RISC-V

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RISC-V is not a single ISA, rather a meta-ISA. It defines basics and boundaries for a family of implementations. An implementation consists of one of the Base ISAs and zero or more Extensions.

Architecture

The RISC-V ISA has fixed-length 32-bit instructions aligned on their natural boundaries, but is designed to encode variable-length instructions.
The base ISA operates on a little-endian memory system, but non-standard extensions may add support for big-endian or bi-endian.

Hardware Threads

The RISC-V ISA specifies hardware threads, also known as harts, that allow the processor to execute multiple independent streams of instructions. It is up to the processor to decide how to schedule these threads and among which of it's cores. Each hart has an ID associated with it

Exceptions, Traps and Interrupts

In RISC-V the term Exception refers to an unusual condition at run-time associated with an instruction in the current hardware thread.
A Trap is a synchronous transfer of control to a trap handler and is caused by an exceptional condition within a RISC-V thread. The trap handlers usually execute in a more privileged environment.
An external event that occurs asynchronously to the current thread will cause an Interrupt. When an interrupt occurs, some instruction is selected to experience a trap.

Base ISA

The base ISA specifies RV32I and RV64I, 32 and 64-bit respectively, most of what is said about RV32I also applies to RV64I. Additionally, there is also RV32E, a reduced version of RV32I for embedded systems and RV128I, a 128-bit version, which is mostly a placeholder so far.

RV32I

RV32I offers 31 general-purpose registers (x1-x31) which hold integer values, the x0 register is hardwired to zero, all registers are 32 bits wide. It specifies a number of logical and arithmetic operations (and, or, xor, shift left and right, addition and subtraction), all of which are available with a source register or an immediate.

RV32E

RV32E reduces the number of general-purpose registers to 15 (x1-x15), and x0 is still hardwired to constant zero.

RV64I

RV64I is very similar to its 32-bit counterpart, but offers 64-bit wide registers and can read the CSRs in one operation instead of requiring the programmer to read the upper and lower half separately. Additionaly, there are some instructions to work with words (32 bit) instead of double-words.

Extensions

An extensions can be one of the officialy defined ones or a vendor-specific one (there is opcode space explicitely reserved for this) and can define additional elements, including opcodes and registers.

The following Standard Extensions are noteworthy here:

RVM (Multiply-Divide Instruction Extension)

The RVM Extension adds instructions for multiplying, dividing and computing the remainder of a division.

RVA (Atomic Instruction Extension)

The RVA Extension adds instruction to work atomically with memory, including reserved load and conditional store.

RVF & RVD & RVQ (Floating Point Extensions)

Adds additional instructions to work with floating points and also the floating point registers. The different extensions differ in the length|precision of the floating point numbers.

See Also

• HiFive-1 Bare Bones

Wikipedia

• RISC-V

External Links

• riscv.org
• RISC-V Specifications
• RISC-V ISA Simulator "Spike"