AP startup issue

[worldsapart]

Hi guys,

I've been trying to start up my APs. what i did initially ws jus make them increment a counter in memory. And that worked and i ws getting them al started up. But after that I tried setting up gdt and tryin to jump them into protected mode. It jus doesnt seem to work. it breaks... Triple fault! I'd really appreciate it if u guys could take a look at my code and tel me wat's wrong. What i do, is i copy the AP startup code to a low memory location and the APs jump to it on startup. I know it's something to do wit me referencing symbols not accesible to the APs.. but i jus cant seem to find a way around it.. Thanks for ur time guys..

Code: Select all

bits	16							; We are still in 16 bit Real Mode
global AP_startup
extern AP_main
extern AP_stage3
extern Stage3

;*************************************************;
;	Bootloader Entry Point
;*************************************************;

AP_startup:

	; Incrementing processor count -  THIS WORKS FINE. SO APs ARE STARTING UP
	mov	bx, 500h
	lock
	inc byte [bx]

	mov ax, cs
	mov ds, ax

	;EFLAGS
	push 2
	popf
	
	mov bx, 0x600
	;Loading GDT and IDT
	lgdt [bx]

	; Enabling Protected mode

	mov eax, cr0
	or al, 1
	mov cr0, eax

	; Far jump... THIS IS WAT IS SCREWED.. I GET A RELOCATION ERROR
	jmp 0x8:Pmode
	 

bits 32
	
Pmode:

	mov ax, 0x10 
	mov ds, ax
	mov es, ax
	mov fs, ax
	mov gs, ax
	mov ss, ax

	call AP_main
	cli							; Clear all Interrupts
	hlt							; halt the system

Hope it's readable. Thanks again.

[01000101]

you should setup *all* of your segment registers and get a stack setup before you start using any memory offsets and the PUSH/POP instructions.

Also, I don't see where you unlock your bus lock... you do an "inc [lockspace]", but you never alter that memory location again.

[worldsapart]

well... I am not sure if this is rite.. but the manual says that the bus is locked only for the duration of the instruction accompanying the LOCK prefix... please correct me if, i'm wrong..

[Combuster]

True, the lock prefix forces ordering and atomicity around one read-modify-write instruction. In between locked instructions, the processor can do other things (not doing so once resulted in Cyrix' infamous coma bug).

[worldsapart]

Thanks combustor for clearing that up.. did u find anythin wrong wit my code? I really need some help... thanks.

[Brendan]

Hi,

Code: Select all

bits	16							; We are still in 16 bit Real Mode
global AP_startup
extern AP_main
extern AP_stage3
extern Stage3

;*************************************************;
;	Bootloader Entry Point
;*************************************************;

AP_startup:

	; Incrementing processor count -  THIS WORKS FINE. SO APs ARE STARTING UP
	mov	bx, 500h
	lock
	inc byte [bx]

You can't assume DS is set to anything here, so "lock inc byte [bx]" might increment the byte at the address 0x0000:0x0500 or it might try to increment the byte at the address 0xFFFF:0x0500 (or any other segment). Also, it's probably better to write "lock inc byte [0x0500]" rather than spreading it over 3 separate lines. Incrementing a word or dword might be even better (e.g. not much point limiting it to 256 CPUs when computers that exceed this limit are on the way).

Note: because you only use DS a few times, maybe "lock inc byte [cs:0x0500]" would be better than setting up DS.

Code: Select all

	mov ax, cs
	mov ds, ax

	;EFLAGS
	push 2
	popf

You can't assume SS or ESP is set to anything here either - the PUSH could trash your code or data, or the POP could get a value from ROM, etc.

Code: Select all

	mov bx, 0x600
	;Loading GDT and IDT
	lgdt [bx]

Could maybe change this to "lgdt [cs:0x0600]" too (and get rid of the "mov ds,ax" above).

