GCC-Inline-Assembly-HOWTO
FreeGuide to GCC inline assembly for x86
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About GCC-Inline-Assembly-HOWTO
A comprehensive HOWTO document by Sandeep S (v0.1, March 2003) that explains the use and usage of GCC's inline assembly feature for x86 processors. It covers both basic and extended inline assembly using AT&T syntax, including the assembler template, operands, clobber list, volatile qualifier, and constraint modifiers. The document assumes basic knowledge of x86 assembly and C, and is intended for system programmers who need to optimize critical code sections or interface with hardware directly. It is licensed under the GNU General Public License.
Key Features
Explains AT&T vs Intel syntax differences
Covers basic inline assembly with asm keyword
Detailed explanation of extended asm: assembler template, operands, clobber list, volatile
Comprehensive coverage of operand constraints and constraint modifiers
Includes useful code recipes and examples
Discusses how inline assembly reduces function-call overhead
Pros & Cons
Pros
- Reduces function-call overhead by inlining assembly
- Allows direct access to CPU instructions and registers
- Integrates seamlessly with C variables and expressions
- Provides detailed explanation of constraints for operand matching
- Free and open-source (GPL licensed)
Cons
- Requires prior knowledge of x86 assembly and C
- Document is from 2003 and may not cover latest GCC changes
- Only covers x86 architecture (no ARM, RISC-V, etc.)
- Assumes AT&T syntax, not Intel syntax
- Not a tutorial for beginners; assumes familiarity with assembly
Best For
System programming where performance is criticalEmbedded systems programmingWriting device driversOptimizing critical code sectionsInterfacing assembly with C variables
FAQ
What is inline assembly?
Inline assembly is assembly code written as inline functions using the 'asm' keyword, allowing the compiler to insert assembly instructions directly into the caller's code, reducing function-call overhead.
What syntax does GCC use for inline assembly?
GCC uses AT&T UNIX assembly syntax for inline assembly, which differs from Intel syntax in operand ordering and other conventions.
What are the prerequisites for this HOWTO?
Basic knowledge of x86 assembly language and the C programming language are required.
Is this document free to use and distribute?
Yes, it is licensed under the GNU General Public License (version 2 or later), allowing free redistribution and modification.