Re: [libdv-dev] Assembler for vlc

[Erik W. <om...@cs...>]

On Wed, 26 Apr 2000, James Bowman wrote:

Some issues:

> .text
> 	.align 4
> .globl __dv_decode_vlc 
> 	.type	 __dv_decode_vlc,@function
> __dv_decode_vlc:
> 	pushl %ebx
> 
> 	/* Args are at 20(%esp). */
> 	movl  8(%esp),%eax		/* %eax is bits */
> 	
> 	movl  %eax,%edx			/* %edx is class */
> 	andl  $0xfe00,%edx
> 	sarl  $9,%edx
> 	movsbl dv_vlc_class_lookup5(%edx),%edx

I'm confused.  What does this do?

> 	
> 	movl  dv_vlc_index_mask(,%edx,4),%ebx
> 	movl  dv_vlc_index_rshift(,%edx,4),%ecx
> 	andl  %eax,%ebx
> 	sarl  %cl,%ebx
> 
> 	movl  dv_vlc_lookups(,%edx,4),%edx
> 	movl  (%edx,%ebx,4),%edx

This is where it segfaults right now.  I can't seem to get gdb to tell me
what's in the register, though.

> 
> 	/* Now %edx holds result, like this:
> 	   bits 0-7   run
> 	   bits 8-15  len
> 	   bits 16-31 amp
> 	*/
> 	/* code needs to do this with result:
> 	   if ((amp > 0) &&
> 	     if ((bits >> sign_rshift[result->len]) & 1)
> 	         amp = -amp;
> 	   }
> 	*/
> 	/* if (amp < 0) %edx is 0, else 0xffff0000. */
> 	movl  %edx,%ecx
> 	sarl  $8,%ecx
> 	andl  $0xff,%ecx
> 	movl  sign_mask(,%ecx,4),%ecx
> 	andl  %ecx,%eax
> 	negl  %eax
> 	sarl  $31,%eax
> 
> 	movl  %edx,%ebx
> 	sarl  $31,%ebx
> 	xorl  $0xffffffff,%ebx
> 	andl  $0xffff0000,%ebx
> 
> 	andl  %ebx,%eax
> 	
> 	xorl  %eax,%edx
> 	subl  %eax,%edx
> 
> 	movl  12(%esp),%eax
> 	movl  %edx,(%eax)
> 	
> 	popl  %ebx
> 	ret

         Erik Walthinsen <om...@cs...> - Staff Programmer @ OGI
        Quasar project - http://www.cse.ogi.edu/DISC/projects/quasar/
   Video4Linux Two drivers and stuff - http://www.cse.ogi.edu/~omega/v4l2/
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