Writing A Simple ARM OS - Part 4

Introduction

In Part 3, we explored ARM calling conventions, debugging, and cleaned up our UART driver. While assembly has given us fine control over the hardware, writing an entire OS in assembly would be painful.

It’s time to enter C land.

This post covers:

• Modifying the bootloader to transition from assembly to C
• Updating the UART driver to be callable from C
• Writing our first C function (kmain())
• Adjusting the Makefile and linker script for C support

Booting into C

We still need a bit of assembly to set up the stack and call kmain(). Let’s start by modifying our bootloader.

Updated bootloader.s

.section .text
.global _start

_start:
    LDR sp, =stack_top  @ Set up the stack
    BL kmain            @ Call the C kernel entry function
    B .                 @ Hang forever

.section .bss
.align 4
stack_top:
.space 1024

What’s changed?

• We load the stack pointer (sp) before calling kmain(). This ensures C has a valid stack to work with.
• We branch-and-link (BL) to kmain(), following ARM’s calling conventions.
• The infinite loop (B .) prevents execution from continuing into unknown memory if kmain() ever returns.

With this setup, execution will jump to kmain()—which we’ll define next.

Our First C Function: kmain()

Now that we can transition from assembly to C, let’s create our first function.

kmain.c

#include "uart.h"

void kmain() {
    uart_puts("Hello from C!\n");
    while (1);
}

What’s happening?

• We include our uart.h header so we can call uart_puts().
• kmain() prints "Hello from C!" using our UART driver.
• The infinite while(1); loop prevents execution from continuing into unknown territory.

At this point, our OS will boot from assembly, call kmain(), and print text using our UART driver—but we need to make a few more changes before this compiles.

Making the UART Driver Callable from C

Right now, uart_puts and uart_putc are assembly functions. To call them from C, we need to:

1. Ensure they follow the ARM calling convention.
2. Declare them properly in a header file.

uart.h (Header File)

#ifndef UART_H
#define UART_H

void uart_putc(char c);
void uart_puts(const char *str);

#endif

Updated uart.s

.section .text
.global uart_putc
.global uart_puts

uart_putc:
    PUSH {lr}
    ldr r1, =0x101f1000  @ UART0 Data Register
    STRB r0, [r1]        @ Store byte
    POP {lr}
    BX lr

uart_puts:
    PUSH {lr}

next_char:
    LDRB r1, [r0], #1    @ Load byte from string
    CMP r1, #0
    BEQ done

wait_uart:
    LDR r2, =0x101f1018  @ UART0 Flag Register
    LDR r3, [r2]
    TST r3, #0x20        @ Check if TX FIFO is full
    BNE wait_uart

    LDR r2, =0x101f1000  @ UART0 Data Register
    STR r1, [r2]         @ Write character
    B next_char

done:
    POP {lr}
    BX lr

How this works:

• Function names are declared .global so they are visible to the linker.
• uart_putc(char c)
  • Expects a character in r0 (following ARM’s C calling convention).
  • Writes r0 to the UART data register.
• uart_puts(const char *str)
  • Expects a pointer in r0.
  • Iterates through the string, sending each character until it reaches the null terminator (\0).
• Preserving Registers
  • PUSH {lr} ensures lr is restored before returning.

Updating the Build System

To compile both assembly and C, we need to adjust the Makefile and linker script.

Updated Makefile

# Makefile for the armos project

# Cross-compiler tools (assuming arm-none-eabi toolchain)
AS = arm-none-eabi-as
CC = arm-none-eabi-gcc
LD = arm-none-eabi-ld
OBJCOPY = arm-none-eabi-objcopy

# Compiler and assembler flags
CFLAGS = -ffreestanding -nostdlib -O2 -Wall -Wextra
ASFLAGS =
LDFLAGS = -T linker.ld -nostdlib

# Files and directories
BUILD_DIR = build
TARGET = armos.elf
BIN_TARGET = armos.bin

# Source files
ASM_SRCS = asm/bootloader.s asm/uart.s
C_SRCS = src/kmain.c
OBJS = $(BUILD_DIR)/bootloader.o $(BUILD_DIR)/uart.o $(BUILD_DIR)/kmain.o

# Build rules
all: $(BUILD_DIR)/$(BIN_TARGET)

$(BUILD_DIR):
	@mkdir -p $(BUILD_DIR)

# Assemble the bootloader and UART
$(BUILD_DIR)/bootloader.o: asm/bootloader.s | $(BUILD_DIR)
	$(AS) $(ASFLAGS) -o $@ $<

$(BUILD_DIR)/uart.o: asm/uart.s | $(BUILD_DIR)
	$(AS) $(ASFLAGS) -o $@ $<

# Compile the C source file
$(BUILD_DIR)/kmain.o: src/kmain.c | $(BUILD_DIR)
	$(CC) $(CFLAGS) -c -o $@ $<

# Link everything together
$(BUILD_DIR)/$(TARGET): $(OBJS)
	$(LD) $(LDFLAGS) -o $@ $(OBJS)

# Convert ELF to binary
$(BUILD_DIR)/$(BIN_TARGET): $(BUILD_DIR)/$(TARGET)
	$(OBJCOPY) -O binary $< $@

# Clean build artifacts
clean:
	rm -rf $(BUILD_DIR)

Updating the Linker Script

The linker script ensures that kmain() and our code are properly loaded in memory.

Updated linker.ld

ENTRY(_start)

SECTIONS
{
    . = 0x10000; /* Load address of the kernel */

    .text : {
        *(.text*)
    }

    .rodata : {
        *(.rodata*)
    }

    .data : {
        *(.data*)
    }

    .bss : {
        *(COMMON)
        *(.bss*)
    }
}

Key Changes:

• Code starts at 0x10000, ensuring it is loaded correctly.
• .text, .rodata, .data, and .bss sections are properly defined.

Build

Now that all of these changes in place, we can make our kernel and run it. If everything has gone to plan, you should see our kernel telling us that it’s jumped to C.

Hello from C!

Conclusion

We’ve successfully transitioned from pure assembly to using C for higher-level logic, while keeping low-level hardware interaction in assembly.

The code for this article is available in the GitHub repo.