Author Topic: GDT position/length calculation failing.  (Read 9750 times)

Offline Dulci

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Re: GDT position/length calculation failing.
« Reply #15 on: November 04, 2010, 10:46:31 PM »
Ah, my bad, however, the code still operates the same when changed over.

Offline Keith Kanios

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Re: GDT position/length calculation failing.
« Reply #16 on: November 04, 2010, 11:28:34 PM »
The following files and build process worked for me. I tested it in Bochs as a floppy disk image.

mbr.asm:
Code: [Select]
%macro print 2
mov ah, 0xa
mov al, [%1]
mov bl, 0xf
mov bh, 0
mov cx, %2
int 0x10
%endmacro

%macro printr 2
mov al, %1
mov bl, 0xf
mov bh, 0
mov cl, %2
mov ah, 0xa
int 0x10
%endmacro

[ORG 0x7c00]
[BITS 16]

jmp 0x0000:bootstart ; jump to set up CS:IP

bootstart:
; Set our data segment
xor ax, ax
mov ds, ax
mov es, ax
mov [dlvalue], dl
cli
print greeting, 2
jmp reset
; 512 bytes per sector
; 18 sectors per track
; 63 tracks

reset:
mov ah, 0 ; reset floppy disk function
mov dl, [dlvalue]
int 0x13 ; call BIOS
jmp readCD
print reading, 3
jc reset

readCDerror:
xor ch, ch
xor cl, cl
mov dl, [dlvalue]
print error, 4
jmp reset

readCD: ; Read 0x7700 bytes from the disk to point 0x500 in memory from beginning of cd
mov dh, 0 ; Drive head
mov ax, 0x500
mov es, ax ; Set es to 0x500, where we want to buffer to
xor bx, bx ; buffer is at 500:0, es:bx
mov ch, 0 ; Set track to 0
mov cl, 2 ; sector to 2
mov ah, 0x2 ; What we want to do with 0x13
mov al, 0x2 ; How much we want to read
mov dl, [dlvalue]
clc
int 0x13
jc readCDerror ; There was a problem with reading

jumpOut:
print load, 5
; Load GDT
cli
xor ax, ax
mov ds, ax
mov es, ax
mov ax, 0x9000 ; set stack at 0x9000-0xffff
mov ss, ax
mov sp, 0xFFFF
pusha
lgdt [TGDT]
popa

jmp 0x500:0x0 ; No problem, just jump to the entry point

data:
greeting   db 'G'
error   db 'E'
load   db 'L'
reading   db 'R'
dlvalue db 0

;========================================
;===================GDT==================
;========================================
;align 4

BGDT:
; Null
dd 0
dd 0
; Code
dw 0xFFFF ; limit low
dw 0 ; base low
db 0 ; base middle
db 10011010b ; access
db 11001111b ; granularity
db 0 ; base high
; Data
dw 0xFFFF ; limit low
dw 0 ; base low
db 0 ; base middle
db 10010010b ; access
db 11001111b ; granularity
db 0 ; base high
EGDT:

TGDT:
dw EGDT - BGDT - 1 ; limit (Size of GDT)
dd BGDT

times 510-($-$$) db 0
dw 0xAA55

lbr.asm:
Code: [Select]
ORG 0x5000

%macro print 2
mov ah, 0xa
mov al, [%1]
mov bl, 0xf
mov bh, 0
mov cx, %2
int 0x10
%endmacro

%macro printr 2
mov al, %1
mov bl, 0xf
mov bh, 0
mov cl, %2
mov ah, 0xa
int 0x10
%endmacro

[BITS 16]
ssblstart:
; Set our data segment
mov eax, cr0
or eax, 1
mov cr0, eax
cli
; hlt
jmp 0x8:PmodeE
;    printr [greeting2], 12
; mov ax, cr0
; or ax, 1
; mov cr0, axe

[BITS 32]

PmodeE:
cli
mov ax, 0x10 ; set data segments to data selector (0x10)
mov ds, ax
mov ss, ax
mov es, ax
mov esp, 90000h
cli
.hang:
inc DWORD[0x000B8000]
jmp .hang
hlt

greeting2   db 'A'
error   db '2'
load   db '3'

times 510-($-$$) db 0
dw 0xAA55

floppy.asm:
Code: [Select]
incbin "mbr.bin"
incbin "lbr.bin"

TIMES 1474560-($-$$) DB 0

build process:
Code: [Select]
nasm -f bin -o mbr.bin mbr.asm
nasm -f bin -o lbr.bin lbr.asm
nasm -f bin -o floppy.img floppy.asm

Please note that I commented out align in the MBR because NASM 2.09.03 (and perhaps even earlier versions) is messing up the alignment, padding much more than expected.

Also, the ALIGN directives in your linker script might be causing problems, as Frank expressed in his above post. I'm also not sure if OUTPUT_FORMAT("i386") is working for you, since that might default to ELF. Try OUTPUT_FORMAT("binary") instead.
« Last Edit: November 04, 2010, 11:33:19 PM by Keith Kanios »

Offline Keith Kanios

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Re: GDT position/length calculation failing.
« Reply #17 on: November 05, 2010, 03:37:10 PM »
I'm also not sure if OUTPUT_FORMAT("i386") is working for you, since that might default to ELF. Try OUTPUT_FORMAT("binary") instead.

Scratch that, I see you are supplying --oformat=binary at the command line.

Offline Dulci

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Re: GDT position/length calculation failing.
« Reply #18 on: November 05, 2010, 05:51:48 PM »
Okay, I finally got it working! I realized that while I changed the jmp to 0x0:0x500, I forgot to change es to 0x50 in bootstart, and so it was loading the second stage bootloader into 0x500:0x0, which caused a number of memory problems since the ssbootloader is linked to 0x0:0x500. Thank you all for your help and for your patience in dealing with me and this problem! It is very much appreciated!