/mrisc32

MRSIC32 ISA documentation and development

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This repo has moved to: https://gitlab.com/mrisc32/mrisc32

MRISC32

This is an open and free 32-bit RISC/Vector instruction set architecture (ISA), primarily inspired by the Cray-1 and MIPS architectures. The focus is to create a clean, modern ISA that is equally attractive to software, hardware and compiler developers.

This repository contains LaTeX documentation and databases of architectural information (e.g. instructions and system registers).

Documentation

The latest MRISC32 Instruction Set Manual (PDF) describes the MRISC32 ISA in detail.

Overview documents:

Features

  • Unified scalar/vector/integer/floating-point ISA.
  • There are two register files:
    • R0-R31: 32 scalar registers, each 32 bits wide.
      • Three registers have special meaning in hardware: Z, LR, VL.
      • 29 registers are general purpose (of which three are reserved by the ABI: SP, FP, TP).
      • All registers can be used for all types (integers, addresses and floating-point).
    • V0-V31: 32 vector registers, each with at least 16 32-bit elements.
      • All registers can be used for all types (integers, addresses and floating-point).
  • All instructions are 32 bits wide and easy to decode.
  • Most instructions are non-destructive 3-operand (two sources, one destination).
  • All conditionals are based on register content.
    • There are no condition code flags (carry, overflow, ...).
    • Compare instructions generate bit masks.
    • Branch instructions can act on bit masks (all bits set, all bits zero, etc) as well as signed quantities (less than zero, etc).
    • Bit masks are suitable for masking in conditional operations (for scalars, vectors and packed data types).
  • Powerful addressing modes:
    • Scaled indexed load/store (x1, x2, x4, x8).
    • Gather-scatter and stride-based vector load/store.
    • PC-releative and absolute load/store:
      • ±4 MiB range with one instruction.
      • Full 32-bit range with two instructions.
    • PC-relative and absolute branch:
      • ±4 MiB range with one instruction.
      • Full 32-bit range with two instructions.
  • Many traditional floating-point operations can be handled in whole or partially by integer operations, reducing the number of necessary instructions:
    • Load/store.
    • Branch.
    • Sign and bit manipulation (e.g. neg, abs).
  • Vector operations use a Cray-like model:
    • Vector operations are variable length (1-N elements).
    • Most integer and floating-point instructions come in both scalar and vector variants.
    • Vector instructions can use both vector and scalar operands (including immediate values), which removes the overhead for transfering scalar data into vector registers.
  • In addition to vector operations, there are also packed operations that operate on small data types (byte and half-word).
  • Fixed point operations are supported:
    • Single instruction multiplication of Q31, Q15 and Q7 fixed point numbers.
    • Single instruction conversion between floating-point and fixed point.
    • Saturating and halving addition and subtraction.

Note: There is no support for 64-bit floating-point operations (that is left for a 64-bit version of the ISA).