14.2 Register Conventions

The 32 general purpose registers contained in the PIC32 are shown in the table below. Some of these registers are assigned a dedicated task by the compiler. The name used in assembly code and the usage is indicated.

Table 14-1. Register Conventions
Register Number	Software Name	Use
$0	`zero`	Always 0 when read.
$1	`at`	Assembler temporary variable. Do not use the $at register from source code unless you fully understand the implications.
$2-$3	`v0-v1`	Return value from functions.
$4-$7	`a0-a3`	Used for passing arguments to functions.
$8-$15	`t0-t7`	Temporary registers used by compiler for expression evaluation. Values not saved across function calls.
$16-$23	`s0-s7`	Temporary registers whose values are saved across function calls.
$24-$25	`t8-t9`	Temporary registers used by compiler for expression evaluation. Values not saved across function calls.
$26-$27	`k0-k1`	Reserved for interrupt/trap handler.
$28	`gp`	Global Pointer.
$29	`sp`	Stack Pointer.
$30	`fp` or `s8`	Frame Pointer if needed. Additional temporary saved register if not.
$31	`ra`	Return address for functions.

The PIC32MZ family uses the microMIPS compressed instruction-set architecture. You can use the micromips function attribute to compile the function for the microMIPS compressed mode. This compressed ISA generally results in a ~30% reduction in overall application code size at the expense of ~2% in performance. The microcontroller can switch between the MIPS32 and microMIPS modes on a function call. Consult your device data sheet to determine if your target device supports the microMIPS ISA.

Example function:

#include <xc.h>
void
__attribute__((micromips))
peanut (void)
{
  // function code here
}

Important: Standard function calls can switch between MIPS32 and microMIPS modes. However, when calling a MIPS32 library function from a microMIPS function, the compiler may generate a compressed jals instruction to call the library function. A jals instruction cannot change modes to MIPS32 and upon linking, you may receive an error, “Unsupported jump between ISA modes; consider recompiling with interlinking enabled.” In that case, add the -mno-jals option to the Alternative Options field in your project properties for xc32-gcc, so it is passed to the compiler.