problem with printchar

problem with printchar

[ssams]

i have this code
.globl mulai
mulai:
   movl $4,%eax
   movl $1,%ebx
   movl $'a',%ecx
   movl $1,%edx
   int   $0x80
   xorl  %eax,%eax
   movl  $1,%eax
   xorl  %ebx,%ebx
   int   $0x80
then i compile with
as program1.s -o program1.o
ld program1.o -o program1

by this code i want print one char but there is no output.
can anybody help me why..?

 

Re: problem with printchar

[Scott Lanning]

On Wed, 24 Apr 2002, ssams wrote:
>i have this code
>.globl mulai
>mulai:
>   movl $4,%eax
>   movl $1,%ebx
>   movl $'a',%ecx

Wow, is that valid?
Anyway, %ecx should not be the value of 'a'
but rather a pointer to a buffer.

>   movl $1,%edx
>   int   $0x80
>   xorl  %eax,%eax
>   movl  $1,%eax

ITYM 'incl %eax'.

>   xorl  %ebx,%ebx
>   int   $0x80
>then i compile with
>as program1.s -o program1.o
>ld program1.o -o program1
>
>by this code i want print one char but there is no output.
>can anybody help me why..?

Re: problem with printchar

[Rudolf Marek]

Yes...
Because in ecx should be address (pointer) to char.
Kernel silently ignores bad address so there is no output.
You can use strace utility to watch whats going on

$strace ./program1

execve("./program1", ["./program1"], [/* 30 vars */]) = 0
write(1, ptrace: un: Input/output error
0x61, 1)                       = -1 EFAULT (Bad 
addreexit(0)                                = ?

uh... the solution: you have to declare somewhere in memory a byte where 
you will store that char.

This is not "as" syntax but hope you will get an idea

mov ecx,my_char
mov [ecx],'a' ;you can cachnge it "on-line"

.....

DATASEG
my_char DB 'b' 

Happy assembling,
    
      Regards

		Rudolf


On Wed, 24 Apr 2002, ssams wrote:

> Date: Wed, 24 Apr 2002 14:18:57 +0700
> From: ssams <ssams@telkom.net>
> To: linux-assembly@vger.kernel.org
> Subject: problem with printchar
> 
> i have this code
> .globl mulai
> mulai:
>    movl $4,%eax
>    movl $1,%ebx
>    movl $'a',%ecx
>    movl $1,%edx
>    int   $0x80
>    xorl  %eax,%eax
>    movl  $1,%eax
>    xorl  %ebx,%ebx
>    int   $0x80
> then i compile with
> as program1.s -o program1.o
> ld program1.o -o program1
> 
> by this code i want print one char but there is no output.
> can anybody help me why..?
> 
>  
> -
> To unsubscribe from this list: send the line "unsubscribe linux-assembly" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> 
> 
>