🚀 go-pugleaf

JRS:  In article <12nBGCAEnUm3YwId@pedt.demon.co.uk> of Sat, 24 Jul 1999
06:06:12 in news:comp.lang.pascal.borland, Pedt Scragg
<newsmaster@pedt.demon.co.uk> wrote:
>Robert AH Prins <mailto:prinsra@williscorroon.com> said: 
>
>>To avoid having to load pointers over and over again, the program uses a
>>GETMEM'ed array, I use BASM, and this works without a glitch, until...
>>I had to change the size of the elements from 8 to 16 and didn't realize
>>that I also needed to change a "shl  cx, 3" into "shl  cx, 4". I spend
>>ages trying to find out what was wrong - the equivalent code in Pascal
>>continued to work, be it rather more slowly.
>>
>>Does anyone know, short of what I've done now, adding a constant with
>>the required shift value, if there is a way to get the base-2 log of
>>sizes of datastructures in BASM. (Obviously restricted to sizes that are
>>powers of 2...)
>>
>I can only think of two easy ways to do it as you cannot use SizeOf in
>BASM :

>2. Declare two dummy global variables the same size as one data element
>immediately after each other and use BASM to get load the memory offset
>of each into variables then use SUB to get the memory size. You'll still
>need to SHL <register>, CL


If BASM allows "compile-time" access to offsets, then by declaring ONE
of the same size, then something, and subtracting the offsets, you get
the number you need to take the log2 of - though the original post
implied that you already have that number.

My pas-norm.htm#Bin says :
    Counting bits in an ordinal : "J and Pred(J)" has one "1" bit fewer 
    than "J" does, for J<>0; so :
        while J<>0 do begin
          Inc(Count) ; J := J and Pred(J) end ;

Can you use that, somehow, unrolled, in BASM? Pseudo-code:

        C := 0 ;
        J := Size-1 (Relies on Size a power of 2; count its bits)
        several of ( J := J and Pred(J) ; C := C + Ord(J<>0) ; )

would set C to log2(Size), give or take - and you can ensure that Size
is a power of two by checking that (Size and Pred(Size))=0.

I have a feeling that I've seen an assembler in which one could do that.

  
-- 
 � John Stockton, Surrey, UK.  jrs@merlyn.demon.co.uk   Turnpike v4.00   MIME. �
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