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bootnorm.asm: do not relocate beyond 128k anymore, and...
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... fix invalid drive number provided by BIOS, further try to
make source more readable.
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@boeckmann
boeckmann committed May 11, 2024
1 parent b106753 commit 611de5f
Showing 1 changed file with 128 additions and 165 deletions.
293 changes: 128 additions & 165 deletions source/fdisk/bootnorm.asm
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Original file line number Diff line number Diff line change
@@ -1,5 +1,6 @@
;
; normal DOS boot sector
; MBR boot code installed by Free FDISK
; to assemble use Netwide Assembler (NASM)
;
;-----------------------------------------------------------------------
; ENTRY (copied from freedos bootsector)
Expand All @@ -9,180 +10,142 @@
;
;-----------------------------------------------------------------------

real_start: cli
cld
xor ax, ax
mov ss, ax ; initialize stack
mov ds, ax
mov bp, 0x7c00
lea sp, [bp-0x20]
sti
mov ax, 0x1FE0
mov es, ax
mov si, bp
mov di, bp
mov cx, 0x0100
rep movsw

jmp word 0x1FE0:0x7c00+ cont-real_start

cont: mov ds, ax
mov ss, ax
xor ax,ax
mov es,ax

; search for active partition
lea di, [bp+0x1be] ; start of partition table
test_next_for_active:
test byte [di],0x80
jne active_partition_found
add di,0x10 ; next table
cmp di, 07c00h+0x1fe; scanned beyond end of table ??
jb test_next_for_active

;*****************************************************************
call print
db 'no active partition found',0
WAIT_FOR_REBOOT:
jmp $


;*****************************************************************
trouble_reading_drive:
call print
db 'read error while reading drive',0
jmp WAIT_FOR_REBOOT

;*****************************************************************

invalid_partition_code:
call print
db 'partition signature != 55AA',0
jmp WAIT_FOR_REBOOT

;*****************************************************************

active_partition_found:
; call print
; db 'loading active partition',0

call read_boot_sector
jc trouble_reading_drive
cmp word [es:0x7c00+0x1fe],0xaa55
jne invalid_partition_code
; call print
; db '.jump DOS..',0
jmp word 0x0:0x7c00 ; and jump to boot sector code


;*****************************
RELOC_SEG equ 0x80
PARTTBL_OFFSET equ 0x1be
SIGNATURE_OFFSET equ 0x1fe


org 0x7c00

start:
cli
xor ax, ax
mov ss, ax ; initialize stack
mov sp, start - 0x20 ; SS:SP = 0:7be0
sti
cld
mov bp, start
mov ds, ax ; DS = 0
; move MBR code out of the way to make room for volume boot record
relocate:
mov ax, RELOC_SEG
mov es, ax
mov si, bp
mov di, bp
mov cx, 0x0100
rep movsw
jmp word RELOC_SEG:.cs_changed
; From now on, the code is located 0x800 bytes above 0x7c00
; at absolute address 0x8400, or more accurate 80:7c000.
.cs_changed:
mov ds, ax ; DS = RELOC_SEG
xor ax, ax
mov es, ax ; ES = 0

fix_bootdrive:
test dl, dl ; is boot drive in DL given by BIOS zero?
jnz .dl_good ; if yes this can be considered a BIOS bug,
mov dl, 0x80 ; because we know we boot from HD
.dl_good:

; Test if one of the four primary partitions is active by checkung
; for value 0x80 in the first byte of the entries.
scan_for_active_partition:
lea di, [bp + PARTTBL_OFFSET] ; start of partition table
.l: test byte [di], 0x80
jnz chainload_bootsect
add di, 0x10 ; next table entry
cmp di, start + SIGNATURE_OFFSET ; scanned beyond end of table?
jb .l
.no_active:
call print
db 'no active partition found', 0
jmp $

; We found an active partition. Load its boot sector to 0:7c00,
; test the signature and far jump to 0:7c00
chainload_bootsect:
call read_boot_sector
jc .read_error
cmp word [es:start + SIGNATURE_OFFSET], 0xaa55
jne .invalid_partition_code
;-----------------------------------------------------------------------------
jmp word 0x0:0x7c00 ; jump to volume boot code
;-----------------------------------------------------------------------------
.read_error:
call print
db 'read error while reading drive', 0
jmp $
.invalid_partition_code:
call print
db 'partition signature != 55AA', 0
jmp $


