Re: Switch from User space to Kernel space.

[Ketan Mukadam <kmukadam@xxxxxxxxxxxx>]

This is extended aseembly in gcc syntax.

the prototype is
asm ( assembler template


           : output operands                  /* optional */

           : input operands                   /* optional */

           : list of clobbered registers      /* optional */

           );


The assembler prototype consists of assembly instructions. Each operand is 
described by an operand-constraint string followed by the C expression. 
First colon separates the assembler instruction from the first output 
operand and other separates the last output operand from the input. Commas 
separate the operands in the group.

The 'a' & 'b' are the registers
a  ==>   %eax, %ax, %al
b  ==>   %ebx, %bx, %bl

Read this 
http://www.ibiblio.org/gferg/ldp/GCC-Inline-Assembly-HOWTO.html#s3 to 
understand more about the syntax.

I hope you can now understand the code easily.

Ketan

At 10:10 PM 7/26/2004, Dhiman, Gaurav wrote:
> Can any one tell me what's going on in following code (esp code in RED 
> color) ?
>
>
> #define 
> <http://lxr.linux.no/ident?v=2.4.21;i=_syscall1>_syscall1(<http://lxr.linux.no/ident?v=2.4.21;i=type>type,<http://lxr.linux.no/ident?v=2.4.21;i=name>name,type1,<http://lxr.linux.no/ident?v=2.4.21;i=arg1>arg1) 
> \
> <http://lxr.linux.no/ident?v=2.4.21;i=type>type 
> <http://lxr.linux.no/ident?v=2.4.21;i=name>name(type1 
> <http://lxr.linux.no/ident?v=2.4.21;i=arg1>arg1) \
> { \
>         long <http://lxr.linux.no/ident?v=2.4.21;i=__res>__res; \
>         __asm__ volatile ("int $0x80" \
>         : "=a" (<http://lxr.linux.no/ident?v=2.4.21;i=__res>__res) \
>         : "" (__NR_##name),"b" 
> ((long)(<http://lxr.linux.no/ident?v=2.4.21;i=arg1>arg1))); \
>
> <http://lxr.linux.no/ident?v=2.4.21;i=__syscall_return>__syscall_return(<http://lxr.linux.no/ident?v=2.4.21;i=type>type,<http://lxr.linux.no/ident?v=2.4.21;i=__res>__res); 
> \
> }
> This has been taken from 'unistd.h' file in kernel sources. It's a code 
> which actually creates a stub, so that user process can make a call to 
> system call.
>
> My question is for having an access this code (macro) user process or 
> library function used by user program must include this file ('unistd.h'). 
> Is it like this that this file is included by a library or user program 
> which makes a call to system call. Actually it is the first time that I am 
> studying about how the control switches from user space to kernel space. I 
> want to know exact code which make this switch. I think that its this stub 
> only which allow the control to switch from user space to kernel space.
>
> One more thing, Please let me know what does the code in RED color is 
> doing. Its an assembly code, please explain it.
>
> Regards,
> Gaurav



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