Problem with my floppy bootsector
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Ch H
Oct 7, 2024, 6:04:04 AM10/7/24
to MikeOS
Even when there is the file to load on the disk, it still says non system disk or disk error. I will post the source code here:
;----------------------------------;
; Floppy Boot Record ;
; Written by Christian Henrichsen ;
; 30.09.2024 ;
; Copyright (C) ;
; Christian Henrichsen ;
; 2024 ;
; ;
; Copyright Notice Must be ;
; Included with All Copies of This ;
; Program ;
;----------------------------------;
org 7c00h ; Program origin of 7c00h
bits 16 ; 16-bit programing
cpu 8086 ; And assembling for the iAPX 8086 processor
jmp short start
nop
;------------------------------
; BIOS PARAMETER BLOCK
BPB:
OEMLabel db "MSWIN4.1" ; Disk label
bps dw 512 ; Bytes per sector
spc db 1 ; Sectors per cluster
rs dw 1 ; Reserved sectors for boot record
nFAT db 2 ; Number of copies of the FAT
MRDIR dw 224 ; Number of entries in root dir
; (224 * 32 = 7168 = 14 sectors to read)
tSect dw 2880 ; Number of logical sectors
mdesc db 0F0h ; Medium descriptor byte
spfat dw 9 ; Sectors per FAT
sptrk dw 18 ; Sectors per track (36/cylinder)
nhead dw 2 ; Number of sides/heads
hsect dd 0 ; Number of hidden sectors
tsectl dd 0 ; Number of LBA sectors
drvno dw 0 ; Drive No: 0
ebsig db 41 ; Drive signature: 41 for floppy
vsnum dd 00000000h ; Volume ID: any number
vlabl db "MIKEOS "; Volume Label: any 11 chars
fstyp db "FAT12 " ; File system type: don't change!
;bps dw 512 ; Bytes Per Sector
;spc db 2 ; Sectors Per Cluster
;rs dw 0 ; Reserved Sectors
;nFAT db 2 ; Number of FAT's
;MRDIR dw 224 ; Max Root Directory Entries
;tSect dw 2880 ; Total Sectors
;mdesc db 0f0h ; Media Descriptor Byte
;spfat dw 9 ; Sectors Per FAT
;sptrk dw 18 ; Sectors Per Track
;nhead dw 2 ; Number of heads
;hsect dd 0 ; Hidden Sectors
;tsect dd 0 ; Total Sectors
;drvno db 0 ; Drive Number
;flags db 0 ; Flags
;ebsig db 29h ; Extended Boot Signature
;vsnum dd 12341234h ; Volume Serial Number
;vlabl db 'NO NAME ' ; Volume Label
;fstyp db 'FAT12 ' ; Filesystem Type
FILES_AREA equ 33
ROOT_DIR equ 19
ROOT_SIZE equ 14
LOAD_OFFSET equ 0000h
LOAD_SEGMENT equ 2000h
;-----------------
; Boot Loader
start:
cli ; Clear the interrupt flag, only non-maskable interrupts will be enabled
xor ax, ax ; Zero out accumulator
mov ss, ax ; Set the stack segment register
mov sp, 7c00h ; and the Stack Pointer
push ax ; Pushing AX
push ax ; Pushing AX again
pop es ; And popping it into ES
pop ds ; and DS and setting them both to zero
sti ; Set the interrupts flag back on, now every single interrupt can interrupt my work
jmp 0000:enforce_csip ; Preforming a far jump to enforce CS:IP
enforce_csip: ; Now that CS:IP is enforced
read_rootdir:
;mov byte [drvno], dl ; Sve drive number
mov ax, ROOT_DIR ; AX = the lba of the root directory
call convert_LBA ; Convert it to its appropriate CHS parameters for int 13h
mov al, ROOT_SIZE ; How many sectors? 14, read 14 sectors
mov bx, buffer
call read_sect ; Let's read those sector's
mov cx, 224
check_root_dir:
mov di, buffer
mov si, filename
push cx
mov cx, 11
rep cmpsb
je found_file
pop cx
add di, 32
loop check_root_dir
no_kernel:
mov si, non_sys_disk
jmp print_string
found_file:
pop cx
mov ax, 1
call convert_LBA
mov al, 9
mov bx, buffer
call read_sect
mov si, di
mov al, [si+0bh]
test al, 0d8h ; Bit-mask for bits 11011000
;jnz no_kernel ; The kernel is either invalid or not here
xor ax, ax
mov dx, ax
mov word ax, [si+1ah] ; AX = First cluster of kernel
push ax
mov bx, LOAD_SEGMENT
mov es, bx
mov bx, LOAD_OFFSET
read_cluster:
sub ax, 2 ; Cluster number - 2
mul byte [spc] ; Times Sectors Per Cluster
add ax, FILES_AREA ; AX now equals the actual LBA sector to read
call convert_LBA ; Convert to CHS
mov al, [spc] ; Sectors to read
call read_sect ; Read setctor's
pop ax ; AX = Cluster number for next entry in FAT
push bx ; Save BX since it has the load pointer
mov bx, buffer ; BX = FAT buffer
call get_fat ; Get FAT12 field
cmp ax, 0ff8h ; Is it ff8 or greater?
