Assembly Language Coding Guidelines¶
- General
- ASM-GN-01: One address shall not be declared by two labels
- ASM-GN-02: Names reserved for use by the assembler shall not be used for any other purpose
- ASM-GN-03: All declared labels shall be used
- ASM-GN-04: Magic numbers shall be used with restrictions
- ASM-GN-05: Parentheses shall be used to set the operator precedence explicitly
- ASM-GN-06: .end directive statement shall be the last statement in an Assembly file
- ASM-GN-07: Infinite loop shall not exist
- ASM-GN-08: All code shall be reachable
- ASM-GN-09: Far jump shall be used with restrictions
- ASM-GN-10: Assembler directives shall be used with restrictions
- Functions
- ASM-FN-01: Function shall have return statement
- ASM-FN-02: A function shall only have one entry point
- ASM-FN-03: A function shall only have one return statement
- ASM-FN-04: Function shall only be entered by explicit call
- ASM-FN-05: A jump instruction shall not be used to jump out of a function
- ASM-FN-06: Recursion shall not be used in function calls
- ASM-FN-07: Cyclomatic complexity shall be less than 10
- Coding Style
- ASM-CS-01: One instruction statement shall not be split into multiple lines
- ASM-CS-02: Assembler directive statements shall be aligned
- ASM-CS-03: Labels shall be aligned to the start of the line
- ASM-CS-04: Instruction statements shall be aligned
- ASM-CS-05: ‘//’ shall not be used for comments
- ASM-CS-06: Tabs shall be 8 characters wide
- ASM-CS-07: Each line shall contain at most 120 characters
- ASM-CS-08: Legal entity shall be documented in every file
- Naming Convention
General¶
ASM-GN-01: One address shall not be declared by two labels¶
Compliant example:
asm_showcase_1:
movl $0x1, %eax
Non-compliant example:
asm_showcase_1:
asm_showcase_2:
movl $0x1, %eax
ASM-GN-02: Names reserved for use by the assembler shall not be used for any other purpose¶
Names defined by developers shall not be the same as an assembler directive, instruction prefix, instruction mnemonic, or register name.
Compliant example:
asm_showcase_1:
movl $0x1, %eax
asm_showcase_2:
movl $0x2, %eax
asm_showcase_3:
movl $0x3, %eax
Non-compliant example:
text:
movl $0x1, %eax
mov:
movl $0x2, %eax
eax:
movl $0x3, %eax
ASM-GN-03: All declared labels shall be used¶
Otherwise, the label shall be removed.
Compliant example:
asm_showcase_1:
movl $0x1, %eax
jmp asm_showcase_2
/* do something */
asm_showcase_2:
movl $0x2, %eax
Non-compliant example:
asm_showcase_1:
movl $0x1, %eax
/*
* 'asm_showcase_2' is not used anywhere, including
* all C source/header files and Assembly files.
*/
asm_showcase_2:
movl $0x2, %eax
ASM-GN-04: Magic numbers shall be used with restrictions¶
Only the following cases shall be allowed:
- The magic number is defined as a MACRO with a name clearly indicating its meaning.
- The meaning of the magic number is clearly documented in the comments before its usage.
- The meaning of the magic number is straightforward in the specific context.
Compliant example:
.section .data
showcase_data:
/* 0xff000000 means <something> */
.long 0xff000000
Non-compliant example:
.section .data
showcase_data:
.long 0xff000000
ASM-GN-05: Parentheses shall be used to set the operator precedence explicitly¶
Compliant example:
.section .data
showcase_data:
/* 0x1234 means <something> */
.long 0x1234 * (0x1234 >> 2)
Non-compliant example:
.section .data
showcase_data:
/* 0x1234 means <something> */
.long 0x1234 * 0x1234 >> 2
ASM-GN-06: .end directive statement shall be the last statement in an Assembly file¶
This rule only applies to the Assembly file which uses .end directive. .end directive shall be the last statement in this case. All the statements past .end directive will not be processed by the assembler.
