Scalar Unit Registers; Su Register Format; Register 0; Register 31 - Nintendo Ultra64 Programmer's Manual

RSP Architecture

Scalar Unit Registers

32
The RSP Scalar Unit has 32 general-purpose registers, each 32 bits wide.

SU Register Format

The RSP has big-endian byte ordering.
Figure 2-2
byte 0
31

Register 0

Register 0 ($0) is a special register. It always contains a zero, and cannot be
modified. Attempting to modify $0 is a null operation.
Since DMEM addresses are only 12-bits, it can be convenient to use $0 as the
base register for loads/stores (the entire DMEM address will fit in the 16-bit
offset field).

Register 31

Register 31 ($31) is a special register. The jal and jalr instructions store
their return address in this register.
If these instructions are avoided, this register can be treated as any other SU
register.
SU Register Format
byte 1 byte 2 byte 3
0