Specifications

General-Purpose Registers

The Nios II architecture provides thirty-two 32-bit general-purpose registers, r0 through r31. Some

registers have names recognized by the assembler. For example, the zero register (r0) always returns the

value zero, and writing to zero has no effect. The ra register (r31) holds the return address used by

procedure calls and is implicitly accessed by the call, callr and ret instructions. C and C++ compilers

use a common procedure-call convention, assigning specific meaning to registers r1 through r23 and r26

through r28.

Table 3-5: The Nios II General-Purpose Registers

r0 zero 0x00000000 r16 Callee-saved register

r1 at Assembler temporary r17 Callee-saved register

r2 Return value r18 Callee-saved register

r3 Return value r19 Callee-saved register

r4 Register arguments r20 Callee-saved register

r5 Register arguments r21 Callee-saved register

r6 Register arguments r22 Callee-saved register

r7 Register arguments r23 Callee-saved register

r8 Caller-saved register r24 et Exception temporary

r9 Caller-saved register r25 bt Breakpoint temporary

r10 Caller-saved register r26 gp Global pointer

r11 Caller-saved register r27 sp Stack pointer

r12 Caller-saved register r28 fp Frame pointer

r13 Caller-saved register r29 ea Exception return address

r14 Caller-saved register r30 sstatus Status register

r15 Caller-saved register r31 ra Return address

Note: r25 is used exclusively by the JTAG debug module. It is used as the breakpoint temporary (bt)

Note: r30 is used as the breakpoint return address (ba) in the normal register set, and as the shadow

Status Register section.

For more information, refer to the Application Binary Interface chapter of the Nios II Processor Reference

Handbook.

Related Information

• Application Binary Interface on page 7-1

• Application Binary Interface

3-10

General-Purpose Registers

NII51003

2015.04.02

Altera Corporation

Programming Model

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