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bootnorm.asm: apply most of the suggestions proposed by ecm #84
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#84
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@boeckmann
boeckmann committed May 12, 2024
1 parent f52198a commit 8539f43
Showing 1 changed file with 90 additions and 67 deletions.
157 changes: 90 additions & 67 deletions source/fdisk/bootnorm.asm
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Original file line number Diff line number Diff line change
Expand Up @@ -10,91 +10,74 @@
;
;-----------------------------------------------------------------------

RELOC_SEG equ 0x80
TOP_OF_STACK equ 0x7be0 ; right below original boot code
; with 0x20 bytes of safety margin
BOOTSECT_OFFSET equ 0x7c00
RELOCATED_OFFSET equ 0x0600
PARTTBL_SIZE equ 64
PARTTBL_OFFSET equ 0x1be
SIGNATURE_OFFSET equ 0x1fe


org 0x7c00
org RELOCATED_OFFSET

start:
cli
xor ax, ax
mov ss, ax ; initialize stack
mov sp, start - 0x20 ; SS:SP = 0:7be0
mov sp, TOP_OF_STACK ; SS:SP = 0:7be0
sti
cld
mov bp, start
mov ds, ax ; DS = 0
mov ds, ax ; do not trust BIOS to have the segment
mov es, ax ; registers initialized to zero
; 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 si, BOOTSECT_OFFSET
mov di, RELOCATED_OFFSET
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
jmp 0:.relocated ; far jump to be sure we get CS right
.relocated:

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:
.dl_good:
mov [driveno], dl ; save driveno in case BIOS destroys DL

; Test if one of the four primary partitions is active by checkung
; Test if one of the four primary partitions is active by checking
; 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
mov di, parttbl ; start of partition table
.l: test byte [di], 0x80 ; is partition active?
jnz chainload_bootsect
add di, 0x10 ; next table entry
cmp di, start + SIGNATURE_OFFSET ; scanned beyond end of table?
cmp di, signature ; scanned beyond end of table?
jb .l
.no_active:
call print
call fatal ; does not return
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 $

push di ; save parttbl entry (restore to SI)
call read_boot_sector ; reads one sector
jnc .check_signature ; no read error occured?
call fatal ; does not return
db 'read error while reading drive', 0
.check_signature:
cmp word [signature], 0xaa55
je handoff_to_volume_bootrecord
call fatal ; does not return
db 'partition signature != 55AA', 0

;-----------------------------------------------------------------------------
; 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
handoff_to_volume_bootrecord:
pop si ; restore parttbl entry to SI, to
; comply with lDOS boot protocol
mov dl, [driveno]
jmp 0:0x7c00 ; far jump to volume boot code
;-----------------------------------------------------------------------------


;-----------------------------------------------------------------------------
Expand All @@ -116,43 +99,83 @@ read_boot_sector:
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
shr cl, 1 ; no support if LBA flag not set
jnc .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 ; LBA read function
mov ah, 0x42 ; LBA read function
mov si, dap
stc
int 0x13
ret
jmp short .intcall
.read_chs:
mov ax, 0x0201 ; read one sector
mov bx, 0x7c00 ; to 0:7c00
mov cx, [di + 2]
mov dh, [di + 1]
.intcall:
stc
int 0x13
ret


;-----------------------------------------------------------------------------
; prints text after call to this function.
; Fatal error handler. Displays error message and waits forever
; IN: CS:IP = ASCIIZ with error message to print

print_1char:
__fatal_print_char:
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
fatal: 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
jne __fatal_print_char ; until done
.wait_forever:
jmp short .wait_forever


driveno db 0x0

;-----------------------------------------------------------------------------
; Padding bytes, BIOS disk access packet used by ext. INT13 LBA read function,
; reserved space for partition table and BIOS signature

DAP_PACKET_SIZE equ 16
PADDING_BYTES equ PARTTBL_OFFSET - DAP_PACKET_SIZE - ($ - $$)

%if PADDING_BYTES < 0
; Not strictly needed, because this is catched by the times
; directive below. But this gives a more meaningful error message.
%error "bootsector too large, try decreasing code size"
%endif

; zero-fill rest of sector and put signature into place
times 0x1fe - $ + $$ db 0
db 0x55, 0xaa
; padding bytes to ensure bootsector is 512 bytes in size
times PADDING_BYTES nop

; By prepending the disk access packet to the partition table it is ensured
; that it is word-aligned.
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
dd 0
.end:

%if dap.end - dap != DAP_PACKET_SIZE
; Be sure to get DAP_PACKET_SIZE right. We defined it manually above because
; times directive needs a defined expression value.
%error "Wrong DAP size"
%endif

parttbl:
times PARTTBL_SIZE db 0 ; space for partition table
signature:
db 0x55, 0xaa ; BIOS signature

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