== Introduction

Hazard3 is a configurable 3-stage RISC-V processor, implementing:

* `RV32I`: 32-bit base instruction set
* `M`: integer multiply/divide/modulo
* `A`: atomic memory operations, with AHB5 global exclusives
* `C`: compressed instructions
* `Zicsr`: CSR access
* `Zba`: address generation
* `Zbb`: basic bit manipulation
* `Zbc`: carry-less multiplication
* `Zbs`: single-bit manipulation
* `Zbkb`: basic bit manipulation for scalar cryptography
* `Zcb`: basic additional compressed instructions
* `Zcmp`: push/pop and double-move compressed instructions
* Debug, Machine and User privilege/execution modes
* Privileged instructions `ECALL`, `EBREAK`, `MRET` and `WFI`
* External debug support
* Instruction address trigger unit (hardware breakpoints)

=== Architectural Overview

==== Pipe Stages

The three stages are:

* `F`: Fetch
** Contains the data phase for instruction fetch
** Contains the instruction prefetch buffer
** Predecodes register numbers `rs1`/`rs2`, for faster register file read and register bypass
* `X`: Execute
** Decode and execute instructions
** Drive the address phase for load/store/AMO
** Generate jump/branch addresses
* `M`: Memory
** Contains the data phase for load/store/AMO
** Register writeback is at the end of stage `M`
** Generate exception addresses

The instruction fetch address phase is best thought of as residing in stage `X`. The 2-cycle feedback loop between jump/branch decode into address issue in stage `X`, and the fetch data phase in stage `F`, is what defines Hazard3's jump/branch performance.

==== Bus Interfaces

Hazard3 implements either one or two AHB5 bus master ports. The single-port configuration is used when ease of integration is a priority, since it supports simpler bus topologies. The dual-port configuration adds a dedicated port for instruction fetch, which improves both the maximum frequency and the clock-for-clock performance.

Hazard3 uses AHB5 specifically, rather than older versions of the AHB standard, because of its support for global exclusives. This is a bus feature that allows a processor to perform an ordered read-modify-write sequence with a guarantee that no other processor has written to the same address range in between. Hazard3 uses this to implement multiprocessor support for the A (atomics) extension.

==== Multiply/Divide

For minimal M-extension support, Hazard3 instantiates a sequential multiply/divide circuit (restoring divide, naive repeated-addition multiply). Instructions stall in stage `X` until the multiply/divide completes. Optionally, the circuit can be unrolled by a small factor to produce multiple bits ber clock -- 2 or 4 is achievable in practice.

A single-cycle multiplier can be instantiated, retiring either to stage 3 or stage 2 (configurable). By default only 32-bit `mul` is supported, which is by far the most common of the four multiply instructions.

=== List of RISC-V Specifications

These are links to the ratified versions of the base instruction set and extensions implemented by Hazard3.

[%autowidth.stretch, options="header"]
|===
| Extension         | Specification
| `RV32I` v2.1      | https://github.com/riscv/riscv-isa-manual/releases/download/Ratified-IMAFDQC/riscv-spec-20191213.pdf[Unprivileged ISA 20191213]
| `M` v2.0          | https://github.com/riscv/riscv-isa-manual/releases/download/Ratified-IMAFDQC/riscv-spec-20191213.pdf[Unprivileged ISA 20191213]
| `A` v2.1          | https://github.com/riscv/riscv-isa-manual/releases/download/Ratified-IMAFDQC/riscv-spec-20191213.pdf[Unprivileged ISA 20191213]
| `C` v2.0          | https://github.com/riscv/riscv-isa-manual/releases/download/Ratified-IMAFDQC/riscv-spec-20191213.pdf[Unprivileged ISA 20191213]
| `Zicsr` v2.0      | https://github.com/riscv/riscv-isa-manual/releases/download/Ratified-IMAFDQC/riscv-spec-20191213.pdf[Unprivileged ISA 20191213]
| `Zifencei` v2.0   | https://github.com/riscv/riscv-isa-manual/releases/download/Ratified-IMAFDQC/riscv-spec-20191213.pdf[Unprivileged ISA 20191213]
| `Zba` v1.0.0      | https://github.com/riscv/riscv-bitmanip/releases/download/1.0.0/bitmanip-1.0.0-38-g865e7a7.pdf[Bit Manipulation ISA extensions 20210628]
| `Zbb` v1.0.0      | https://github.com/riscv/riscv-bitmanip/releases/download/1.0.0/bitmanip-1.0.0-38-g865e7a7.pdf[Bit Manipulation ISA extensions 20210628]
| `Zbc` v1.0.0      | https://github.com/riscv/riscv-bitmanip/releases/download/1.0.0/bitmanip-1.0.0-38-g865e7a7.pdf[Bit Manipulation ISA extensions 20210628]
| `Zbs` v1.0.0      | https://github.com/riscv/riscv-bitmanip/releases/download/1.0.0/bitmanip-1.0.0-38-g865e7a7.pdf[Bit Manipulation ISA extensions 20210628]
| `Zbkb` v1.0.1     | https://github.com/riscv/riscv-crypto/releases/download/v1.0.1-scalar/riscv-crypto-spec-scalar-v1.0.1.pdf[Scalar Cryptography ISA extensions 20220218]
| `Zcb` v1.0.3-1    | https://github.com/riscv/riscv-code-size-reduction/releases/download/v1.0.3-1/Zc-v1.0.3-1.pdf[Code Size Reduction extensions frozen v1.0.3-1]
| `Zcmp` v1.0.3-1   | https://github.com/riscv/riscv-code-size-reduction/releases/download/v1.0.3-1/Zc-v1.0.3-1.pdf[Code Size Reduction extensions frozen v1.0.3-1]
| Machine ISA v1.12 | https://github.com/riscv/riscv-isa-manual/releases/download/Priv-v1.12/riscv-privileged-20211203.pdf[Privileged Architecture 20211203]
| Debug v0.13.2     | https://riscv.org/wp-content/uploads/2019/03/riscv-debug-release.pdf[RISC-V External Debug Support 20190322]
|===