Asm: what changed

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Title: Asm

New		Line
1	+	The [[Reserved word\|reserved word]] `asm` starts a [[Frame\|frame]] of inline [[Assembly language\|assembly]] code.
2	+
3	+	``` pascal
4	+	program asmDemo(input, output, stderr);
5	+
6	+	// The $asmMode directive informs the compiler
7	+	// which syntax is used in asm-blocks.
8	+	// Alternatives are 'att' (AT&T syntax) and 'direct'.
9	+	{$asmMode intel}
10	+
11	+	var
12	+	n, m: longint;
13	+	begin
14	+	n := 42;
15	+	m := -7;
16	+	writeLn('n = ', n, '; m = ', m);
17	+
18	+	// instead of declaring another temporary variable
19	+	// and writing "tmp := n; n := m; m := tmp;":
20	+	asm
21	+	mov eax, n // eax := n
22	+	// xchg can only operate at most on one memory address
23	+	xchg eax, m // swaps values in eax and at m
24	+	mov n, eax // n := eax (holding the former m value)
25	+	// an array of strings after the asm-block closing 'end'
26	+	// tells the compiler which registers have changed
27	+	// (you don't wanna mess with the compiler's notion
28	+	// which registers mean what)
29	+	end ['eax'];
30	+
31	+	writeLn('n = ', n, '; m = ', m);
32	+	end.
33	+	```
34	+
35	+	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\|conditional compilation]]:
36	+
37	+	``` pascal
38	+	program sign(input, output, stderr);
39	+
40	+	type
41	+	signumCodomain = -1..1;
42	+
43	+	{ returns the sign of an integer }
44	+	function signum({$ifNDef CPUx86_64} const {$endIf} x: longint): signumCodomain;
45	+	{$ifDef CPUx86_64} // ============= optimized implementation
46	+	assembler;
47	+	{$asmMode intel}
48	+	asm
49	+	xor rax, rax // ensure result is not wrong
50	+	// due to any residue
51	+
52	+	test x, x // x ≟ 0
53	+	setnz al // al ≔ ¬ZF
54	+
55	+	sar x, 63 // propagate sign-bit through reg.
56	+	cmovs rax, x // if SF then rax ≔ −1
57	+	end;
58	+	{$else} // ========================== default implementation
59	+	begin
60	+	// This is what math.sign virtually does.
61	+	// The compiled code requires _two_ cmp instructions, though.
62	+	if x > 0 then
63	+	begin
64	+	signum := 1;
65	+	end
66	+	else if x < 0 then
67	+	begin
68	+	signum := -1;
69	+	end
70	+	else
71	+	begin
72	+	signum := 0;
73	+	end;
74	+	end;
75	+	{$endIf}
76	+
77	+	// M A I N =================================================
78	+	var
79	+	x: longint;
80	+	begin
81	+	readLn(x);
82	+	writeLn(signum(x));
83	+	end.
84	+	```
85	+
86	+	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.
87	+
88	+	## see also
89	+
90	+	general
91	+
92	+	- [[The inline assembler parser]]
93	+	- [[Lazarus inline assembler]]
94	+	- [[Label\|label § “assembler”]]
95	+
96	+	relevant compiler directives
97	+
98	+	- [[$asmMode\|{$asmMode}]]
99	+	- [[$goto\|{$goto}]]
100	+	- [[$stackFrames\|{$stackframes}]]
101	+
102	+	special tasks
103	+
104	+	- [[Hardware Access\|hardware access]]
105	+	- [[AVR Programming\|AVR programming]]
106	+
107	+	---
108	+	From the Free Pascal wiki, page "Asm", as it stood on 2021-02-05 after a change by Kai Burghardt, out of the wiki's dump of 2024-05-26. Copyright stays with the author who wrote it, according to the old wiki's [[Licensing]] page. If you have concerns about this material, [message the admin](/wiki-contact).
109	+	Category: Code
110	+	Category: Reserved words

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