convert simple assembly into C

[fayya]

Hi,

I am working on a project written in C that involves assembly codes. I need some help on converting an assembly function to complete C code, any help or pointer is welcome, thanks!

This is the asm code that I want to convert to C:

SWORD get_svalue(BYTE k)
// k>0 always
// Takes k bits out of the BIT stream (wordval), and makes them a signed value
{
_asm {
xor ecx, ecx
mov cl,k
mov eax,[wordval]
shl eax,cl
shr eax, 16
dec cl
bt eax,ecx
jc end_macro
signed_value:inc cl
mov ebx,[start_neg_pow2]
add ax,word ptr [ebx+ecx*2]
end_macro:
}
}

 

[strettonman]

The registers obviously need replacing with variables
================================
>>= and <<= in place of shr and shl
==================================

mov ebx, [start_neg_pow2]
add ax, word ptr [ebx + ecx * 2 ]

replacing start_neg_pow2 with p, the above becomes something like:

add regax, *( *p + regcx * 2 );

 

[lionelhill]

I think you'd get better code if you look at what this snippet does, and do it in C.
It will be more readable, and more future-proof.

 

[strettonman]

Intel x86 assembly language is as future proof as could be wished for. A program written in 1981 for the very first PC could, in theory, be run on the very latest PC. Provided you installed MS-DOS first of course, or alternatively (like me) have it already installed on a spare partition for old time's sake.

 

[lionelhill]

Absolutely. I didn't mean that at all. Fayya's project is based in C, not intel x86 assembly. What I meant was that different compilers have considerable flexibility in the definition of C about how their various number formats are allowed to be handled behind the scenes.

If you embed assembler code into C with an intention of returning a particular number format, it is great until someone changes the compiler, and the new compiler expects a different format. Hence my suggestion that if you're writing C, and need to fiddle with formats, let the compiler do as much as possible, in standard C.

 

[Nacker]

Hello!
This works pretty well for the code

SWORD get_svalue(BYTE k)
// k>0 always
// Takes k bits out of the BIT stream (wordval), and makes them a signed value
{
/* Original asm code.
_asm {
xor ecx, ecx
mov cl,k
mov eax,[wordval]
shl eax,cl
shr eax, 16
dec cl
bt eax,ecx
jc end_macro
signed_value:inc cl
mov ebx,[start_neg_pow2]
add ax,word ptr [ebx+ecx*2]
end_macro:
}*/
/* Substitute with this */
unsigned long wordtemp;
signed short int ret;

wordtemp = wordval<<k;
wordtemp>>=16;

k--;

if((wordtemp>>k)&0x00000001)
return wordtemp;

else{
k++;
ret = wordtemp+neg_pow2[k];
return ret;
}

}

Njoy!

Nacker

http://eces.colorado.edu/~kaje