You should consider replacing hard-coded constants with defines (for example, use "%define AP_TEMP_GDT_ADDRESS 0x600" somewhere and then "lgdt [cs:AP_TEMP_GDT_ADDRESS]" here, and then do the same for the "lock inc [cs:0x500]").

Code: Select all

	; Enabling Protected mode

	mov eax, cr0
	or al, 1
	mov cr0, eax

	; Far jump... THIS IS WAT IS SCREWED.. I GET A RELOCATION ERROR
	jmp 0x8:Pmode

A relocation error sounds like a problem with the linking. I've got no idea how you're trying to link this, so I've got no idea what's going wrong.


Cheers,

Brendan

[worldsapart]

[Removed post]

Issue explained below..

[worldsapart]

k.. This is wat i have so far.. thanks for those tips Brendan.. I got rid of the triple faults n the relocation errors.. But it stil doesnt work...

Code: Select all

bits	16							; We are still in 16 bit Real Mode
global AP_startup
extern AP_main

%define COUNTER 0x0500
%define GDTR 0x0600
%define CODE_DESC 0x08

;*************************************************;
;	Bootloader Entry Point
;*************************************************;

AP_startup:

	cli
        ; TRIED PUTTING THIS LINE AFTER SETTING UP THE SEGMENTS. BUT DIDNT WORK!!
        ; HERE IT WORKS JUST FINE! WHY IS THAT??
        ; Incrementing processor count
	lock inc byte [COUNTER]
        

	xor ax, ax
	mov cs, ax
	mov ds, ax
	mov ss, ax
	mov sp, 0x100

	;SETTING UP THE STACK GOT RID OF THE TRIPLE FAULT THE PUSH OR POP WAS CREATING
        ;THANKS FOR THAT

	;EFLAGS
	push 2
	popf

	;Setting up GDT
	lgdt [GDTR]

        ; IN THE KERNEL CODE RUNNING ON THE BSP I COPIED THE GDTR ENTRY AND THE GDTABLE TO LOW MEMORY
        ; "GDTR" POINTS TO THE GDTR ENTRY STRUCTURE.

	; Enabling Protected mode

	mov eax, cr0
	or al, 1
	mov cr0, eax

bits 32
        ; CODE_DESC - 0x8
	jmp CODE_DESC:Pmode

Pmode:

	mov ax, 0x10 
	mov ds, ax
	mov es, ax
	mov fs, ax
	mov gs, ax
	mov ss, ax

	mov esp, 0x1000
	call AP_main                                    ; C fUNCTION .... BUT NEVER GETS HERE!
	hlt							; halt the system

I have added comments, in between so that you can understand wat's goin on.. here's some more info abt my implementation..

the COUNTER value is initialized in my C code as:

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unsigned char* counter = (unsigned char*)0x500;

The GDTR entry is initialized as follows:

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typedef struct DTR
{
	unsigned short int limit;
	unsigned short int alignment;
	unsigned int offset;

}DTR_t;

DTR_t *GDT = (DTR_t*)0x600;	

So that's where the 0x600 and 0x500 come from... Am sorry for the trouble guys.. help will be appreciated.. I know I'm doing something terribly wrong!...

[djmauretto]

Code: Select all

   cli
        ; TRIED PUTTING THIS LINE AFTER SETTING UP THE SEGMENTS. BUT DIDNT WORK!!
        ; HERE IT WORKS JUST FINE! WHY IS THAT??
        ; Incrementing processor count
   lock inc byte [COUNTER]
       

   xor ax, ax
   mov cs, ax      ???????????
   mov ds, ax
   mov ss, ax
   mov sp, 0x100

mov cs,ax is invalid instruction

[Combuster]

CS affects execution - you can't set it without setting IP as well. In other words you are trying to do a jump without using a jump instruction.

[worldsapart]

Damn.. that ws stupid! .. corrected that! ... but when i remove that it triple faults somewher.. isolating it now. thanks guys.