Page 1 of 1
Bochs reboots when enter to PM
Posted: Wed Oct 04, 2006 3:53 pm
by muisei
I'm wonderin why this simple code reboots when I set the CR0 register.
Code: Select all
[BITS 16]
[ORG 0x00]
jmp start
start:
cli
mov ax,cs
mov ds,ax
mov es,ax
mov gs,ax
mov fs,ax
mov ss,ax
mov sp,0xffff
sti
lgdt [gdtr] ; Load GDT
mov eax,cr0 ; The lsb of cr0 is the protected mode bit
or al,0x01 ; Set protected mode bit
mov cr0,eax ; Mov modified word to the control register
mov ax,datasel
mov ds,ax ; Initialise ds & es to data segment
mov es,ax
mov ss,ax
mov fs,ax
mov ax,videosel ; Initialise gs to video memory
mov gs,ax
jmp codesel:go_pm
bits 32
go_pm:
mov BYTE [gs:0x00],'P'
mov BYTE [gs:0x01],0x3f
jmp $
;-------------------------------------------------------------------------------------------;
; GDT ;
;-------------------------------------------------------------------------------------------;
gdtr:
dw gdt_end-gdt-1 ; Length of the gdt
dd gdt ; physical address of gdt
gdt:
nullsel equ $-gdt ; $->current location,so nullsel = 0h
gdt0: ; Null descriptor,as per convention gdt0 is 0
dd 0 ; Each gdt entry is 8 bytes, so at 08h it is CS
dd 0
; In all the segment descriptor is 64 bits
codesel equ $-gdt ; This is 8h,ie 2nd descriptor in gdt.Code descriptor 4Gb flat segment at 0000:0000h
dw 0xffff ; Limit 4Gb bits 0-15 of segment descriptor
dw 0x0000 ; Base 0h bits 16-31 of segment descriptor (sd)
db 0x00 ; Base addr of seg 16-23 of 32bit addr,32-39 of sd
db 0x09a ; P,DPL(2),S,TYPE(3),A->Present bit 1,Descriptor
; privilege level 0-3,Segment descriptor 1 ie code
; or data seg descriptor,Type of seg,Accessed bit
db 0x0cf ; Upper 4 bits G,D,0,AVL ->1 segment len is page
; granular, 1 default operation size is 32bit seg
; AVL : Available field for user or OS
; Lower nibble bits 16-19 of segment limit
db 0x00 ; Base addr of seg 24-31 of 32bit addr,56-63 of sd
datasel equ $-gdt ; ie 10h, beginning of next 8 bytes for data sd.Data descriptor 4Gb flat seg at 0000:0000h
dw 0x0ffff ; Limit 4Gb
dw 0x0000 ; Base 0000:0000h
db 0x00 ; Descriptor format same as above
db 0x092
db 0x0cf
db 0x00
videosel equ $-gdt ; ie 18h,next gdt entry
dw 3999 ; Limit 80*25*2-1
dw 0x8000 ; Base 0xb8000
db 0x0b
db 0x92 ; present,ring 0,data,expand-up,writable
db 0x00 ; byte granularity 16 bit
db 0x00
gdt_end
10x in advance
Posted: Wed Oct 04, 2006 6:42 pm
by Dex
You need to disable interrupts on entering pmode (eg: cli ,)as you have not set idt up yet, also set these:
Code: Select all
mov ax,datasel
mov ds,ax ; Initialise ds & es to data segment
mov es,ax
mov ss,ax
mov fs,ax
mov ax,videosel ; Initialise gs to video memory
mov gs,ax
After the jump to pmode.
Posted: Thu Oct 05, 2006 3:03 am
by muisei
I rewrote the code like this but still no success.
