asked on

asm functions and pointers

i'm learning to make asm functions inside pascal 7 and
i've figured out that the return value for the function
is the value in AX. but that is at most a word. how do
i send the function a pointer as parameter and read/modify
its value?

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snirad

Hi acerola, you said that you have learned how to return values in the AX but at most you can only return a word. This isn't fully true. For larger DWord size object such as pointers and LongInts you can use the DX:AX pair. The following program should give you a good example of how to do this along with a few other tricks you may find helpful.

{ This Program is a simple demonstration of how to use Pointers from within}
{ assembler functions in Pascal.}
PROGRAM TestPtr;

{ Even though NIL in Borland Pascal should always be the value of 0:0 it }
{ is good to make your code as compatibly with other versions as possible. }
{ Defining constants for the segment and offset values of NIL and assigning }
{ them formulas will both make it easier to change you code if you need to }
{ and increase the chances of not needing to change it at all. }
CONST
kNilOfs = LongInt(NIL) AND $0000FFFF;
kNilSeg = LongInt(NIL) SHR 16;

{ This function returns the value of P and then clears P by assigning it NIL}
FUNCTION ClearPointer(VAR P: Pointer): Pointer; ASSEMBLER;
ASM
LES DI,P {Get the Pointers address}
MOV AX,ES:[DI] {Load it’s offset}
MOV DX,ES:[DI+2] {Load it’s segment}
{DX:AX now has P loaded and is ready to be returned}
MOV CX,kNilOfs {Load CX with the offset of Nil}
MOV ES:[DI],CX {Store the offset}
MOV CX,kNilSeg {Load CX with the segment of Nil}
MOV ES:[DI+2],CX {Store the segment}
{We're done!}
END;

VAR
OriginalPtr, ReturnedPtr: Pointer;

BEGIN
{This point on is an example of how to use ClearPointer}
OriginalPtr:= Ptr(1234, 5678);
ReturnedPtr := ClearPointer(OriginalPtr);
IF OriginalPtr<>NIL THEN
WriteLn('ERROR: ClearPointer failed to place the value of NIL in OriginalPtr!')
ELSE
WriteLn('SUCCESS: ClearPointer successfully place the value of NIL in OriginalPtr!');
WriteLn('The value of ReturnedPtr is ',Seg(ReturnedPtr^),':',Ofs(ReturnedPtr^),'.');
END.