os/boot.asm at master · PandaDex/os · GitHub

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[org 0x7C00]
[bits 16]

start:
    xor ax, ax
    mov ds, ax
    mov es, ax
    mov ss, ax
    mov sp, 0x7C00
    KERNEL_LOAD_ADDR equ 0x7E00

    mov [boot_drive], dl   ; BIOS passes boot drive number in dl — save it

    mov si, msg
    call print_string

    mov bx, 0x7E00      ; destination offset (es already 0)
    mov dh, 128         ; number of sectors to read (64KB kernel budget)
    mov dl, [boot_drive]
    call disk_load
    call switch_to_pm
    jmp $               ; jump into protected mode here

enable_a20:
    in al, 0x92
    or al, 2
    out 0x92, al        ; fast A20 enable via system control port
    ret

gdt_start:
    dq 0x0                          ; null descriptor

gdt_code:                           ; code segment descriptor
    dw 0xFFFF                       ; limit (low)
    dw 0x0                          ; base (low)
    db 0x0                          ; base (mid)
    db 10011010b                    ; access: present, ring0, code, exec/read
    db 11001111b                    ; flags + limit (high): 4K granularity, 32-bit
    db 0x0                          ; base (high)

gdt_data:                           ; data segment descriptor
    dw 0xFFFF
    dw 0x0
    db 0x0
    db 10010010b                    ; access: present, ring0, data, read/write
    db 11001111b
    db 0x0

gdt_end:

gdt_descriptor:
    dw gdt_end - gdt_start - 1       ; size of GDT, minus 1
    dd gdt_start                     ; linear address of GDT

CODE_SEG equ gdt_code - gdt_start
DATA_SEG equ gdt_data - gdt_start

switch_to_pm:
    call enable_a20
    cli                              ; no interrupts during the switch
    lgdt [gdt_descriptor]

    mov eax, cr0
    or eax, 0x1
    mov cr0, eax                     ; set PE bit — we are now in protected mode

    jmp CODE_SEG:init_pm             ; far jump: flushes CPU pipeline, loads CS

[bits 32]
init_pm:
    mov ax, DATA_SEG                 ; reload all data-segment registers
    mov ds, ax
    mov es, ax
    mov fs, ax
    mov gs, ax
    mov ss, ax

    mov esp, 0x90000                 ; new 32-bit stack
    mov ebp, esp

    jmp KERNEL_LOAD_ADDR              ; hand off to loaded kernel

disk_load:
    push dx
    mov ah, 0x02
    mov al, dh
    mov ch, 0x00
    mov cl, 0x02
    mov dh, 0x00
    int 0x13
    jc disk_error
    pop dx              ; restore original dx (with requested sector count in dh)
    cmp al, dh
    jne disk_error
    ret

disk_error:
    mov si, disk_err_msg
    call print_string
    jmp $

print_string:
    lodsb
    or al, al
    jz .done
    mov ah, 0x0E
    int 0x10
    jmp print_string
.done:
    ret

boot_drive db 0
msg db 'Booting...', 0
disk_err_msg db 'Disk error!', 0

times 510-($-$$) db 0
dw 0xAA55