What instruction set does the Nvidia GeForce 6xx S

Question

Does the GeForce 6xx Series GPUS use RISC, CISC or VLIW style instructions?

In one source, at http://www.motherboardpoint.com/risc-cisc-t241234.html someone said
"GPUs are probably closer to VLIW than to RISC or CISC".

In another source, at http://en.wikipedia.org/wiki/Very_long_instruction_word#implementations it says "both Nvidia and AMD have since moved to RISC architectures in order to improve performance on non-graphics workload"

Answer 1

AFAIK, Nvidia does not publicly document it's hardware instruction sets.

The best you can see officially is PTX ISA which is the instruction set of a virtual machine which Nvidia's compiler (or drivers) then convert to the real instruction set to be executed on specific GPU. cuobjdump utility can show you disassembled GPU code. IMHO it looks like a fairly typical RISC -- load+store+operations on registers.

On the other hand, some operations are very complex. For instance, texture lookup instruction does a lot -- it may interpolate coordinates, deal with coordinates being out of range, fetch required data and convert it to desired data type. While the syntax remains RISC-y, the substance feels like CISC.

Answer 2

They use a combination of VLIW and SIMID, this allows them to complete several processes at once rather than doing them one by one like RISC or CISC.