Asm

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The reserved word asm starts a block of inline assembly code.

 1 program asmDemo(input, output, stderr);
 2 
 3 // The $asmMode directive informs the compiler
 4 // which syntax is used in asm-blocks.
 5 // Alternatives are 'att' (AT&T syntax) and 'direct'.
 6 {$asmMode intel}
 7 
 8 var
 9 	n, m: longint;
10 begin
11 	n := 42;
12 	m := -7;
13 	writeLn('n = ', n, '; m = ', m);
14 	
15 	// instead of declaring another temporary variable
16 	// and writing "tmp := n; n := m; m := tmp;":
17 	asm
18 		mov eax, n  // eax := n
19 		// xchg can only operate at most on one memory address
20 		xchg eax, m // swaps values in eax and at m
21 		mov n, eax  // n := eax (holding the former m value)
22 	// an array of strings after the asm-block closing 'end'
23 	// tells the compiler which registers have changed
24 	// (you don't wanna mess with the compiler's notion
25 	// which registers mean what)
26 	end ['eax'];
27 	
28 	writeLn('n = ', n, '; m = ', m);
29 end.

In order to maintain portability between platforms (i.e. your code still compiles for many targets), while optimizing for specific targets, you want to set up conditional compilation:

 1 program sign(input, output, stderr);
 2 
 3 type
 4 	signumCodomain = -1..1;
 5 
 6 { returns the sign of an integer }
 7 function signum({$ifNDef CPUx86_64} const {$endIf} x: longint): signumCodomain;
 8 {$ifDef CPUx86_64} // ============= optimized implementation
 9 assembler;
10 {$asmMode intel}
11 asm
12 	xor rax, rax                  // ensure result is not wrong
13 	                              // due to any residue
14 	
15 	test x, x                     // x ≟ 0
16 	setnz al                      // al ≔ ¬ZF
17 	
18 	sar x, 63                     // propagate sign-bit through reg.
19 	cmovs rax, x                  // if SF then rax ≔ −1
20 end;
21 {$else} // ========================== default implementation
22 begin
23 	// This is what math.sign virtually does.
24 	// The compiled code requires _two_ cmp instructions, though. 
25 	if x > 0 then
26 	begin
27 		signum := 1;
28 	end
29 	else if x < 0 then
30 	begin
31 		signum := -1;
32 	end
33 	else
34 	begin
35 		signum := 0;
36 	end;
37 end;
38 {$endIf}
39 
40 // M A I N =================================================
41 var
42 	x: longint;
43 begin
44 	readLn(x);
45 	writeLn(signum(x));
46 end.

As you can see, you can implement whole routines in assembly language, by adding the assembler modifier and writing asm instead of begin for the implementation block.


see also

general

relevant compiler directives

special tasks