Interfaces in QEMU: UART and MMIO Concepts

TL;DR

• We use QEMU virt UART for all output.
• UART is a simple MMIO device and ideal for bare-metal learning.
• JTAG, SPI, and I2C are introduced conceptually, but we do not require real hardware.

1. UART on QEMU virt

The virt machine exposes a UART at a fixed MMIO address:

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UART0 base: 0x10000000

Writing a byte to that address prints a character.

Minimal UART write

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// src/uart.c (see full file in src/)
#define UART0_BASE 0x10000000u
static volatile u8 *const uart0 = (volatile u8 *)UART0_BASE;

void uart_putc(char c) {
  *uart0 = (u8)c;
}

2. A runnable UART example

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// src/hello_uart.c
#include "types.h"
#include "uart.h"

int main(void) {
  uart_puts("UART is alive!\n");
  return 0;
}

Build and run:

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riscv64-unknown-elf-gcc -O0 -g -ffreestanding -nostdlib \
  -march=rv32im -mabi=ilp32 -T src/link.ld \
  src/start.s src/uart.c src/hello_uart.c -o build/hello_uart.elf

qemu-system-riscv32 -M virt -nographic -bios none -kernel build/hello_uart.elf

3. Conceptual overview: JTAG, SPI, I2C

These are common hardware interfaces, but in this book we only describe them:

• JTAG: debug and boundary scan; typically used by hardware debuggers.
• SPI: fast serial bus for flash and sensors.
• I2C: simple two-wire bus for low-speed peripherals.

Because we run everything in QEMU, we do not require physical wiring, adapters, or boards.

Exercises

1. Change the UART string and verify the output.
2. Add a function that prints a hex value with uart_puthex32.
3. Search the QEMU virt memory map and list one more MMIO device.

Summary

• UART output via MMIO is the simplest reliable output path.
• QEMU virt provides a stable, predictable UART at 0x10000000.
• Physical interfaces are introduced conceptually but not required here.