Some notes on MIPS assembler

ATLAS presently supports only the 64-bit ISAs (MIPS III and MIPS IV) using the ABIs discussed below, which seem to work for Linux and IRIX. This information is presently priliminary, but as far as I can tell, both IRIX and Linux use the same ABIs. The only difference appears to be in their comment character, which is easily worked around by using C-style comments (assuming you use cpp, as I do).

There seem to be a lot of MIPS ABIs, and different archs use different ones. As far as I know, Linux follows the SGI/IRIX ABIs, which can be sorted into the following catagories, based on the IRIX cc flag or gcc's -mabi= flag:

-32

: this is the classic 32 ABI build for old MIPS I MIPS II 32 bit hardware. Has only 16 fp regs, and integer regs are 32 bits long. Not presently supported by ATLAS.

-o64

: Seems to be a 64-bit extension of -32, which is also not presently supported by ATLAS.

-64

: 64 bit ABI for MIPS III and MIPS IV ISA machines. Integer regs are 64 bits long, and 32 fp regs are allowed. Supported by ATLAS.

-n32

: 32 bit ABI for MIPS III and MIPS IV ISA machines. Integer regs are 64 bits long, and 32 fp regs are allowed. Has same register usage as -64, but integers, longs, and pointers all 32 bit.

NOTE: rest of this document describes my understanding of -64 and -n32 ABIs only.

MIPS assembler routines in ATLAS should include:

#if !defined(ATL_GAS_MIPS)
   #error "This kernel requires MIPS assembler!"
#endif

This quick error exit keeps a non-MIPS assembler from generating hundreds or thousands of spurious error messages during install.