hello!

I'm new to assembler so please forgive my beginner question. I'm using FDBG to debug a program I created. I'm simply trying to push words onto the stack a total of 10 times, but when I trace the program using FDBG, I get results on the stack that don't make sense (see attachment)

1) After the first push the stack gets updated. But after the second push, it seems like the first push is undone and there is only one qword pushed on the stack. Could someone explain what is going on?

2) The instruction is pushd 0x00. But I see it is only pushing qwords. In 64 bit, can you push only qwords or can you push other amounts?

thanks for the help

If your code is 32-bit then all pushes can be 2 bytes or 4 bytes. In 64-bit mode all pushes can be either 2 bytes or 8 bytes (never 4 bytes).

So I guess your code is 32-bit, FDBG is displaying the stack as 64-bit values, and each push is 4 bytes.

I'm not sure I understand. If it is displaying it as 64bit values, after the first push, shouldn't it show:
0000000000000000
FFED62CA00000001

But instead it results in:
0000000100000000
00000000FFED62CA

What is all that extra stuff? Sorry for my confusion.

If the push is only 4 bytes then all the values shift by half in the 64 bit display.

Ideally FDBG should display 32 bit values and then each push would simply move everything down one slot, instead of what you see where it moves everything horizontally and then down. If you follow the path of the 32 bit value "FFED62CA" you can see it go from one side the the other side and move down one slot for each two pushes.

Can you change FDBG to show 32 bit values?

I don't know if it's possible to change FDBG to do that. Are there other debuggers for Linux? Could you recommend some?

hi, the e command always displays qwords coz fdbg is 64 bit debugger
if you need to display dwords, use command d
d d rsp
(just from my head, maybe I'm mistaken in the syntax)

gdb is pretty much the "standard" though I dislike it.

I recommend https://github.com/hugsy/gef

You guys will really like it. You need to first install GDB version 7.7+ (8 works too) and then you run that script and then run gdb -q somebinary.

This is great for an assembly programmer on Linux. See screenshot attached to this post.