diff options
Diffstat (limited to 'examples/bootloader/boot.S')
| -rw-r--r-- | examples/bootloader/boot.S | 84 |
1 files changed, 84 insertions, 0 deletions
diff --git a/examples/bootloader/boot.S b/examples/bootloader/boot.S new file mode 100644 index 0000000..4938689 --- /dev/null +++ b/examples/bootloader/boot.S @@ -0,0 +1,84 @@ + +.code16 +.global start +.extern kmain + +start: + cli + mov $0, %ax + mov %ax, %ds + mov %ax, %es + mov %ax, %ss + mov $0x7c00, %sp + + # Read sectors from disk + mov $0x02, %ah # BIOS read sector function + mov $1, %al # Number of sectors to read (Load 1 sector/512 bytes) + mov $0, %ch # Cylinder 0 + mov $0, %dh # Head 0 + mov $2, %cl # Start from sector 2 + # dl already contains the boot drive number + mov $0, %bx # Segment 0 + mov %bx, %es + mov $0x7e00, %bx # Destination buffer + int $0x13 # BIOS interrupt + jc disk_error + + # Load GDT + lgdt gdt_descriptor + + # Switch to Protected Mode + mov %cr0, %eax + or $1, %eax + mov %eax, %cr0 + + # Jump to 32-bit code segment + ljmp $0x08, $protected_mode + +disk_error: + jmp . + +.code32 +protected_mode: + # Set up 32-bit data segments + mov $0x10, %ax + mov %ax, %ds + mov %ax, %es + mov %ax, %fs + mov %ax, %gs + mov %ax, %ss + + # Set up stack + mov $0x7c00, %esp + + call kmain + hlt + +gdt_start: + .quad 0 # Null descriptor + + # Code Segment (Offset 0x08) + .word 0xffff # Limit (low) + .word 0x0 # Base (low) + .byte 0x0 # Base (middle) + .byte 0x9a # Access (exec, read) + .byte 0xcf # Granularity + .byte 0x0 # Base (high) + + # Data Segment (Offset 0x10) + .word 0xffff # Limit (low) + .word 0x0 # Base (low) + .byte 0x0 # Base (middle) + .byte 0x92 # Access (read/write) + .byte 0xcf # Granularity + .byte 0x0 # Base (high) + +gdt_end: + +gdt_descriptor: + .word gdt_end - gdt_start - 1 + .long gdt_start + +.fill 510-(.-start), 1, 0 +.word 0xaa55 + |