Compliant example:
#include <types.h>
#include <spinlock.h>
.macro asm_showcase_mov
movl $0x1, %eax
.endm
.end
Non-compliant example:
#include <types.h>
.end
#include <spinlock.h>
.macro asm_showcase_mov
movl $0x1, %eax
.endm
ASM-GN-07: Infinite loop shall not exist¶
Compliant example:
asm_showcase_1:
movl $0x1, %eax
jmp asm_showcase_2
/* do something */
asm_showcase_2:
movl $0x2, %eax
Non-compliant example:
asm_showcase_1:
movl $0x1, %eax
jmp asm_showcase_1
ASM-GN-08: All code shall be reachable¶
Compliant example:
asm_showcase:
movl %ebx, %eax
test $0x400, %eax
jne asm_test
movl $0x2, %eax
movl $0x3, %eax
asm_test:
movl $0x6, %eax
Non-compliant example:
asm_showcase:
movl %ebx, %eax
jmp asm_test
/* the following two lines have no chance to be executed */
movl $0x2, %eax
movl $0x3, %eax
asm_test:
movl $0x6, %eax
ASM-GN-09: Far jump shall be used with restrictions¶
Jumping to an instruction located in a different segment shall only be used for the following two cases:
- Code bit changes, such as change from 32-bit mode to 64-bit mode.
- System resumes from S3. In this case, Global Descriptor Table (GDT) is set by Bootloader/BIOS and far jump has to be used to correct the Code Segment (CS).
Compliant example:
.code32
execution_32:
/*
* do something in 32-bit mode,
* then,
* perform a far jump to start executing in 64-bit mode
*/
ljmp $0x0008, $execution_64_2
.code64
execution_64_1:
/* do something in 64-bit mode */
execution_64_2:
/* do something in 64-bit mode */
Non-compliant example:
.data
asm_showcase_data:
.word 0x0008
.code32
execution_32:
/* do something in 32-bit mode */
ljmp $0x0008, $asm_showcase_data
ASM-GN-10: Assembler directives shall be used with restrictions¶
Usage of the assembler directive refers to GNU assembler ‘as’ user manual. Only the following assembler directives may be used:
- .align
- .end
- .extern
- repeat related directives, including .rept and .endr
- global related directives, including .global and .globl
- macro related directives, including .altmacro, .macro, and .endm
- code bit related directives, including .code16, .code32, and .code64
- section related directives, including .section, .data, and .text
- number emission related directives, including .byte, .word, .short, .long, and .quad
- .org, which shall be used with restrictions. It shall only be used to advance the location counter due to code bit changes, such as change from 32-bit mode to 64-bit mode.
Functions¶
ASM-FN-01: Function shall have return statement¶
Compliant example:
asm_func_showcase:
movl $0x2, %eax
ret
asm_showcase:
movl $0x1, %eax
call asm_func_showcase
Non-compliant example:
asm_func_showcase:
movl $0x2, %eax
asm_showcase:
movl $0x1, %eax
call asm_func_showcase
ASM-FN-02: A function shall only have one entry point¶
The label in a function shall only be used inside. Jumping into the function from outside via this label shall not be allowed. This rule applies to both conditional jump and unconditional jump.
Compliant example:
asm_func_showcase:
test $0x400, %eax
jne tmp
movl $0x1, %eax
tmp:
movl $0x2, %eax
ret
asm_showcase:
movl $0x1, %eax
call asm_func_showcase
Non-compliant example:
asm_func_showcase:
movl $0x1, %eax
tmp:
movl $0x2, %eax
ret
asm_showcase:
movl $0x1, %eax
call asm_func_showcase
jmp tmp
ASM-FN-03: A function shall only have one return statement¶
Compliant example:
asm_func_showcase:
test $0x400, %eax
jne tmp
movl $0x2, %eax
jmp showcase_return
tmp:
movl $0x3, %eax
showcase_return:
ret
Non-compliant example:
asm_func_showcase:
test $0x400, %eax
jne tmp
movl $0x2, %eax
ret
tmp:
movl $0x3, %eax
ret
ASM-FN-04: Function shall only be entered by explicit call¶
Falling through from prior instruction shall not be allowed.
