Contributor: PHIL NICKELL
(*
ウ{$F+}
ウPROCEDURE DoNothing; INTERRUPT;
ウBEGIN
ウEND;
ウ{$F-}
Would you believe that the code in your DoNothing procedure can be
improved for smaller size and better speed? (No, I'm not kidding,
please read on.) The standard preamble and postamble code generated by
Turbo Pascal for a procedure of type Interrupt pushes a whole wad of
registers, sets the BP and DS registers, and then undoes it all before
the IRET. Your DoNothing procedure compiles to code that looks
something like this:
{ preamble }
PUSH AX BX CX DX SI DI DS ES BP
MOV BP, SP
MOV AX, @DATA
MOV DS, AX
{ postamble }
POP BP ES DS DI SI DX CX BX AX
IRET
The following procedure provides identical results and kills the
overhead.
*)
{$f+}
PROCEDURE DoNothing; ASSEMBLER; { Coded as Int Handler }
asm
IRET { return from interrupt }
end;
{$f-}
(*
With no parameters and no local vars Turbo Pascal generates no preamble
code, and generates only a long return as postamble. The resulting
compiled code from my DoNothing proc looks like this:
IRET
RET
The difference: 26 bytes and many stack memory accesses for the null
Interrupt procedure versus only 2 bytes in the null Assembler procedure
with Iret. The RET never gets executed, of course.
*)