Accumulator; Vu Control Registers; Vector Compare Code Register (Vcc); Figure 2-4 Vu Accumulator Format - Nintendo Ultra64 Programmer's Manual

RSP Architecture
high
byte 0
47
36

Accumulator

Each vector slice has a 48-bit accumulator associated with it. Each 16-bit
element of a vector register maps to a vector slice, and therefore to a different
48-bit accumulator.
VU Accumulator Format
Figure 2-4
byte 1 byte 2
The accumulator is modified by most VU computational instructions, but it
is used most heavily by the multiply-accumulate instructions. For these
instructions, 16-bits of the accumulator is written out after accumulation.
"Which" 16-bits to be written is usually an accumulator element. Consult
"VU Multiply Instructions" in Chapter 3 for more information.
One VU instruction, vsar, can directly reference the accumulator directly.

VU Control Registers

Vector Compare Code Register (VCC)

This 16-bit register contains 2 bits per 16-bit slice of the VU and is used by
the select instructions.
middle
byte 3
low
byte 4 byte 5
0