CS/DS limit?

[okaox]

hi my friends ...

- I activated Protected Mode x86
- I enabled a20
- Set descriptors CS/DS kernel

Code: Select all

descr_so_nulo:
    dw 0x0, 0x0, 0x0, 0x0

descr_so_code:
    dw 0xffff       ; limit[0..15]
    dw 0x0000       ; base[0..15]
    db 0x00         ; base[16..23]
    db 0x9a         ; P=1 DPL=00 S=1 Type=1010 (code read/exec)
    db 0x4f         ; G=0 D/B=1 L=0 AVL=0 Limit[16..19]
    db 0x00         ; base[24..31]

descr_so_data:
    dw 0xffff       ; limit[0..15]
    dw 0x0000       ; base[0..15]
    db 0x00         ; base[16..23]
    db 0x92         ; P=1 DPL=00 S=1 Type=0010 (data read/write)
    db 0x4f         ; G=0 D/B=1 L=0 AVL=0 Limit[16..19]
    db 0x00         ; Base[24..31]

is assumed that I can do this, but I get as output error limits in the debugger:

Code: Select all

    ; code load offset 0:800
    mov     ax, 0x08*2		; DS
    mov     ds, ax
    mov     es, ax
    mov     fs, ax
    mov     gs, ax
    mov     ss, ax

    mov     esp, 0xfffff-4	; Why? ... if Limit = 0xfffff !!!
    jmp     0x08:.aqui
.aqui:
    
    ret

What happens?

greetings!!!

PD: with [mov esp, 0xA0010] have no problem, more than this I get limit error ...

[Brendan]

Hi,

Not sure if this is your problem or not, but...

The G (granularity) flag is used to tell the CPU if the limit has byte granularity (e.g. "limit = 0xFFFFF" means 1 MiB limit) or if the limit has page granularity (e.g. "limit = 0xFFFFF" means 1048575 pages or 4 GiB).

Basically the G (granularity) flag is clear so the segment limit is 1 MiB. For "mov esp, 0xA0010" your stack will be below the 1 MiB limit, so you'd be able to use the stack without general protection faults.


Cheers,

Brendan

[okaox]

Thank you for the response...

If the granularity flag is clear, the segment size can range from 1 byte to 1 MByte, in byte increments.
If the granularity flag is set, the segment size can range from 4 KBytes to 4 GBytes, in 4-KByte increments.

But I am still wondering, Why occur the error?, if the range from A0010 to 0xFFFFF is < 1MB

greetings!!!

pd: Sorry for the english, I speak Spanish

[pcmattman]

Code: Select all

    dw 0xffff       ; limit[0..15]

Correct me if I'm wrong, but shouldn't there be five f's there, not four?

[Brendan]

Hi,

PD: with [mov esp, 0xA0010] have no problem, more than this I get limit error ...

But I am still wondering, Why occur the error?, if the range from A0010 to 0xFFFFF is < 1MB

Are you *sure* it's a problem with the segment limit?

For example, is it possible that you're trying to put your stack in the middle of video display memory or ROM, and the error is caused by the CPU pushing stuff onto the stack (e.g. the return address for "call" instructions) and popping off something different later (e.g. some bytes of ROM that causes RET to return to a dodgy address and makes the CPU execute unknown instructions, that leads to lots of new and exciting ways to crash)?

Code: Select all

    dw 0xffff       ; limit[0..15]

Correct me if I'm wrong, but shouldn't there be five f's there, not four?

In the lowest 16 bits of the limit there's only four F's. In the next 4 bits of the limit (e.g. in the "Limit[16..19]" field) there's another F...


Cheers,

Brendan

[pcmattman]

In the lowest 16 bits of the limit there's only four F's. In the next 4 bits of the limit (e.g. in the "Limit[16..19]" field) there's another F...

Ah, of course. The whole [0..15] thing didn't really compute

[qw]

Are you *sure* it's a problem with the segment limit?

For example, is it possible that you're trying to put your stack in the middle of video display memory or ROM, and the error is caused by the CPU pushing stuff onto the stack (e.g. the return address for "call" instructions) and popping off something different later (e.g. some bytes of ROM that causes RET to return to a dodgy address and makes the CPU execute unknown instructions, that leads to lots of new and exciting ways to crash)?

I'm pretty confident that Brendan is right. With a base of zero, 0xfffff - 4 is in the middle of ROM. By the way, you'd better align the stack to 0xffffc.

[okaox]

I changed stak = 0xAFFFF
--> mov esp, 0xAFFFF

BOCHS: ¿?

[okaox]

thanks my friends !!!

I also thought something like this , I'm accessing an area of read-only (Upper memory area) used by some devices i/o (video, etc)

sorry for the loss of time

greetings !!!

[AJ]

Hi,

Your stack pointer is fine, the return value on the stack is not. It's one of two things:

* You are deliberately trying to RET to code outside of the 0xFFFFF limit. In this case, set the granularity bit(s) in your GDT.
* You are intending to run code within the correct limits, but your stack is corrupted. What is the last value on your stack? Is it what you expect it to be?

Cheers,
Adam