Issues loading VBR into memory in MBR using int 13h
Posted: Thu Jan 26, 2017 10:08 pm
Hello folks,
I'm attempting to implement a basic MBR, and everything seems to be going fine until I attempt to load the VBR from the active partition into memory. The VM hangs when calling int 31h function code 42. I have a feeling its in the construction of my DAP, and is likely due to my shaky understanding of the fine details little-endian formatted values. Thanks for taking a look!
I'm attempting to implement a basic MBR, and everything seems to be going fine until I attempt to load the VBR from the active partition into memory. The VM hangs when calling int 31h function code 42. I have a feeling its in the construction of my DAP, and is likely due to my shaky understanding of the fine details little-endian formatted values. Thanks for taking a look!
Code: Select all
BITS 16
ORG 0x7C00
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
Constants:;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
%define NEW_OFFSET 0x0600
%define OLD_OFFSET 0x7C00
%define MBR_SIZE 512
%define PARTITION_TABLE_OFFSET 446
%define PARTITION_ENTRY_SIZE 16
%define ENTRY_COUNT 4
%define BOOTABLE_BIT_MASK 0x80
%define ENTRY_CYLINDER_OFFSET 3
%define ENTRY_HEAD_OFFSET 1
%define ENTRY_SECTOR_OFFSET 2
%define LBA_OFFSET 0x08
%define DAP_LOC 0x800
%define DAP_SIZE 0x10
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
PreCopy:;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
DisableInterrupts:
cli
SetupStack:
mov ax, 0
mov ss, ax ; Stack segment SS:SP is 0
mov sp, OLD_OFFSET ; bottom of stack (grows downwards) starts just below code
InitSegRegs: ; Initialize all segment registers to zero
xor ax, ax
mov ds, ax
mov es, ax
mov fs, ax
mov gs, ax
RelocateMBR:
xor ax, ax ; Zero ax for segment register zeroing
mov es, ax ; Set copy location to 0:0x0600
mov di, NEW_OFFSET
mov ds, ax ; Set origin location to 0:0x7C00
mov si, OLD_OFFSET
mov cx, MBR_SIZE ; Copy all 512 bytes of MBR
rep movsb ; Copy MBR one byte at a time
jmp 0:PostCopy ; Jump to copied MBR after copy code
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
PostCopy:;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
ReenableInterrupts:
sti
ReadPartitionTable:
mov [bootDrive], dl ; save boot drive from bios
mov bx, NEW_OFFSET + PARTITION_TABLE_OFFSET ; mov table addr into bx
mov cx, ENTRY_COUNT ; 4 partitions to test
.bootableScan:
mov al, [bx] ; get status byte
test al, BOOTABLE_BIT_MASK ; test if its bootable
jnz LoadPartition ; bootable -> load vbr
add bx, PARTITION_ENTRY_SIZE ; go to next partition
loop .bootableScan
NoBootFound:
int 0x18 ; Report not bootable to bios and return control
LoadPartition:
;;; Check if bios supports lba reading from disk
.checkExtension:
push bx ; we need the partition entry address, save it
mov ah, 0x41
mov bx, 0x55AA
mov dl, [bootDrive]
int 0x13
pop bx ; restore partition entry address
jc .chsLoad ; unsupported -> load via chs
jmp .lbaLoad ; supported -> load via lba
.lbaLoad:
mov si, DAP_LOC
mov [si], BYTE DAP_SIZE ; 0-1 size of dap (16)
inc si
mov [si], BYTE 0x00 ; 1-2 0
inc si
mov [si], WORD 0x0001 ; 2-4 number of sectors to read (1)
add si, 2
mov [si], WORD OLD_OFFSET ; Offset to read to (0x7c00)
add si, 2
mov [si], WORD 0x0000 ; Segment to read to (0)
add si, 2
mov cx, WORD [bx+LBA_OFFSET]
mov [si], cx
add si, 2
mov cx, WORD [bx+LBA_OFFSET+2]
mov [si], cx
add si, 2
mov [si], DWORD 0x00000000
mov ah, 0x42
mov dl, [bootDrive]
mov si, DAP_LOC
int 0x13
jnc VBRHandoff
.chsLoad:
mov ch, [bx + ENTRY_CYLINDER_OFFSET] ; Cylinder number
mov cl, [bx + ENTRY_SECTOR_OFFSET] ; Sector Number
mov dh, [bx + ENTRY_HEAD_OFFSET] ; Head Number
mov dl, [bootDrive] ; drive number
xor bx, bx
mov es, bx ; Segment to write to (0)
mov bx, OLD_OFFSET ; offset to write to (0x7c00)
mov ah, 0x02 ; function code
mov al, 1 ; sectors to read
int 0x13 ; read from disk
jmp VBRHandoff ; Hand off control to loaded vbr
VBRHandoff:
mov dl, [bootDrive] ; Restore boot drive for vbr
jmp OLD_OFFSET ; Jump to vbr code
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; String address in si
PrintString:
push ax
push si
.printloop:
mov al, [si]
inc si
test al, al
jz .done
call PrintChar
jmp .printloop
.done:
pop si
pop ax
ret
;; Char in al
PrintChar:
push ax
push bx
mov ah, 0x0E
mov bh, 0
int 0x10
pop bx
pop ax
ret
;;; Assume byte in ah
PrintHex:
push ax
push bx
xor bx, bx
mov bl, ah
and bl, 0xF0
shr bl, 4
mov al, [hexstring + bx]
call PrintChar
mov bl, ah
and bl, 0x0F
mov al, [hexstring + bx]
call PrintChar
pop bx
pop ax
ret
PrintNL:
push si
mov si, NL
call PrintString
pop si
ret
;;; Reg is ax
PrintReg:
push ax
call PrintHex
mov ah, al
call PrintHex
pop ax
ret
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
Data:;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
bootDrive db 0
hexstring db "0123456789ABCDEF"
NL db 0x0A,0x0D,0
InLBA: db "In Lba",0x0A,0x0D,0
HERE: db "HERE",0x0A,0x0D,0
;;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
times 446 - ($ - $$) db 0
