Nintendo Ultra64 Programmer's Manual page 88

RSP Coprocessor 0
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$c5
This register maps to bit 3 of the RSP status register, DMA_FULL. It is read
only.
On power-up, this register is 0x0.
$c6
This register maps to bit 2 of the RSP status register, DMA_BUSY. It is read
only.
On power-up, this register is 0x0.
$c7
This register is a hardware semaphore for synchronization with the CPU,
typically used to share the DMA activity. If this register is 0, the semaphore
may be acquired. This register is set on read, so the test and set is atomic.
Writing 0 to this register releases the semaphore.
GetSema:
mfc0
bne
nop
# do critical work
ReleaseSema:
mtc0
On power-up, this register is 0x0.
$c8
This register holds the RDP command buffer START address. Depending on
the state of the RDP STATUS register, this address is interpreted by the RDP
$1, $c7
$1, $0, GetSema
$0, $7