OS Thing Code

BITS 16
ORG 7C00h

start:
    cli

    mov [drive], dl

    mov ah, 01h
    mov ch, 3Fh
    int 10h

    mov ah, 13h
    mov al, 01h
    mov bh, 00h
    mov bl, 0Fh
    mov cx, 0Ah
    mov dh, 00h
    mov dl, 00h
    mov bp, loading
    int 10h

    mov dl, [drive]

    mov ah, 02h
    mov al, 2
    mov ch, 0
    mov dh, 0
    mov cl, 2
    mov bx, loadLocation
    int 13h

    xor ax, ax
    push ax
    popf
    mov ds, ax
    lgdt [gdt_desc]
    mov eax, cr0
    or eax, 1
    mov cr0, eax
    jmp 08h:protectedModeBegin

loading:
    db "Loading...", 0

drive: db 0

BITS 32

protectedModeBegin:
    mov ax, 10h
    mov ds, ax
    mov ss, ax
    mov esp, 90000h

    jmp loadLocation

gdt:
gdt_null:
    dq 0
gdt_code:
    dw 0FFFFh
    dw 0
    db 0
    db 10011010b
    db 11001111b
    db 0
gdt_data:
    dw 0FFFFh
    dw 0
    db 0
    db 10010010b
    db 11001111b
    db 0
gdt_end:

gdt_desc:
    dw gdt_end-gdt
    dd gdt

idtbegin:
zerodivide:
    dw 0000h
    dw 0008h
    db 00h
    db 8Eh
    dw 0000h
idtend:

idt:
    dw idtend-idtbegin
    dq idtbegin

    times 510-($-$$) db 0
    dw 0AA55h

loadLocation:
    mov eax, zeroDivide_exception
    and eax, 0000FFFFh
    mov [zerodivide], ax
    mov eax, zeroDivide_exception
    and eax, 0FFFF0000h
    mov [zerodivide+6], ax
    lidt [idt]

    mov bl, 00h
    div bl      ;Cause a divide-by-zero exception ;)

    call clearTextScreen

    mov si, whoopsie
    mov ax, 01h
    mov bx, 01h
    call placeText
    mov si, oops
    mov ax, 02h
    mov bx, 02h
    call placeText
    mov si, gurumeditation
    mov ax, 03h
    mov bx, 03h
    call placeText
    mov si, abortretryfail
    mov ax, 04h
    mov bx, 04h
    call placeText
    mov si, notatypewriter
    mov ax, 05h
    mov bx, 05h
    call placeText
    mov si, nocarrier
    mov ax, 06h
    mov bx, 06h
    call placeText
    mov si, pcloadletter
    mov ax, 07h
    mov bx, 07h
    call placeText
    mov si, lp0onfire
    mov ax, 08h
    mov bx, 08h
    call placeText

.loop:
    jmp .loop

lp0onfire:
    db "lp0 on fire", 0
pcloadletter:
    db "PC LOAD LETTER", 0
notatypewriter:
    db "Not a typewriter", 0
abortretryfail:
    db "Abort, Retry, Fail?", 0
gurumeditation:
    db "Guru Meditation #00000004.0000AAC0", 0
nocarrier:
    db "NO CARRIER", 0
oops:
    db "thing(0): Oops", 0
whoopsie:
    db "whoopsie", 0

tempx: dw 0
tempy: dw 0
tempattrib: db 0
tempval: dq 0

;esi - Pointer to text
;ax - X Pos.
;bx - Y Pos.
;cx - Attrib.
placeText:
    mov [tempattrib], cx
    mov [tempx], ax
    mov ax, bx
    mov bx, 160
    mul bx
    mov [tempy], ax
    mov ax, [tempx]
    mov bx, 2
    mul bx
    mov bx, ax
    mov ax, [tempy]
    add eax, ebx

    mov ecx, 0B8000h
    add ecx, eax
.placeTextLoop:
    lodsb
    cmp al, 00h
    je .donePlacingText
    mov [ecx], BYTE al
    add ecx, 1h
    mov dx, [tempattrib]
    mov [ecx], dx
    add ecx, 1h
    jmp .placeTextLoop
.donePlacingText:
    ret

clearTextScreen:
    mov ecx, 0B8000h
    mov ax, (80*25)*2
.loop:
    mov [ecx], BYTE 00h
    add ecx, 01h
    sub ax, 01h
    cmp ax, 00h
    je .doneClearingTextScreen
    jmp .loop
.doneClearingTextScreen:
    ret

divideByZero:
    db "DIVIDE-BY-ZERO EXCEPTION", 0
pcTxt:
    db "PC: ", 0
espTxt:
    db "ESP: ", 0
eaxTxt:
    db "EAX: ", 0
ebxTxt:
    db "EBX: ", 0
ecxTxt:
    db "ECX: ", 0
edxTxt:
    db "EDX: ", 0
hexTable:
    db "0", 0
    db "1", 0
    db "2", 0
    db "3", 0
    db "4", 0
    db "5", 0
    db "6", 0
    db "7", 0
    db "8", 0
    db "9", 0
    db "A", 0
    db "B", 0
    db "C", 0
    db "D", 0
    db "E", 0
    db "F", 0
hexEnd:
    db "H", 0

temppc: dq 0
tempesp: dq 0
tempeax: dq 0
tempebx: dq 0
tempecx: dq 0
tempedx: dq 0

copyRegsToMem:
    mov [tempeax], eax
    mov [tempebx], ebx
    mov [tempecx], ecx
    mov [tempedx], edx
    pop ebx ;Pop the return address from calling this subroutine into EBX
    pop eax
    mov [temppc], eax
    push ebx ;Push the return address back onto the stack
    mov [tempesp], esp
    ret

zeroDivide_exception:
    call copyRegsToMem

    call clearTextScreen
    mov si, divideByZero
    mov ax, 00h
    mov bx, 00h
    mov cx, 0Ch
    call placeText

    call showRegisters

    jmp hang

showRegisters:
    mov si, pcTxt
    mov ax, 00h
    mov bx, 01h
    mov cx, 0Ch
    call placeText
    mov si, espTxt
    mov ax, 00h
    mov bx, 02h
    mov cx, 0Ch
    call placeText
    mov si, eaxTxt
    mov ax, 00h
    mov bx, 03h
    mov cx, 0Ch
    call placeText
    mov si, ebxTxt
    mov ax, 00h
    mov bx, 04h
    mov cx, 0Ch
    call placeText
    mov si, ecxTxt
    mov ax, 00h
    mov bx, 05h
    mov cx, 0Ch
    call placeText
    mov si, edxTxt
    mov ax, 00h
    mov bx, 06h
    mov cx, 0Ch
    call placeText

    mov ax, 05h
    mov bx, 01h
    mov ecx, [temppc]
    call print32Bit
    mov ax, 05h
    mov bx, 02h
    mov ecx, [tempesp]
    call print32Bit
    mov ax, 05h
    mov bx, 03h
    mov ecx, [tempeax]
    call print32Bit
    mov ax, 05h
    mov bx, 04h
    mov ecx, [tempebx]
    call print32Bit
    mov ax, 05h
    mov bx, 05h
    mov ecx, [tempecx]
    call print32Bit
    mov ax, 05h
    mov bx, 06h
    mov ecx, [tempedx]
    call print32Bit
    ret

;ax - X Pos.
;bx - Y Pos.
;ecx - Value to print
print32Bit:
    mov cx, 0Fh
    mov si, hexTable
    call placeText
    ret

hang:
    jmp hang