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Assembly Files: Difference between .a .s .asm

In the MIPS simulator "QTSpim", three types of files are accepted:

  • .a
  • .s
  • .asm

Is there a difference between those filetypes, and if yes, what is the difference?

Solution

  • In Unix/Linux systems:

    • .a is the usual extension for static libraries (aka Archives of multiple .o files, made with ar(1)). Dynamic libraries, aka Shared Objects, use .so.
    • .s is used for asm compiler output. (gcc -S foo.c produces asm output, with a default filename of foo.s)
    • .S is used for hand-written asm source files. gcc -c foo.S runs it through the C preprocessor (so you can use #include<>, #if, #define, and C-style comments.) Some C headers, like asm/unistd.h only have #defines, and so can be included in a .S to get definitions like __NR_write system call numbers, for example.

    In x86, there are two separate versions of asm syntax: AT&T (used by Unix compilers like gcc), and Intel/NASM (with a couple dialects, like MASM vs. NASM itself).

    .S is appropriate for asm in GNU as syntax, whether you use any C preprocessor features or not.

    In x86, .asm is more often associated with Intel-syntax NASM/YASM, or MASM, source code. Outside of x86, it's probably a good choice for asm source files that could be assembled by the platform-specific assembler, if it uses different directives than GNU as.

    The glibc source tree uses .S for all asm source files.

    People with a gcc background might put their MIPS asm into .S or .s files, while people with more NASM/YASM experience (or Windows), might go for .asm.

    I'd recommend against .s files, because it's easy to accidentally overwrite with gcc -S foo.c.