jae short jump_to_kernel ; Yes, it's the end of our kernel. Let's jump to it
pop bx ; Nope keep reading
add bx, 512 ; Add 512 since we are to not overwrite what we have read
jmp short read_cluster
jump_to_kernel:
pop bx
jmp LOAD_SEGMENT:LOAD_OFFSET
;-----------------
; Loader Subroutines
get_fat:
push bx
push cx
mov cx, ax
shl ax, 1
add ax, cx
test ax, 1
pushf
shr ax, 1
add bx, ax
mov ax, [bx]
popf
jnz .getfat1
and ax, 0fffh
jmp .getfat2
.getfat1:
mov cl, 4
shr ax, cl
.getfat2:
pop cx
pop bx
ret
print_string:
mov ah, 0eh
.print_loop:
lodsb
cmp al, 0
je .done
int 10h
jmp short .print_loop
.done:
reboot:
xor ax, ax
int 16h
mov word [ds:472h], 1234h ; To simulate a ctrl+alt+del
jmp 0ffffh:0000h ; Reset the system
read_sect: ; IN: call to convert_LBA, DL = Drive to read, BX = Buffer address, ES = Segment
mov ah, 02h
int 13h
ret
convert_LBA: ; Converts LBA to CHS tuple ready for int 13h call
push bx
push ax
mov bx, ax ; Save logical sector
mov dx, 0 ; First the sector
div word [sptrk]
add dl, 01h ; Physical sectors start at 1
mov cl, dl ; Sectors belong in CL for int 13h
mov ax, bx
mov dx, 0 ; Now calculate the head
div word [sptrk]
mov dx, 0
div word [nhead]
mov dh, dl ; Head/side
mov ch, al ; Track
pop ax
pop bx
ret
non_sys_disk db 'Non system disk or disk error!', 0
filename db 'AC-DOS SYS'
times 510-($-$$) db 0 ; Pad remainder of boot sector with zeros
dw 0AA55h ; Boot signature (DO NOT CHANGE!)
buffer:
Dan
Oct 7, 2024, 7:04:06 AM10/7/24
to mik...@googlegroups.com
Hello Christian,
Firstly, you have chosen to write your own bootloader rather than use an
existing bootloader, likely to learn, so it does you no good to tell you
the solution.
For a fresh floppy you know the kernel (`AC-DOS.SYS`) will be loaded
into the very first part of the file allocation table as the first
entry. When you load it to `buffer` you can just print it (fixed length
as there might not be a termination character) and see what is there.
You should make use of printing to check your programming assumptions.
The other thing you can do is use the MikeOS functions for loading the
kernel (which works), and then you can supplement back your code to see
when it breaks.
Kind Regards,
Dan
Firstly, you have chosen to write your own bootloader rather than use an
existing bootloader, likely to learn, so it does you no good to tell you
the solution.
For a fresh floppy you know the kernel (`AC-DOS.SYS`) will be loaded
into the very first part of the file allocation table as the first
entry. When you load it to `buffer` you can just print it (fixed length
as there might not be a termination character) and see what is there.
You should make use of printing to check your programming assumptions.
The other thing you can do is use the MikeOS functions for loading the
kernel (which works), and then you can supplement back your code to see
when it breaks.
Kind Regards,
Dan
James Huddle
Oct 9, 2024, 9:04:17 AM10/9/24
to Ch H, MikeOS
Not sure if I'm replying correctly to the listserv...
I just spent 5 min looking at your code, and it occurred to me:
If you recently expanded your code, and if the size of the code exceeded 510 bytes,
The BIOS gulps in 512 bytes which does not end in 0xAA55 so it goes south.
Kind of a long shot, and I would like to spend more than 5 minutes with your code.
But if you *have* recently added to your code, that could be an issue.
Maybe make your non_sys_disk string shorter, like 'NS-Disk!!'
That might save enough space to put things back in order.
Best of luck.
-Jim
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