RISC-V Architecture - From Basics to Building a Simple OS

What is RISC-V (risk-five)?

RISC-V is an open-source instruction set architecture (ISA) based on principles of Reduced Instruction Set Computing. Unlike proprietary architectures like ARM or x86, RISC-V is freely available for anyone to use, modify, and implement without licensing fees.

• Supports 32-bit (RV32), 64-bit (RV64), and 128-bit (RV128) addressing
• Modular design with base ISA and optional extensions
• Growing ecosystem with hardware implementations and software tools

Key Differences from ARM Cortex

1. Licensing Model
   • RISC-V: Open-source and royalty-free
   • ARM: Proprietary, requires licensing fees
2. Instruction Set
   • RISC-V: Clean-slate design with minimal, orthogonal instruction set
   • ARM: Evolution-based design with historical compatibility requirements
3. Extensibility
   • RISC-V: Modular design allows custom extensions
   • ARM: Limited customization options

Building a Minimal RISC-V Operating System

Let’s create a simple OS that handles basic file operations and task scheduling. We’ll break this down into steps:

1. Boot Sequence

boot.S - Initial boot sequence
.section .text
.global start
start:
# Set up stack pointer
li sp, 0x80000000
# Jump to kernel main
call kernel_main

2. Basic Kernel Implementation

// kernel.c
#include <stdint.h>
void kernel_main() {
// Initialize hardware
init_uart();
init_interrupts();
// Initialize task scheduler
init_scheduler();
// Start system
while(1) {
schedule_next_task();
}
}

3. Simple Task Scheduler

// scheduler.c
#define MAX_TASKS 16
struct Task {
uint32_t sp;
uint32_t priority;
enum TaskState state;
};
static struct Task tasks[MAX_TASKS];
static int current_task = 0;
void init_scheduler() {
// Initialize task array
for(int i = 0; i < MAX_TASKS; i++) {
tasks[i].state = TASK_EMPTY;
}
}
void schedule_next_task() {
// Simple round-robin scheduling
current_task = (current_task + 1) % MAX_TASKS;
if (tasks[current_task].state == TASK_READY) {
context_switch(tasks[current_task].sp);
}
}

4. Basic File System

// filesystem.c
#define MAX_FILES 32
#define MAX_FILE_SIZE 4096
struct File {
char name[32];
uint8_t data[MAX_FILE_SIZE];
uint32_t size;
};
static struct File files[MAX_FILES];
int create_file(const char name) {
// Find empty slot
for(int i = 0; i < MAX_FILES; i++) {
if(files[i].size == 0) {
strncpy(files[i].name, name, 31);
files[i].size = 0;
return i;
}
}
return -1; // No space available
}

Hardware Implementation

RISC-V can be implemented on FPGAs or ASICs. Here’s a basic overview of the pipeline stages:

1. Fetch: Retrieve instruction from memory
2. Decode: Parse instruction fields
3. Execute: Perform ALU operations
4. Memory: Handle memory access
5. Writeback: Update registers

Getting Started

To run this minimal OS:

1. Install RISC-V toolchain:

sudo apt-get install riscv64-linux-gnu

1. Compile the code:

riscv64-linux-gnu-gcc -o kernel kernel.c

1. Compile the kernel:

riscv64-linux-gnu-gcc -march=rv64gc -mabi=lp64d -static -mcmodel=medany -fvisibility=hidden -nostdlib -nostartfiles -T linker.ld boot.S kernel.c scheduler.c filesystem.c -o kernel.elf

1. Run the kernel(QEMU):

qemu-system-riscv64 -M virt -nographic -bios none -kernel kernel.elf

This will boot the RISC-V OS and display the UART output.