OSDev.org

I just wanted to set and test a interrupt in my bootimage code. But something seems to be wrong. Can someone help and find whats wrong ?


[BITS 16]
[ORG 0x7C0]

jmp loadOS
     loadOS:

        cli             ;Clear Interrupt

        mov ax,0x07C0
        mov ds,ax
        mov es,ax
        mov fs,ax
        mov gs,ax

        mov ax,0x0000
        mov ss,ax
        mov sp,0xFFFF   ;Maximum Limit for the Stack Pointer in 16bit mode
                        ;i.e. 64kb = 2^16
        sti

        push dx
        push es

        xor ax,ax
        mov es,ax

        cli

        mov word [es:0x21*4], _int0x21
        mov [es:0x21*4+2], cs

        sti             ;Set Interrupt

        pop es
        pop dx

        int 0x16

        mov si, msgBoot
        mov al,0x01
        int 0x21
        jmp $


_int0x21:
  _int0x21_ser0x01:
        cmp al,0x01
        jne _int0x21_end

        _int0x21_ser0x01_start:

                lodsb
                or al,al
                jz _int0x21_ser0x01_end
                mov ah,0x0E
                mov bh,0x00
                mov bl,0x07
                int 0x10
                jmp _int0x21_ser0x01_start
                
        _int0x21_ser0x01_end:

        _int0x21_end:

iret                    ; return from INT 0x21

        msgBoot db 0x0D,0x0A,"OS Booting ....",0x00

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

the mistake is in the 2nd line, your code is loaded to 0000:7c00 but you have [org 0x7c0]

greets, hartyl

hi hartyl,

yea u were right. My code worked after I changed that and a related           'mov ax,0x07C' code. As planned it prints the code when I hit a key. But after that Bochs reports a fatal error saying

"FATAL: KBD: dropped key scan=1F, ascii=73"

The corrected code:
-------------------

[BITS 16]
[ORG 0x7C00]

jmp loadOS
     loadOS:

        cli             ;Clear Interrupt

        push cs
        pop ax
        mov ds,ax
        mov es,ax
        mov fs,ax
        mov gs,ax

        mov ax,0x0000
        mov ss,ax
        mov sp,0xFFFF   ;Maximum Limit for the Stack Pointer in 16bit mode
                        ;i.e. 64kb = 2^16
        sti

        push dx
        push es

        xor ax,ax
        mov es,ax

        cli

        mov word [es:0x21*4], _int0x21
        mov [es:0x21*4+2], cs

        sti             ;Set Interrupt

        pop es
        pop dx

        int 0x16

        mov si, msgBoot
        mov al,0x01
        int 0x21
        jmp $


_int0x21:
  _int0x21_ser0x01:
        cmp al,0x01
        jne _int0x21_end

        _int0x21_ser0x01_start:

                lodsb
                or al,al
                jz _int0x21_ser0x01_end
                mov ah,0x0E
                mov bh,0x00
                mov bl,0x07
                int 0x10
                jmp _int0x21_ser0x01_start
                
        _int0x21_ser0x01_end:

        _int0x21_end:

iret                    ; return from INT 0x21

        msgBoot db 0x0D,0x0A,"OS Booting ....",0x00

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