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

CMakeLists.txt

@@ -0,0 +1,18 @@
+cmake_minimum_required(VERSION 3.16)
+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 "${CMAKE_C_FLAGS} -ffreestanding -m32 -fno-pie -fno-pic -fno-builtin -fno-stack-protector -g -O2 -Wall -Wextra")
+set(CMAKE_ASM_FLAGS "${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)
+set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
+set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
+
+add_subdirectory(src)
+# add_subdirectory(libs)
+# add_subdirectory(apps)

README.md

@@ -37,9 +37,9 @@ The git commit should describe what is done and why certain choices were made.
 The title of the commit should always end with (AI), to indicate that it was written by an AI.
 Once a task is completed, it should be checked off.
 
-- [ ] Create directory structure
-- [ ] Create initial build system
-- [ ] Setup a simple kernel that writes `Hello, world` to Qemu debug port
+- [x] Create directory structure
+- [x] Create initial build system
+- [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

@@ -0,0 +1,14 @@
+cmake_minimum_required(VERSION 3.16)
+
+add_executable(kernel 
+    boot.S
+    kernel.c
+)
+
+# Use our custom linker script
+target_link_options(kernel PRIVATE -T ${CMAKE_CURRENT_SOURCE_DIR}/linker.ld)
+
+target_include_directories(kernel PRIVATE 
+    ${CMAKE_SOURCE_DIR}/vendor
+    ${CMAKE_SOURCE_DIR}/include # If created later
+)

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

@@ -0,0 +1,31 @@
+#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");
+}

src/linker.ld

@@ -0,0 +1,28 @@
+ENTRY(_start)
+
+SECTIONS
+{
+	. = 1M;
+
+	.text BLOCK(4K) : ALIGN(4K)
+	{
+		KEEP(*(.multiboot))
+		*(.text)
+	}
+
+	.rodata BLOCK(4K) : ALIGN(4K)
+	{
+		*(.rodata)
+	}
+
+	.data BLOCK(4K) : ALIGN(4K)
+	{
+		*(.data)
+	}
+
+	.bss BLOCK(4K) : ALIGN(4K)
+	{
+		*(COMMON)
+		*(.bss)
+	}
+}