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; GDT
gdt_start:

gdt_null: ; Mandatory null descriptor
    dd 0x0 ; (Define double)
    dd 0x0

gdt_code: ; code segment descriptor
    ; base=0x0, limit=0xfffff,
    ; 1st flags: (present)1 (privilege)00 (descriptor type)1 -> 1001b
    ; type flags: (code)1 (conforming)0 (readable)1 (accesed)0 -> 1010b
    ; 2nd flags: (granularity)1 (32-bit default)1 (64-bit seg)0 (AVL)0 -> 1100b
    dw 0xffff ; Limit (0-15)
    dw 0x0 ; Base (0-15)
    db 00000000b ; Base (16-23)
    db 10011010b ; 1st flags and type flags
    db 11001111b ; 2nd flags and Limit (16-19)
    db 0x0 ; Base (24-31)

gdt_data: ; data segment descriptor
    ; Same as code segment except type flags
    ; type flags: (code)0 (expand down)0 (writable)1 (accesed)0 -> 0010b
    dw 0xffff ; Limit (0-15)
    dw 0x0 ; Base (0-15)
    db 00000000b ; Base (16-23)
    db 10010010b ; 1st flags and type flags
    db 11001111b ; 2nd flags and Limit (16-19)
    db 0x0 ; Base (24-31)

gdt_end: ; Have the assembler calculate the size of the GDT for the GDT descriptor (below)


; GDT Descriptor
gdt_descriptor:
    dw gdt_end - gdt_start - 1 ; Size of the GDT minus one
    dd gdt_start ; Start adress of the GDT

; Define some handy constants for the GDT segment descriptor offsets , which
; are what segment registers must contain when in protected mode. For example ,
; when we set DS = 0 x10 in PM , the CPU knows that we mean it to use the
; segment described at offset 0 x10 ( i.e. 16 bytes ) in our GDT , which in our
; case is the DATA segment (0 x0 -> NULL ; 0 x08 -> CODE ; 0 x10 -> DATA )
CODE_SEG equ gdt_code - gdt_start
DATA_SEG equ gdt_data - gdt_start

Code: Select all

switch_to_pm:
    cli ; Disable interrupts

    lgdt [gdt_descriptor] ; Load GDT

    mov eax, cr0 ; Make the 1st bit of cr0 to be a one
    or al, 0x1
    mov cr0, eax ; We are in PM

    jmp CODE_SEG:init_pm ; Far jump to 32-bit code, setting CS

[bits 32]
; Initialize registers and stack once in PM
init_pm:
    sti
    mov ax, DATA_SEG ; Point segment registers to the data segment
    mov ds, ax
    mov ss, ax
    mov es, ax
    mov fs, ax
    mov gs, ax

    mov ebp, 0x90000 ; Update stack position
    mov esp, ebp

    call begin_pm

Code: Select all

__asm__ __volatile__(
        "    push %%eax\n"
        "    push %%ebx\n"
        "    mov  $0xA5, %%ebx\n"
        "    mov  $0x100000, %%eax\n" // 0x107D19 crashes
        "1:  \n"
        "    mov  %%ebx, (%%eax)\n"
        "    inc  %%eax\n"
        "    cmp  $0x600000, %%eax\n"
        "    je   2f\n"
        "    jmp  1b\n"
        "2:  \n"
        "    pop  %%ebx\n"
        "    pop  %%eax\n"
        :
        :
        : "memory", "esi", "edi", "eax", "ebx", "ecx", "edx");