Assembly Language Coding Guidelines

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:

  1. The magic number is defined as a MACRO with a name clearly indicating its meaning.
  2. The meaning of the magic number is clearly documented in the comments before its usage.
  3. 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:

  1. Code bit changes, such as change from 32-bit mode to 64-bit mode.
  2. 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:

  1. .align
  2. .end
  3. .extern
  4. repeat related directives, including .rept and .endr
  5. global related directives, including .global and .globl
  6. macro related directives, including .altmacro, .macro, and .endm
  7. code bit related directives, including .code16, .code32, and .code64
  8. section related directives, including .section, .data, and .text
  9. number emission related directives, including .byte, .word, .short, .long, and .quad
  10. .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:

  1. 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.
  2. 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.
  3. 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:

  1. 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.
  2. One space shall be used to separate the instruction prefix and the instruction mnemonic, where applicable.
  3. 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.
  4. 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. */

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

ABI Conformance

ASM-ABI-01: The implementation of Assembly code shall conform to the System V x86/AMD ABI

The implementation of Assembly code shall conform to the function calling sequence defined in System V Application Binary Interface AMD64 Architecture Processor Supplement.

Refer to the System V Application Binary Interface AMD64 Architecture Processor Supplement.