Can't access real mode memory after pmode?

[Anon]

Heres the simple code...

mov ax, 0x7000
mov

[Anon]

The code is:

mov ax, 0x7000
mov es, ax
mov bx, 0x0000

mov dword [es:bx], [code]

mov bx, 0x0004

mov dword [es:bx], [code + 4]

jmp short aftercode

code:
mov byte [gs:0xB8002], 'C'    ; this ends up compiled as 8 bytes

aftercode:
;code to switch to pmode, then...

jmp 0x70000

jmp short $

; gdt, idt, etc...



when it jumps to the code (in pmode), it should print the letter C as the second character on screen... but it just triple-faults... Got any ideas?


Anon

[anton]

First, did you set up GDT or LDT.
Second, where does gs poit to.
Third, (if you did set up GDT or LDT and gs is valid data segment), is the code "mov byte..."
compiled correctly for the setted up descriptor(16 or 32 bit).
PS you should also put a loop after "mov byte ...",or it is probobly triple-faults after "mov ..."

[Anon]

GDT, IDT are set up. First part is [BITS 16], after pmode is [BITS 32] gs is LINEAR_SEL, which has base 0. It doesn't seem that 0x70000 is the correct mem for 0x7000:0x0000.

Anon

[anton]

jmp 0x70000 should be 16 bit if there was no change of controll before it(jmp or call) after setting
the pmode bit.

[Anon]

It goes to full pmode and resets the registers.

Anon

[Anon]

There was really nothing wrong... The selector wasn't set as executable! If anyone else ever has this problem, check if the selector is executable! (Man, why didn't I think of that before?)

Thanks anyway,
Anon

[carbonBased]

Huh!?  Last I checked, there was no "executable" bit in a descriptor...?

Anyway, glad to hear all's fixed!  Believe me, dude, those kinda tiny mistakes are the basis of OS Development, I swear!   Makes it all the more interesting, though, eh?

Jeff