Bochs says: "exception(): 3rd (13) exception with no resolution, shutdown status is 00h, resetting"
It continues to reboot at "mov cr0,eax"
Code: Select all
[BITS 16]
[ORG 0x00]
jmp start
start:
cli
mov ax,cs
mov ds,ax
mov es,ax
mov gs,ax
mov fs,ax
mov ss,ax
mov sp,0xffff
xor eax,eax <- Added
mov ax,ds <- Added
shl eax,4 <- Added
add eax,gdt <- Added
mov [gdtr+2],eax <- Added
lgdt [gdtr] ; Load GDT
mov eax,cr0 ; The lsb of cr0 is the protected mode bit
or al,0x01 ; Set protected mode bit
mov cr0,eax
jmp codesel:go_pm
bits 32
go_pm:
sti <- Added here
mov ax,datasel
mov ds,ax ; Initialise ds & es to data segment
mov es,ax
mov ss,ax
mov fs,ax
mov ax,videosel ; Initialise gs to video memory
mov gs,ax
mov BYTE [gs:0x00],'P'
mov BYTE [gs:0x01],0x3f
jmp $
Posted: Thu Oct 05, 2006 9:03 am
by Dex
What if you add "org 0x7C00" to the start ?.
As this code works fine for me
Code: Select all
org 0x7C00
use16
;****************************
; Realmode startup code.
;****************************
start:
xor ax,ax
mov ds,ax
mov es,ax
mov ss,ax
mov sp,0x7C00
;*****************************
; Setting up, to enter pmode.
;*****************************
cli
lgdt [gdtr]
mov eax, cr0
or al,0x1
mov cr0,eax
jmp 0x10: protected
;*****************************
; Pmode. ;-)
;*****************************
use32
protected:
mov ax,0x8
mov ds,ax
mov es,ax
mov ss,ax
mov esp,0x7C00
;*****************************
; Turn floppy off (if space).
;*****************************
mov dx,3F2h
mov al,0
out dx,al
mov byte [es:0xB8000], "P"
jmp $
;*************************************
; GDT.
;*************************************
gdt: dw 0x0000, 0x0000, 0x0000, 0x0000
sys_data: dw 0xFFFF, 0x0000, 0x9200, 0x00CF
sys_code: dw 0xFFFF, 0x0000, 0x9800, 0x00CF
gdt_end:
gdtr: dw gdt_end - gdt - 1
dd gdt
;*************************************
; Make program 510 byte's + 0xaa55
;*************************************
times 510- ($-start) db 0
dw 0xaa55
NOTE: assembled with fasm.
Posted: Thu Oct 05, 2006 10:22 am
by muisei
The code is loaded by the MBR at address 0x50:0x00.Coud that be the problem?
Posted: Thu Oct 05, 2006 10:29 am
by Dex
Yes as the address will be wrong, here are the seting for a com file loaded to that address "COM program defaults: cs = ds = es = ss = 50h, sp = 0, ip = 100h"
Posted: Thu Oct 05, 2006 1:01 pm
by muisei
But the free convencional memory resides from 0x500 to 0x9FBFF, isn't it.
Posted: Fri Oct 06, 2006 9:54 am
by Dex
But 50h is not real 50h, but 500h, somthing like this
;; ;;
;; Memory Map: ;;
;; ~~~~~~~~~~~ ;;
;; ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ ;;
;; ³ Interrupt Vector Table ³ 0000 ;;
;; ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ;;
;; ³ BIOS Data Area ³ 0040 ;;
;; ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ;;
;; ³ PrtScr Status / Unused ³ 0050 ;;
;; ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ;;
;; ³ Image Load Address ³ 0060 ;;
;; ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ;;
;; ³ Available Memory ³ nnnn ;;
;; ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ;;
;; ³ 2KB Boot Stack ³ A000 - 512 - 2KB ;;
;; ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ;;
;; ³ Boot Sector ³ A000 - 512 ;;
;; ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ;;
;; A000 ;;
;; ;;
;; ;;
;; Boot Image Startup (register values): ;;
;; ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ;;
;; dl = boot drive number ;;
;; cs:ip = program entry point ;;
;; ss:sp = program stack (don't confuse with boot sector's stack) ;;
;; COM program defaults: cs = ds = es = ss = 50h, sp = 0, ip = 100h ;;
;; EXE program defaults: ds = es = 50h, other stuff depends on EXE header ;;
Posted: Fri Oct 06, 2006 1:47 pm
by muisei
Also found this document which may be useful.
http://www.frontiernet.net/~fys/rombios.htm
Acording to what you said and this document, it seems perfectly legal to me to load my kernel at linear address from 0x600 to 0x9fbff whitout worring that I can overlap some useful code.Is that true?
Posted: Fri Oct 06, 2006 5:14 pm
by Dex
Yes
Posted: Sat Oct 07, 2006 1:49 am
by muisei
thanks for the enlightening.
