Microchip Technology SW006023-1N Data Sheet

®

 2012 Microchip Technology Inc.

DS51686E-page 121

This chapter examines registers used by the compiler to generate assembly from 
C/C++ source code.

• Register Usage

• Register Conventions

The assembly generated from C/C++ source code by the compiler will use certain reg-
isters that are present on the PIC MCU device. The compiler assumes that nothing 
other than code it generates can alter the contents of these registers, but an extended 
assembly language format can be used to indicate to the compiler registers used in 
assembly code so that code can be adjusted accordingly.

The 32 general purpose registers contained in the PIC32 are shown in Table 9-1. Some 
of these registers are assigned a dedicated task by the compiler. The name used in 
assembly code and the usage is indicated.

$0

zero

Always 0 when read.

$1

at

Assembler temporary variable.

$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.