os/kernel32/src/entrypoint.s at main · reindeer002/os · GitHub

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[ORG 0x00]
[BITS 16]

SECTION .text

START:
    mov ax, 0x1000

    mov ds, ax
    mov es, ax

    mov ax, 0x2401 ; A20 gate active
    int 0x15 ; with BIOS interrupt service

    jc .A20GATEERROR
    jmp .A20GATESUCCESS

.A20GATEERROR:
    in al, 0x92 ; read system control port
    or al, 0x02 ; set A20 gate bit in 1
    and al, 0xFE
    out 0x92, al ; write system control port

.A20GATESUCCESS:
    cli
    lgdt [ GDTR ]

    mov eax, 0x4000003B
    mov cr0, eax

    jmp dword 0x18: ( PROTECTEDMODE - $$ + 0x10000 )

[BITS 32]
PROTECTEDMODE:
    mov ax, 0x20
    mov ds, ax
    mov es, ax
    mov fs, ax
    mov gs, ax

    mov ss, ax
    mov esp, 0xFFFE
    mov ebp, 0xFFFE

    push ( SWITCHSUCCESSMESSAGE - $$ + 0x10000 )
    push 2
    push 0
    call PRINTMESSAGE
    add esp, 12

    jmp dword 0x18: 0x10200 ; jump to starting point of C based kernel

PRINTMESSAGE:
    push ebp
    mov ebp, esp
    push esi
    push edi
    push eax
    push ecx
    push edx

    mov eax, dword [ ebp + 12 ]
    mov esi, 160
    mul esi
    mov edi, eax

    mov eax, dword [ ebp + 8 ]
    mov esi, 2
    mul esi
    add edi, eax

    mov esi, dword [ ebp + 16 ]

.MESSAGELOOP:
    mov cl, byte [ esi ]
    cmp cl, 0
    je .MESSAGEEND

    mov byte [ edi + 0xB8000 ], cl

    add esi, 1
    add edi, 2

    jmp .MESSAGELOOP

.MESSAGEEND:
    pop edx
    pop ecx
    pop eax
    pop edi
    pop esi
    pop ebp
    ret

align 8, db 0

dw 0x0000
GDTR:
    dw GDTEND - GDT - 1
    dd ( GDT - $$ + 0x10000 )

GDT:
    NULLDescriptor:
    dw 0x0000
    dw 0x0000
    db 0x00
    db 0x00
    db 0x00
    db 0x00

IA_32eCODEDESCRIPTOR:
    dw 0xFFFF
    dw 0x0000
    db 0x00
    db 0x9A
    db 0xAF
    db 0x00

IA_32eDATADESCRIPTOR:
    dw 0xFFFF
    dw 0x0000
    db 0x00
    db 0x92
    db 0xAF
    db 0x00

CODEDESCRIPTOR:
    dw 0xFFFF
    dw 0x0000
    db 0x00
    db 0x9A
    db 0xCF
    db 0x00

DATADESCRIPTOR:
    dw 0xFFFF
    dw 0x0000
    db 0x00
    db 0x92
    db 0xCF
    db 0x00
GDTEND:

SWITCHSUCCESSMESSAGE: db 'Switch Success', 0

times 512 - ( $ - $$ ) db 0x00