1 May
2020
1 May
'20
6:12 a.m.
The syscall skips over a location for reasons not fully clear to me. I guess if you dug
down into the libc functions that call it you’d figure out why. As far as the kernel is
concerned, it just doesn’t look at it.
The zero is just a spacer, other code just does a tst -(sp) there which just decrs the
stack poitner.
On Apr 30, 2020, at 5:49 PM, Alexander Voropay
<alec(a)sensi.org> wrote:
Can anyone please explain the last $0 pushed to the stack ?
Early SysIII ans SYSV on the i386 (and may be on i286) used
similar syscall convention.
I wrote about this:
https://minnie.tuhs.org/pipermail/tuhs/2019-October/019274.html
https://minnie.tuhs.org/pipermail/tuhs/2019-October/019294.html
Example:
===
.file "test.s"
.version "02.01"
.set WRITE,4
.set EXIT,1
.text
.align 4
.globl entry
entry:
pushl %ebp
movl %esp,%ebp
subl $8,%esp
pushl $14 /length
pushl $hello
pushl $1 /STDOUT
pushl $0
movl $WRITE,%eax
lcall $0x07,$0
addl $16,%esp
pushl $0
movl $EXIT,%eax
lcall 0x07,$0
.data
.align 4
hello:
.byte 0x48,0x65,0x6c,0x6c,0x6f,0x2c, 0x20,0x77,0x6f,0x72
.byte 0x6c,0x64,0x21,0x0a,0x00
ср, 29 апр. 2020 г. в 17:19, <ron(a)ronnatalie.com>:
>
> Thanks for the link. With that help, I fixed the bug in the program:
>
> mov $6., -(sp)
> mov $1f, -(sp)
> mov $1,-(sp)
> mov $0,-(sp)
> sys 4
> add $8., sp
> mov $0,-(sp)
> mov $0,-(sp)
> sys 1
> 1: <hello>
>
>
>>> Sorry, I typed that in haste without testing. I don’t have a 2.11 system
>>> to try it on. However, reading the source code, I did that wrong. The
>>> args go on the stack, not in line with the code.
>>> mov $6, -(sp)
>>> mov a, -(sp)
>>> mov $1,-(sp)
>>> sys 4
>>
>> Without suggesting that every helpful post should be tested, I find the
>> superb https://unix50.org web emulator excellent for such things.
>>
>> Many thanks to the folks hosting & maintaining this great resource!
>>
>>
>
>