Setup simple kernel with Hello World output via debugcon (AI)

CMakeLists.txt

@@ -4,9 +4,9 @@ project(ClaudeOS C ASM)
 set(CMAKE_C_STANDARD 99)
 
 # We are building a kernel, so we don't want standard libraries
-set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -ffreestanding -m32 -g -O2 -Wall -Wextra")
-set(CMAKE_ASM_FLAGS "${CMAKE_ASM_FLAGS} -m32")
-set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -m32 -nostdlib")
+set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -ffreestanding -m32 -fno-pie -fno-pic -fno-builtin -fno-stack-protector -g -O2 -Wall -Wextra")
+set(CMAKE_ASM_FLAGS "${CMAKE_ASM_FLAGS} -m32 -fno-pie -fno-pic")
+set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -m32 -nostdlib -no-pie")
 
 # Define build output directory
 set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)

README.md

@@ -39,7 +39,7 @@ Once a task is completed, it should be checked off.
 
 - [x] Create directory structure
 - [x] Create initial build system
-- [ ] Setup a simple kernel that writes `Hello, world` to Qemu debug port
+- [x] Setup a simple kernel that writes `Hello, world` to Qemu debug port
 - [ ] Update the build system to create both ISO and Floppy images. Verify these work using a test script. The standard CMake build target should automatically generate both images.
 - [ ] Update the kernel to correctly setup the GDT
 - [ ] Create a physical memory allocator and mapper. The kernel should live in the upper last gigabyte of virtual memory. It should support different zones (e.g.: `SUB_16M`, `DEFAULT`, ...) These zones describe the region of memory that memory should be allocated in. If it is not possible to allocate in that region (because it is full, or has 0 capacity to begin with), it should fallback to another zone.

output.txt

@@ -0,0 +1 @@
+Hello, world

src/CMakeLists.txt

@@ -1,6 +1,7 @@
 cmake_minimum_required(VERSION 3.16)
 
 add_executable(kernel 
+    boot.S
     kernel.c
 )
 

src/boot.S

@@ -0,0 +1,65 @@
+#define ASM_FILE
+#include <multiboot2.h>
+
+/* Multiboot 1 header constants */
+.set ALIGN,    1<<0             /* align loaded modules on page boundaries */
+.set MEMINFO,  1<<1             /* provide memory map */
+.set FLAGS,    ALIGN | MEMINFO  /* this is the Multiboot 'flag' field */
+.set MAGIC,    0x1BADB002       /* 'magic number' lets bootloader find the header */
+.set CHECKSUM, -(MAGIC + FLAGS) /* checksum of above, to prove we are multiboot */
+
+.section .multiboot
+.align 4
+multiboot1_header:
+    .long MAGIC
+    .long FLAGS
+    .long CHECKSUM
+
+.align 8
+multiboot_header:
+    /* magic */
+    .long MULTIBOOT2_HEADER_MAGIC
+    /* architecture: 0 (protected mode i386) */
+    .long MULTIBOOT_ARCHITECTURE_I386
+    /* header length */
+    .long multiboot_header_end - multiboot_header
+    /* checksum */
+    .long -(MULTIBOOT2_HEADER_MAGIC + MULTIBOOT_ARCHITECTURE_I386 + (multiboot_header_end - multiboot_header))
+
+    /* Tags here */
+
+    /* End tag */
+    .short MULTIBOOT_HEADER_TAG_END
+    .short 0
+    .long 8
+multiboot_header_end:
+
+.section .bss
+.align 16
+stack_bottom:
+.skip 16384 # 16 KiB
+stack_top:
+
+.section .text
+.global _start
+.type _start, @function
+_start:
+    /* Set up stack */
+    mov $stack_top, %esp
+
+    /* Reset EFLAGS */
+    push $0
+    popf
+
+    /* Push magic and multiboot info pointer onto stack for kernel_main */
+    /* Multiboot2 puts magic in EAX, pointer in EBX */
+    push %ebx 
+    push %eax
+
+    call kernel_main
+
+    cli
+1:  hlt
+    jmp 1b
+
+.size _start, . - _start

src/kernel.c

@@ -1,3 +1,31 @@
-void _start() {
-    while(1) {}
+#include <multiboot2.h>
+#include <stdint.h>
+#include <stddef.h>
+
+static inline void outb(uint16_t port, uint8_t val)
+{
+    asm volatile ( "outb %b0, %w1" : : "a"(val), "Nd"(port) );
+}
+
+void offset_print(const char *str)
+{
+    while (*str) {
+        outb(0xE9, *str);
+        str++;
+    }
+}
+
+#define MULTIBOOT_BOOTLOADER_MAGIC 0x2BADB002
+
+void kernel_main(uint32_t magic, uint32_t addr) {
+    (void)addr; // Unused for now
+
+    if (magic != MULTIBOOT2_BOOTLOADER_MAGIC && magic != MULTIBOOT_BOOTLOADER_MAGIC) {
+        offset_print("Invalid magic number: ");
+        // I don't have hex print yet, but I can print something generic
+        offset_print("Unknown\n");
+        return;
+    }
+
+    offset_print("Hello, world\n");
 }