Compliant example:
asm_func_showcase:
movl $0x2, %eax
ret
asm_showcase:
movl $0x1, %eax
call asm_func_showcase
Non-compliant example:
asm_showcase:
movl $0x1, %eax
asm_func_showcase:
movl $0x2, %eax
ret
ASM-FN-05: A jump instruction shall not be used to jump out of a function¶
This rule applies to both conditional jump and unconditional jump.
Compliant example:
asm_func_showcase:
movl $0x2, %eax
ret
asm_showcase:
movl $0x1, %eax
call asm_func_showcase
Non-compliant example:
asm_func_showcase:
movl $0x2, %eax
jmp asm_test
ret
asm_showcase:
movl $0x1, %ebx
call asm_func_showcase
asm_test:
cli
ASM-FN-06: Recursion shall not be used in function calls¶
Compliant example:
asm_func_showcase:
movl $0x2, %eax
ret
asm_showcase:
movl $0x1, %eax
call asm_func_showcase
Non-compliant example:
asm_func_showcase:
movl $0x2, %eax
call asm_func_showcase
ret
asm_showcase:
movl $0x1, %eax
call asm_func_showcase
ASM-FN-07: Cyclomatic complexity shall be less than 10¶
A function with cyclomatic complexity greater than 10 shall be split into multiple sub-functions to simplify the function logic.
Compliant example:
asm_func_showcase:
/* do something */
cmpl $0x0, %eax
je tmp
cmpl $0x1, %eax
je tmp
cmpl $0x2, %eax
je tmp
/* do something */
tmp:
/* do something */
ret
Non-compliant example:
asm_func_showcase:
/* do something */
cmpl $0x0, %eax
je tmp
cmpl $0x1, %eax
je tmp
cmpl $0x2, %eax
je tmp
cmpl $0x3, %eax
je tmp
cmpl $0x4, %eax
je tmp
cmpl $0x5, %eax
je tmp
cmpl $0x6, %eax
je tmp
cmpl $0x7, %eax
je tmp
cmpl $0x8, %eax
je tmp
cmpl $0x9, %eax
je tmp
cmpl $0xa, %eax
je tmp
cmpl $0xb, %eax
je tmp
cmpl $0xc, %eax
je tmp
cmpl $0xd, %eax
je tmp
cmpl $0xe, %eax
je tmp
/* do something */
tmp:
/* do something */
ret
Coding Style¶
ASM-CS-01: One instruction statement shall not be split into multiple lines¶
Compliant example:
movl $0x2, %eax
Non-compliant example:
movl $0x2, \
%eax
ASM-CS-02: Assembler directive statements shall be aligned¶
An assembler directive statement is composed of directive and arguments. Arguments are optional depending on the use case. Some detailed rules about the alignment are listed below:
- Assembler directives shall be aligned with one tab if the statement is in the code block under any label from functional perspective. Otherwise, assembler directives shall be aligned to the start of the line.
- Tabs shall be used to separate the directive and the first argument, where applicable. The number of tabs could be decided by the developers based on each case and it shall guarantee that the first argument is aligned in each directive block.
- A single space shall be used to separate multiple arguments.
Compliant example:
.extern cpu_primary_save32
.extern cpu_primary_save64
.section multiboot_header, "a"
.align 4
.long 0x0008
.long 0x0018
.section entry, "ax"
.align 8
.code32
Non-compliant example:
.extern cpu_primary_save32
.extern cpu_primary_save64
.section multiboot_header, "a"
.align 4
.long 0x0008
.long 0x0018
.section entry, "ax"
.align 8
.code32
ASM-CS-03: Labels shall be aligned to the start of the line¶
Compliant example:
asm_showcase_1:
movl $0x1, %eax
asm_showcase_2:
movl $0x2, %eax
Non-compliant example:
asm_showcase_1:
movl $0x1, %eax
asm_showcase_2:
movl $0x2, %eax
ASM-CS-04: Instruction statements shall be aligned¶
An instruction statement is composed of instruction prefix, instruction mnemonic, and instruction operands. Instruction prefix and instruction operands are optional depending on the use case. Some detailed rules about the alignment are listed below:
- The start of instruction statements shall be aligned with one tab if the instruction statement is in the code block under any label from functional perspective. Otherwise, the start of instruction statements shall be aligned to the start of the line. The start of the instruction could either be the instruction mnemonic or the instruction prefix.
