RISC-V Overview

RISC-V is an open Instruction Set Architecture (ISA) based on the principles of Reduced Instruction Set Computing (RISC)

It defines how software communicates with the processor through:

• machine instructions
• registers
• memory

ÆRIS implements the RV32I set, which is the base integer set of the RISC-V architecture and represents the most common starting point for learning the architecture

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RV32I

The simulator implements RV32I, the base integer set of the RISC-V architecture

Property	Value
Word size	32 bits
Register width	32 bits
Number of registers	32
Instruction size	32 bits
Instruction set	Base integer

The RV32I set includes instructions for:

• arithmetic operations
• memory access
• conditional branches
• jumps
• system calls

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Registers

RISC-V defines 32 general-purpose registers:

x0 - x31

Each register stores a 32-bit value

ABI Convention

In addition to the numeric name, many registers have ABI names, which indicate their conventional use

Register	ABI Name	Typical Use
x0	zero	constant zero
x1	ra	return address
x2	sp	stack pointer
x3	gp	global pointer
x4	tp	thread pointer
x5–x7	t0–t2	temporaries
x8–x9	s0–s1	saved registers
x10–x17	a0–a7	function arguments
x18–x27	s2–s11	saved registers
x28–x31	t3–t6	temporaries

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Instruction Formats

All RV32I instructions are 32 bits, but the internal organization of these bits depends on the instruction format

The main formats are:

Format	Purpose
R-type	operations between registers
I-type	operations with immediate and loads
S-type	stores
B-type	conditional branches
U-type	upper immediate
J-type	jumps

Each format positions the instruction fields in different locations within the 32-bit word

Common fields include:

• opcode
• source registers
• destination register
• immediate

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Program Counter

The Program Counter (PC) stores the address of the current instruction

Normally:
PC = PC + 4

This happens because each instruction occupies 4 bytes.

Branch and jump instructions modify the value of the PC to change the program execution flow

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System Calls

The ecall instruction allows the program to request services from the execution environment

In ÆRIS, syscalls are used to interact with the console

Example:

li a7, 1
li a0, 42
ecall

This code prints the integer stored in a0