path: root/pmgdt.s

global _main
global _pm_bootstrap
global gdt_start
global gdt_null
global gdt_code
global gdt_data
global gdt_user_code
global gdt_user_data
global gdt_end
global gdt_descriptor

[bits 16]

_main:
_pm_bootstrap:
    mov     dx, msg_title
    call    putstr
    mov     dx, msg_ldgdt
    call    putstr

    cli
    lgdt    [gdt_descriptor]

    in      al, 0x92
    or      al, 2
    out     0x92, al

    mov     eax, cr0
    or      eax, 0x1
    mov     cr0, eax

    jmp     dword CODE_SEGMENT:pmkernel

msg_title: db 13, 10, 'YOTOS protected mode loader', 13, 10, 0
msg_ldgdt: db 'Loading Global Descriptor Table (GDT)... ', 0

putstr:             ; dx=argument
    mov     cx, bx  ; we must preserve bx
putstr_start:
    mov     bx, dx  ; [dx] is not a effective address
    mov     al, byte [bx]
    test    al, al
    jz      putstr_end
    mov     ah, 0x0e
    mov     bx, 0x0007
    int     0x10
    inc     dx
    jmp     putstr_start
putstr_end:
    mov     bx, cx
    ret



; GDT entry


gdt_start:

gdt_null:   ; null descriptor
    dd  0x00
    dd  0x00

gdt_code:   ; code segment descriptor
    dw  0xffff      ; segment limit (bits 0-15)
    dw  0x0         ; base (bits 0-15)
    db  0x0         ; base (bits 16-23)

    db  10011010b   ; (present) 1,
                    ; (privilege) 00,
                    ; (descriptor type) 1,
                    ; type = 
                    ; (code) 1, 
                    ; (conforming) 0, 
                    ; (readable) 1,
                    ; (accessed) 0

    db  11001111b   ; (granularity) 1,
                    ; (32-bit default) 1,
                    ; (64-bit segment) 0, 
                    ; (AVL) 0,
                    ; segment limit (bits 16-19)
    
    db  0x0         ; base (bits 24-31)

gdt_data:   ; data segment descriptor
    dw  0xffff      ; segment limit (bits 0-15)
    dw  0x0         ; base (bits 0-15)
    db  0x0         ; base (bits 16-23)

    db  10010010b   ; (present) 1,
                    ; (privilege) 00,
                    ; (descriptor type) 1,
                    ; type = 
                    ; (code) 0, 
                    ; (expand down) 0, 
                    ; (writable) 1,
                    ; (accessed) 0

    db  11001111b   ; (granularity) 1,
                    ; (32-bit default) 1,
                    ; (64-bit segment) 0, 
                    ; (AVL) 0,
                    ; segment limit (bits 16-19)
    
    db  0x0         ; base (bits 24-31)

gdt_user_code:  ; user mode code segment descriptor
    dw  0xffff      
    dw  0x0         
    db  0x0         
    db  11111010b   
    db  11001111b   
    db  0x0         

gdt_user_data:  ; user mode data segment descriptor
    dw  0xffff
    dw  0x0
    db  0x0
    db  11110010b
    db  11001111b  
    db  0x0  

gdt_end:

gdt_descriptor:
    dw  gdt_end - gdt_start - 1
    dd  load_offset + gdt_start

CODE_SEGMENT    equ     gdt_code - gdt_start
DATA_SEGMENT    equ     gdt_data - gdt_start