SAL and SAR by 0 errors

Question

I have discovered a bug in some assembly code I have been working with but can't figure how to fix it. When shifting left by 0 the result ends up being 0 instead of jut the number. The same applies when shifting to the right. Any and all help is much appreciated.

function sal(n,k:integer):integer;
begin
 asm
 cld
 mov    cx,     k
@1:
  sal n, 1
  loop @1
 end;
 sal:= n;
end;

function sar(n,k:integer):integer;
begin
 asm
  cld
  mov   cx,     k
@1:
  sar   n,      1
  loop  @1
 end;
 sar:=n;
end;

I have tried to changed them in the following way and it still does not work properly.

function sal(n,k:integer):integer;
begin
 asm
 cld
 mov    cx,     k
 jcxz @done
@1:
  sal n, 1
  loop @1
@done:
 end;
 sal:= n;
end;

function sar(n,k:integer):integer;
begin
 asm
  cld
  mov   cx,     k
 jcxz @done
@1:
  sar   n,      1
  loop  @1
@done:
 end;
 sar:=n;
end;

Answer 1

what about shr and shl? http://www.delphibasics.co.uk/RTL.asp?Name=Shr http://www.delphibasics.co.uk/RTL.asp?Name=Shl

Answer 2

about your error: if k is zero then loop will decrease it and get negative value. loop checks only for zero. you number gets decreased until it gets zero again eventually. this shifts your number so much that all bits are shifted out.

Answer 3

The probmem is that the mov instruction doesn't set any of the flags. You probably want to add a test instruction before the conditional jump.

Also, I assume there's a reason you're doing this in a loop instead of a with a single instruction? It is possible to stick the count in CL.

Answer 4

You have a typo... Use clc instead cld, cld instruction will clear direction flag and clc will clear carry flag.

Right asm code...

asm
      mov    cx, k         //ecx for 32bit cpu mode
@1:
      clc 
      sal    n, 1
      loop   @1
end;

or

asm
      mov    cx, k         //ecx for 32bit cpu mode
@1: 
      shl    n, 1
      loop   @1
end;

or

asm
      mov    cl, k        
      shl    n, cl
end;

This code works under 16bit cpu mode!

EDIT: To prevent zero shift first check if k = 0

 asm
      mov    cx, k         //ecx for 32bit cpu mode
      and    cl, cl        //zero test  
      jz     @Done
@1:
      clc
      sal    n, 1
      loop   @1
@Done:
 end;