My BSOD

Edwin · Post by **Edwin** » Fri Jun 29, 2007 11:53 am

[quote]
4.2.2. Stack Alignment
The stack pointer for a stack segment should be aligned on 16-bit (word) or 32-bit (double-word)
boundaries, depending on the width of the stack segment. The D flag in the segment descriptor
for the current code segment sets the stack-segment width (see â€œSegment Descriptorsâ€

Pyrofan1 · Post by **Pyrofan1** » Sat Jun 30, 2007 12:26 am

my RSOD, http://s176.photobucket.com/albums/w199 ... 0_1992.jpg

inflater · Post by **inflater** » Sat Jun 30, 2007 3:31 am

If a 16-bit value is pushed onto a 32-bit wide
stack, the value is automatically padded with zeros out to 32 bits.

Intel manuals say all!

Okay okay, I will read them more...

inflater

Aali · Post by **Aali** » Sat Jun 30, 2007 6:40 am

i hate to break it to you, but thats just wrong

i have code that DEPENDS on this behaviour

the following code:

push word ax
push word 0x4040
inc esp
push 0x5b585252
push word ax

produces this stack:

0xbfe0ac73: 0xcd 0x80 0x52 0x52 0x58 0x5b 0x40 0xcd 0x80

notice the complete lack of zero padding

Edwin · Post by **Edwin** » Sat Jun 30, 2007 6:49 am

perhaps removing "word" will do the trick..

Tyler · Post by **Tyler** » Sat Jun 30, 2007 6:59 am

Edwin wrote:perhaps removing "word" will do the trick..

The "word" is there to force 16-bit and prove that on his processor stack alignment is not forced with padding.

Edwin · Post by **Edwin** » Sat Jun 30, 2007 8:37 am

Okay, that may be right, but there is no reason why an ordinary push bx or pop bx should screw up stack alignment.

inflater · Post by **inflater** » Sat Jun 30, 2007 8:54 am

The "word" is there to force 16-bit

I'm confused.. Why to force 16-bit? The stack will become, logically, unaligned then...

Never mind that, I heard MOV EAX, [ESP - X] should do the POP operation without screwing the stack. What I need to appoint for the X when I want to pop the last dword on the stack? 8? And for word value, 4 ?

inflater

Aali · Post by **Aali** » Sat Jun 30, 2007 10:26 am

Edwin wrote:Okay, that may be right, but there is no reason why an ordinary push bx or pop bx should screw up stack alignment.

the problem is, there is no "ordinary" push bx or pop bx, you have to use the 0x66 prefix (which turns it into a 16bit operation)

56 00000042 6650 push word ax
60 0000004E 6650 push ax

exactly the same opcode, the word is just there to clarify things a bit

Edwin · Post by **Edwin** » Sat Jun 30, 2007 11:19 am

Hmm I tried it myself and it seems you're right Aali, it seems that successive 16-bit push and pops indeed screw up the alignment, but when I do a push ax and then a 32-bit push, stack alignment seems normal.
So I think that the CPU tries to align the stack to the size of the pushed or popped value. It's confusing because the Intel-manual is not clear about this issue (as far as i've seen).