Appendix D Inline Assembler

If you need to write low-level software that interacts directly with the hardware, Ada provides two ways to incorporate assembly language code into your program. First, you can import and invoke external routines written in assembly language, an Ada feature fully supported by GNAT. However, for small sections of code it may be simpler or more efficient to include assembly language statements directly in your Ada source program, using the facilities of the implementation-defined package System.Machine_Code, which incorporates the gcc Inline Assembler. The Inline Assembler approach offers a number of advantages, including the following:

• No need to use non-Ada tools
• Consistent interface over different targets
• Automatic usage of the proper calling conventions
• Access to Ada constants and variables
• Definition of intrinsic routines
• Possibility of inlining a subprogram comprising assembler code
• Code optimizer can take Inline Assembler code into account

This chapter presents a series of examples to show you how to use the Inline Assembler. Although it focuses on the Intel x86, the general approach applies also to other processors. It is assumed that you are familiar with Ada and with assembly language programming.

• Basic Assembler Syntax
• A Simple Example of Inline Assembler
• Output Variables in Inline Assembler
• Input Variables in Inline Assembler
• Inlining Inline Assembler Code
• Other Asm Functionality