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Chapter 1: Introduction

This is a draft of the RISC-V microcontroller architecture description document. Feedback welcome.

Mission Statement

Define a modern C/C++ friendly microcontroller architecture based on the RISC-V instruction set, that makes writing embedded software easier and more productive. And... enjoy the process!

In technical terms, the mission statement can be rephrased as: define a set of specifications for RISC-V microcontrollers intended for embedded real-time / low power / IoT applications that do not require an operating system. Favour C/C++ multi-threaded RTOS systems.

A secondary goal is to improve the RISC-V microcontroller profile to the point where it can be adopted by the RISC-V foundation as a alternate standard for microcontroller devices.

Limitations

These specifications intentionally do not include application class devices which use virtual memory and/or have supervisor/hypervisor modes which are intended to run operating systems kernels. For this class of devices, see the "RISC-V Privileged Architecture" specifications.

Sub-profiles

Since there are many microcontroller configurations, 3 classes were identified:

  • ES (embedded small) ES-RV32E if possible, otherwise ES-RV32I[M][C]: low end, single hart, 32-bits, no floating point, no unprivileged mode (intended to support legacy PIC & AVR class applications; comparable with Cortex-M0)
  • EM (embedded medium) EM-RV32IM[F[D]]C/EM-RV64IM[F[D]]C: regular, single hart, 32/64-bits, possibly with floating point (intended to support common multi-threaded applications; comparable with Cortex-M3/M4)
  • EL (embedded large) EL-RV32IMA[F[D]]C/EL-RV64IMA[F[D]]C: high end, multi-hart, 32/64-bits, atomics, possibly with floating point (intended to support hard real-time, high performance applications)

Benefits

One of the mantras used during the RISC-V design was "if it can be done in software, it should not be done in hardware."

The microcontroller profile reconsidered the implementation of some core features (like stack handling), and pushed then back to hardware, where they belong.

Some of the benefits are:

  • best interrupt latency, more appropriate for real-time applications
  • improved robustness for multi-threaded applications
  • much easier to use directly in C/C++

Definitions

Hart

Hart is a contraction of hardware thread and represents a hardware resource.

Technically, a hart is a resource abstraction representing an independently advancing RISC-V execution context within a RISC-V execution environment.

A RISC-V execution context contains a full set of RISC-V architectural registers.

A hart executes its program independently from other harts in a RISC-V system. "Execute independently" means that each hart will eventually fetch and execute its next instruction in program order regardless of the activity of other harts (at least at user level).

RISC-V microcontroller specifics

Harts are identified by a Hart ID, a small unsigned integer. Hart IDs are unique. The rule used to assign hart IDs is implementation specific, but it is recommended to keep it simple, preferably within a continuous small range. There should always be a hart with ID=0, which will have slightly more duties, for example to process the NMIs.

To help applications auto-configure themselves, the largest hart ID is stored in a register in the Device Control Bloc (dcb.hartidmax).

Core

A RISC-V device can contain one or more RISC-V-compatible processing cores together with other non-RISC-V-compatible cores.

A core is usually considered a purely physical thing.

A core implements one or more harts, where if there are multiple harts, they are time-multiplexing some common hardware components (e.g., instruction fetch, physical registers, ALUs, predictor state, etc.)

CSRs

Control and Status Registers (CSRs) are used for hart-specific state only. CSRs are not memory mapped - they are accessed by CSR instructions.