RISC-V Basics

Complete RV32I ISA

Here is the complete RV32I ISA. We gonna implement most part of it except the gray part.

Complete RV32I ISA

We are not going to handle exceptions.

RISC-V Instruction Formats

The RV32I ISA can be divided into six intruction formats(R/I/S/B/U/J), as shown in the figure below.

RISC-V Instruction Formats

"State" Required by RV32I ISA

Processors are basically FSMs. Each instruction reads and updates this state during execution:

• Registers (x0 .. x31)

• Program Counter (PC)

• Memory (MEM)

x0 is hardwired to 0 (i.e., always 0). Writes to Reg[0] are ignored.

RISC-V state elements

Microarchitecture Organization

RISC-V state elements

Microarchitecture

There are multiple implementations for a single instruction set architecture:

• Single-cycle

  • Each instruction executes in a single clock cycle.

• Multicycle

  • Each instruction is broken up into a series of shorter steps with one step per clock cycle.

• Pipelined (variant on "multicycle")

  • Each instruction is broken up into a series of steps with one step per clock cycle.

  • Multiple instructions execute at once by overlapping in time.

• Superscalar

  • Multiple functional units to execute at once by overlapping in time.

• Out of order

  • Instructions are reordered by the hardware.

We will implement a single-cycle processor first, and then a pipelined one.

References

• The RISC-V related lectures of EECS151/251A
• The RV32I specification.