How close is RISC-V to RISC-I?

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As part of the celebration of the 50th anniversary of IEEE Computer magazine (see below), I’ve been asked to write a retrospective on “A VLSI RISC,” that was published originally in 1982. (The retrospective appeared December 2017.)

It’s an interesting experience to (re)learn from a paper that you wrote 35 years ago. For example, which computer first had a register hardwired to zero? (The answer: the 1951 Ferranti-Manchester MADM, which was the first machine with index registers, also used a register to supply zero.)

By far the biggest surprise was how close is the original instruction set of the RISC-I to the base instruction set of RISC-V (RV32I). The figure below compares the two instruction sets.  In fact, RISC-I is may be the closest instruction set to RISC-V of any era; it is certainly much closer than the original Stanford MIPS and IBM 801 instruction sets.

While architects talk about the differences (register windows, condition codes, delayed branch),  it’s amazing that there aren’t more after 30 years of innovation in computer architecture fueled by Moore’s Law and Dennard Scaling (1981 to 2011). Common features of RISC-I and RV32I:

  • A 32-bit byte-addressable address space
  • All instructions are 32-bit long
  • 31 registers, with register 0 hardwired to zero, all 32 bits wide
  • All operations are register-to-register (none are register-to-memory)
  • The same arithmetic, logical, and shift operations
  • The same load word and store word instructions
  • Signed and unsigned versions of load and store byte and halfword (called “short” in RISC-I)
  • Immediate option for all arithmetic, logical, and shift instructions
  • Immediates are always sign-extended
  • One data addressing mode (register + immediate)
  • PC-relative branch addressing
  • No multiply or divide instructions
  • An instruction to load a wide immediate into the upper part of register so that a 32-bit constant takes only two instructions

Below is the complete ISA for both architectures, aligned by operation.

Equivalent RISC-V and RISC-I instructions. For RISC-I, “S2” can be either a 13-bit sign-extended immediate or a second source register.

From the Archives: Computer’s Legacy