added a build.sh and code to kernel.c

build.sh

@@ -0,0 +1,44 @@
+#!/bin/bash
+
+# Exit immediately if a command exits with a non-zero status.
+set -e
+
+# Function to check if a command exists
+command_exists() {
+    command -v "$1" >/dev/null 2>&1
+}
+
+# Check for required commands
+for cmd in nasm gcc qemu-system-i386; do
+    if ! command_exists "$cmd"; then
+        echo "Error: $cmd is not installed or not in PATH" >&2
+        exit 1
+    fi
+done
+
+# Compile the assembly file
+echo "Compiling boot4.asm..."
+nasm -f elf32 boot4.asm -o boot4.o
+
+# Compile and link the kernel
+echo "Compiling and linking kernel..."
+gcc -m32 -ffreestanding -nostdlib kernel.c boot4.o -o kernel.bin -T linker.ld
+
+# Check if compilation was successful
+if [ -f kernel.bin ]; then
+    echo "Build successful. kernel.bin created."
+
+    # Ask user if they want to run QEMU
+    read -p "Do you want to run QEMU now? (y/n) " -n 1 -r
+    echo    # Move to a new line
+    if [[ $REPLY =~ ^[Yy]$ ]]; then
+        echo "Running QEMU..."
+        qemu-system-i386 -enable-kvm -net none -fda kernel.bin
+    else
+        echo "QEMU not started. You can run it later with:"
+        echo "qemu-system-i386 -enable-kvm -net none -fda kernel.bin"
+    fi
+else
+    echo "Build failed. Check for errors above." >&2
+    exit 1
+fi

kernel.c

@@ -0,0 +1,13 @@
+#include <stdint.h>
+
+void kmain(void)
+{
+    const uint16_t color = 0x0F00;
+    const char *hello = "Hello C world!";
+    volatile uint16_t *vga = (volatile uint16_t *)0xb8000;
+
+    for (int i = 0; i < 16; ++i)
+    {
+        vga[i + 80] = color | (uint16_t)hello[i];
+    }
+}