Defined partially in software, partially in hardware.
R0 - R3: input (color #0) R4 - R7: input (color #1) R8 - R15: general purpose R48: return address
Anything live during BLEND must respect blend shader registers.
R0 - R3: preloaded (message #0) R4 - R7: preloaded (message #1) R57 - R63: preloaded (various)
R0 - R15: general purpose (full threads) R48 - R63: general purpose (full threads)
R32 - R47: general purpose (half threads, or v6)