Use ELF kernel image.

Makefile

@@ -7,17 +7,17 @@ run: image.bin
 
 debug: image.bin
 	qemu-system-i386 -drive format=raw,file=$< -s -S &
-	i386-elf-gdb \
+	x86_64-elf-gdb \
 		-ex "target remote localhost:1234" \
 		-ex "set architecture i8086" \
-		-ex "break *0x7c00" \
+		-ex "break *0x7c5d" \
 		-ex "continue"
 
 image.bin: mbr.bin kernel.bin
 	cat $^ >$@
 
-kernel.bin: kernel.o vga.o string.o
+kernel.bin: kernel.o vga.o string.o drivers/ata.o
-	$(LD) -m elf_i386 -o $@ -Ttext 0x1000 $^ --oformat binary
+	$(LD) -m elf_i386 -o $@ -Ttext 0x1000 $^
 
 %.o: %.c
 	$(CC) -m32 -ffreestanding -c $< -o $@

drivers/ata.c

@@ -3,7 +3,7 @@
 #include <stdint.h>
 
 #include "ata.h"
-#include "../ports.h"
+#include "../port.h"
 
 /*
  BSY: a 1 means that the controller is busy executing a command. No register should be accessed (except the digital output register) while this bit is set.

elf.h

@@ -0,0 +1,38 @@
+#pragma once
+
+#include <stdint.h>
+
+enum {
+	EI_NIDENT = 16,
+};
+
+typedef uint32_t Elf32_Addr;
+typedef uint32_t Elf32_Off;
+
+typedef struct {
+	unsigned char e_ident[EI_NIDENT];
+	uint16_t      e_type;
+	uint16_t      e_machine;
+	uint32_t      e_version;
+	Elf32_Addr    e_entry;
+	Elf32_Off     e_phoff;
+	Elf32_Off     e_shoff;
+	uint32_t      e_flags;
+	uint16_t      e_ehsize;
+	uint16_t      e_phentsize;
+	uint16_t      e_phnum;
+	uint16_t      e_shentsize;
+	uint16_t      e_shnum;
+	uint16_t      e_shstrndx;
+} Elf32_Ehdr;
+
+typedef struct {
+	uint32_t   p_type;
+	Elf32_Off  p_offset;
+	Elf32_Addr p_vaddr;
+	Elf32_Addr p_paddr;
+	uint32_t   p_filesz;
+	uint32_t   p_memsz;
+	uint32_t   p_flags;
+	uint32_t   p_align;
+} Elf32_Phdr;

kernel.c

@@ -1,11 +1,8 @@
-asm("jmp main");
-
 #include "vga.h"
 
-int main() {
+void _start() {
     vga_clear_screen();
     for (int i = 0; i < 24; i++) {
         vga_print_string("hello world\n");
     }
-    return 0;
 }

mbr.S

@@ -11,11 +11,39 @@ _start:
     jmp . // loop forever
 
 
+.equ ELF32_ENTRY_OFFSET, 0x18
+.equ ELF32_PHDR_OFFSET, 0x1c
+.equ ELF32_PHDR_FILESZ_OFFSET, 4*4
 .equ KERNEL_OFFSET, 0x1000
+
+.equ MBR_SECTORS, 1
+.equ SECTOR_BASE, 1
+.equ ELFHDR_SECTORS, 8
+
+.equ SECTOR_SIZE, 512
+.equ SECTOR_SHIFT, 9
+
 load_kernel:
+    mov $1, %al // sectors to read
+    mov $SECTOR_BASE + MBR_SECTORS, %cl // start after MBR
+    call bios_disk_read
+
+    mov KERNEL_OFFSET + ELF32_ENTRY_OFFSET, %si
+    mov %si, entry // store entry point
+
+    mov KERNEL_OFFSET + ELF32_PHDR_OFFSET, %di
+    mov KERNEL_OFFSET + ELF32_PHDR_FILESZ_OFFSET(%di), %ax
+    sub $0x1000, %ax // we won't load the header
+    add $SECTOR_SIZE - 1, %ax
+    shr $SECTOR_SHIFT, %ax // round up to sector count
+
+    mov $SECTOR_BASE + MBR_SECTORS + ELFHDR_SECTORS, %cl //start after ELF header
+    call bios_disk_read
+    ret
+
+bios_disk_read:
+    // expects %al to specify number of sectors, %cl the initial sector
     mov $2, %ah             // read mode
-    mov $2, %al             // sectors to read
-    mov $2, %cl             // start from sector 2
     mov $0, %ch             // cylinder 0
     mov $0, %dh             // head 0
     mov $KERNEL_OFFSET, %bx // bx -> destination
@@ -46,7 +74,8 @@ init_32bit:
     mov $0x90000, %ebp      // 6. setup stack
     mov %ebp, %esp
 
-    call KERNEL_OFFSET      // 7. jump to the kernel
+    movzwl entry, %esi
+    call *%esi              // 7. jump to the kernel
     jmp .                   // 8. loop forever
 
 
@@ -70,6 +99,11 @@ boot_drive:
 banner:
     .asciz "YABLOKO bootloader started\r\n"
 
+    .balign 2
+entry:
+    .word 0
+
+    .balign 4
 gdt_start:
     .quad 0x0 // null descriptor
 

port.h

@@ -6,6 +6,16 @@ static unsigned char port_byte_in(unsigned short port) {
     return result;
 }
 
+static unsigned short port_word_in(unsigned short port) {
+    unsigned short result;
+    __asm__("in %%dx, %%ax" : "=a" (result) : "d" (port));
+    return result;
+}
+
 static void port_byte_out(unsigned short port, unsigned char data) {
     __asm__("out %%al, %%dx" : : "a" (data), "d" (port));
 }
+
+static void port_long_out(unsigned short port, unsigned int data) {
+    __asm__("out %%eax, %%dx" : : "a" (data), "d" (port));
+}