +00:00

160 lines
2.7 KiB
NASM

org 0x0000
jmp 0x07C0:main
times (0x0B - ($-$$)) db 0 ; space-padded
fdc:
.ss:
dw 0x200 ; sector size
.sc:
db 2 ; sectors per cluster
.rsc:
dw 1 ; reserved sector count
.fn:
db 2 ; number of file allocation tables
.rde:
dw 0x70 ; number of root directory entries
.ts:
dw 720 ; total number of sectors
.mi: ; medium identifier
db 0xFD ; 5.25-inch Double sided, 40 tracks per side, 9 sectors per track (360 KB)
.sf: ; sectors per fat
dw 2
.spt:
dw 9 ; sectors per track
.nos:
dw 2 ; number of sides (heads)
.po:
dd 0 ; partition offset (in LBA blocks)
.lrgts:
dd 0
.drv:
db 0 ; drive number
db 0
db 0x29 ; efdc signature
.vid:
dd 0 ; volume id
.vlabel:
db "2B"
times (54 - ($-$$)) db " "
.fstype:
db "FAT12"
times (62 - ($-$$)) db " "
main:
cli
; setup stack area growing down from directly below us
xor ax, ax
mov ss, ax
mov sp, 0x7C00
; setup base segments
mov ax, cs
mov ds, ax
mov es, ax
; save initial environment
push bx
push cx
push dx
mov [fdc.drv], dl ; fix drive number in fdc
%include "chs.inc"
mov di, 0x1000 ; loading address for roms
call search_rootdir
mov ax, 0xFFFF
call error
db "THING", 0
search_rootdir:
mov ax, [fdc.sf]
mul byte [fdc.fn]
add ax, [fdc.rsc]
mul word [fdc.ss]
mov si, ax ; now pointing to start of rootdir
mov cx, [fdc.rde]
jmp .start
.next:
dec cx
jz .end
add si, 0x20
.start:
call lazy_load
cmp byte [si], 0 ; end of directory
je .end
cmp byte [si], 0xe5 ; deleted entry
je .next
cmp word [si+8], 0x4F52 ; first 2 chars of extension: "RO"
jne .next
mov ax, [si+10] ; 3rd char of extension, file flags
and ax, 0x14FF ; zero out flags except directory and system
cmp ax, 0x044D ; "M" , system flag set, directory flag unset
jne .next
mov ax, [si+26]
push si
push cx
call loadrom
pop cx
pop si
jmp .next
.end:
ret
; in ax cluster number
loadrom:
call print16
ret
; Makes sure the block starting with (or under SI) is
; already fetched from disk
; in ds:si data expected to be fetched from disk
; trashes ax, bx and dx
lazy_load:
cmp si, [max_si]
jc .end
; calculate next sector number
mov ax, [max_si]
xor dx, dx
div word [fdc.ss]
; write to es:bx
xor dx, dx
push cs
pop es
mov bx, [max_si]
call load_sector
; update max_si counter
mov ax, [max_si]
add ax, [fdc.ss]
mov [max_si], ax
; loop until requirement is satisfied
jmp lazy_load
.end:
ret
error:
call print16
pop si
mov bx, 0x0000
mov al, 0x20
.loop:
mov ah, 0x0e
int 0x10
lodsb
cmp al, 0x00
jne .loop
.hlt:
hlt
jmp .hlt
%include "print.asm"
times (0x1FA - ($-$$)) db 0
dw 0x2B2B
max_si:
dw 0x200
dw 0xAA55