Reality Signal Processor/CPU Core

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Scalar unit (SU)

The scalar is the half of the RSP core that is similar to a standard MIPS R4000 32-bit CPU. It has 32 32-bit registers (conventionally called r0-r31) and implement most standard opcodes. This page does not describe the whole scalar unit as standard MIPS documentation suffices, but it highlights the main difference.

Missing opcodes

The following opcodes are not implemented by RSP:

  • Multiplication units. RSP does not have a multiplication unit so there is no MULT, MULTU, DIV, DIVU, MFHI, MFLO, MTHI, MTLO.
  • 64-bit instructions. RSP only has 32-bit scalar registers in SU, so there is no 64-bit opcodes (the ones starting with D such as DADDIU, DSRL, etc.) nor 64-bit memory accesses such as LD, SD, LDL, SDL.
  • No opcodes for misaligned memory accesses. All memory accesses to DMEM can be correctly performed also to misaligned addresses, using the standard opcodes like LW / SW or LH / LHU / SH, so there is no LWL, LWR, SWL, SWR.
  • No traps or exceptions. RSP does not implement any form of interrupt or exception handling, so there is no SYSCALL nor trap instructions (TGE, TLT, etc.). BREAK is available but it has a special behavior (see below).
  • No support for likely branches. The "likely" variant of all branches is not supported. The missing opcodes are the ones ending with L (such as BEQL, BLEZL, etc.)

Memory access

RSP is a harvard architecture. All opcodes are fetched from IMEM (4KB) and all data is access in DMEM (4KB).

The PC register is 12-bit. All higher address bits in branch / call instructions are thus ignored. When PC reaches the last opcode (at 0xFFC), execution continues to the first opcode in IMEM (PC wraps to 0x000).

All accesses to DMEM are performed using the lowest 12 bits of the address calculated by the load/store instruction (higher bits are ignored). Moreover, contrary to standard MIPS architecture, the RSP can correctly perform misaligned memory accesses (eg: it is possibly to fetch a 32-bit word at address 0x001, that will contain the 4 bytes at 0x1-0x5). Standard MIPS architecture allows to do misaligned addresses only using the LWL/LWR or SWL/SWR couples, which are not required on the RSP.