;-----------------------------------------------------------------------------
; BIOS disk access packet used by ext. INT13 LBA read function

dap:
.packet_size db 0x10
.reserved1 db 0
.sector_count db 1
.reserved2 db 0
.buf_off dw 0x7c00
.buf_seg dw 0x0000
.lba_low dw 0
.lba_high dw 0
dw 0,0


;-----------------------------------------------------------------------------
; read_boot_sector
;
; IN: DI--> partition info
;OUT:CARRY
;*****************************
; IN : DI--> partition info
; OUT: CARRY

read_boot_sector:
; /* check for LBA support */
mov bx,0x55aa
mov ah,0x41
int 0x13

jc StandardBios ; if (regs.b.x != 0xaa55 || (regs.flags & 0x01))
cmp bx,0xaa55 ; goto StandardBios;
jne StandardBios

; /* if DAP cannot be used, don't use LBA */
; if ((regs.c.x & 1) == 0)
; goto StandardBios;
test cl,1
jz StandardBios
jmp short LBABios



;struct _bios_LBA_address_packet /* Used to access a hard disk via LBA */
;{
; unsigned char packet_size; /* size of this packet...set to 16 */
; unsigned char reserved_1; /* set to 0...unused */
; unsigned char number_of_blocks; /* 0 < number_of_blocks < 128 */
; unsigned char reserved_2; /* set to 0...unused */
; UBYTE far *buffer_address; /* addr of transfer buffer */
; unsigned long block_address; /* LBA address */
; unsigned long block_address_high; /* high bytes of LBA addr...unused */
;};

_bios_LBA_address_packet:
db 0x10
db 0
db 4 ; read four sectors - why not
db 0
dw 0x7c00 ; fixed boot address for DOS sector
dw 0x0000
_bios_LBA_low dw 0
_bios_LBA_high dw 0
dw 0,0


LBABios:
; copy start address of partition to DAP
mov ax,[di+8]
mov [0x7c00+ (_bios_LBA_low-real_start)],ax
mov ax,[di+8+2]
mov [0x7c00+ (_bios_LBA_high-real_start)],ax

mov ax,0x4200 ; regs.a.x = LBA_READ;
mov si,0x7c00+ (_bios_LBA_address_packet-real_start); regs.si = FP_OFF(&dap);

.check_lba:
mov bx, 0x55aa ; magic value changed after call
mov ah, 0x41
int 0x13 ; test for ext. INT13 LBA support
jc .read_chs ; no support if carry set
cmp bx, 0xaa55 ; no support if 55aa not swapped
jne .read_chs
test cl, 1 ; no support if LBA flag not set
jz .read_chs
.read_lba:
mov ax, [di + 8] ; copy start sector of partition to DAP
mov [dap.lba_low], ax
mov ax, [di + 10]
mov [dap.lba_high], ax
mov ax, 0x4200
mov si, dap
int 0x13
ret

;*****************************************************************
; read disk, using standard BIOS
;
StandardBios:
; call print
; db 'standard BIOS',0


mov ax,0x0204 ; regs.a.x = 0x0201;
mov bx,0x7c00 ; regs.b.x = FP_OFF(buffer);
mov cx,[di+2] ; regs.c.x =
; ((chs.Cylinder & 0xff) << 8) + ((chs.Cylinder & 0x300) >> 2) +
; chs.Sector;
; that was easy ;-)
mov dh,[di+1] ; regs.d.b.h = chs.Head;
; regs.es = FP_SEG(buffer);
.read_chs:
mov ax, 0x0201 ; read one sector
mov bx, 0x7c00 ; to 0:7c00
mov cx, [di + 2]
mov dh, [di + 1]
int 0x13
ret


;****** PRINT
;-----------------------------------------------------------------------------
; prints text after call to this function.

print_1char:
xor bx, bx ; video page 0
mov ah, 0x0E ; else print it
int 0x10 ; via TTY mode
print: pop si ; this is the first character
print1: lodsb ; get token
push si ; stack up potential return address
cmp al, 0 ; end of string?
jne print_1char ; until done
ret ; and jump to it



times 0x1fe-$+$$ db 0
db 0x55,0xaa
xor bx, bx ; video page 0
mov ah, 0x0E ; else print it
int 0x10 ; via TTY mode
print: pop si ; this is the first character
lodsb ; get token
push si ; stack up potential return address
cmp al, 0 ; end of string?
jne print_1char ; until done
ret ; and jump to it


; zero-fill rest of sector and put signature into place
times 0x1fe - $ + $$ db 0
db 0x55, 0xaa

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