- One space shall be used to separate the instruction prefix and the instruction mnemonic, where applicable.
- Tabs shall be used to separate the instruction mnemonic and the first instruction operand, where applicable. The number of tabs could be decided by the developers based on each case and it shall guarantee that the first instruction operand in the code block under one label is aligned.
- A single space shall be used to separate multiple operands.
Compliant example:
asm_showcase_1:
movl $0x1, %eax
lock and %rcx, (%rdx)
asm_showcase_2:
movl $0x3, %eax
Non-compliant example:
asm_showcase_1:
movl $0x1, %eax
lock and %rcx, (%rdx)
asm_showcase_2:
movl $0x2, %eax
ASM-CS-05: ‘//’ shall not be used for comments¶
‘/* */’ shall be used to replace ‘//’ for comments.
Compliant example:
/* This is a comment */
movl $0x1, %eax
Non-compliant example:
// This is a comment
movl $0x1, %eax
ASM-CS-06: Tabs shall be 8 characters wide¶
A tab character shall be considered 8-character wide when limiting the line width.
ASM-CS-07: Each line shall contain at most 120 characters¶
No more than 120 characters shall be on a line, with tab stops every 8 characters.
Compliant example:
/*
* This is a comment. This is a comment. This is a comment. This is a comment.
* This is a comment. This is a comment. This is a comment.
*/
Non-compliant example:
/* This is a comment. This is a comment. This is a comment. This is a comment. This is a comment. This is a comment. This is a comment. */
ASM-CS-08: Legal entity shall be documented in every file¶
Legal entity shall be documented in a separate comments block at the start of every file. The following information shall be included:
- Copyright
- License (using an SPDX-License-Identifier)
Compliant example:
/* Legal entity shall be placed at the start of the file. */
-------------File Contents Start After This Line------------
/*
* Copyright (C) 2019 Intel Corporation.
*
* SPDX-License-Identifier: BSD-3-Clause
*/
/* Coding or implementation related comments start after the legal entity. */
.code64
Non-compliant example:
/* Neither copyright nor license information is included in the file. */
-------------------File Contents Start After This Line------------------
/* Coding or implementation related comments start directly. */
.code64
Naming Convention¶
ASM-NC-01: Lower case letters shall be used for case insensitive names¶
This rule applies to assembler directive, instruction prefix, instruction mnemonic, and register name.
Compliant example:
.code64
lock and %rcx, (%rdx)
Non-compliant example:
.CODE64
LOCK AND %RCX, (%RDX)
ASM-NC-02: Names defined by developers shall use lower case letters¶
Names defined by developers shall use lower case letters with the following exception. If an object-like MACRO is defined with ‘#define’, it shall be named with full upper case.
Compliant example:
asm_showcase:
movl $0x1, %eax
Non-compliant example:
ASM_SHOWCASE:
movl $0x1, %eax
ASM-NC-03: Label name shall be unique¶
Label name shall be unique with the following exception. Usage of local labels is allowed. Local label is defined with the format ‘N:’, where N represents any non-negative integer. Using ‘Nb’ to refer to the most recent previous definition of that label. Using ‘Nf’ to refer to the next definition of a local label.
Compliant example:
asm_showcase_1:
movl $0x1, %eax
asm_showcase_2:
movl $0x2, %eax
Non-compliant example:
asm_showcase:
movl $0x1, %eax
asm_showcase:
movl $0x2, %eax
ASM-NC-04: Names defined by developers shall be less than 31 characters¶
Compliant example:
asm_showcase:
movl $0x1, %eax
Non-compliant example:
asm_showcase_asm_showcase_asm_showcase:
movl $0x1, %eax