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Interrupts
This code is meant to show how the hardware interrupt generated when you press a key can be handled by replacing the seg:offset specified in the IVT (interrupt vector table). This normally points to a BIOS routine. To find the entry in the IVT, multiply the interrupt number by 4 (which is the size of each entry).
This key handler just displays the scan code without conversion to ASCII, buffering, or handling extended keys. The reason for doing this is to not muddle up the basic idea, which is to provide input, as well as output, in its most simple form.
I will not go into the hows and whys of reading the ports involved in a key press. Suffice it to say that you are communicating with actual chips (or parts of chips), not some software intermediary. I personally feel it is good to remember that, no matter what level of abstraction you work at, you are ultimately telling hardware what to do.
I will point out the turning the keyboard on/off through port 0x61 is given in its complete form, some of which might not be needed, depending on the system.
;==========================================
; nasmw boot.asm -f bin -o boot.bin
; partcopy boot.bin 0 200 -f0
[ORG 0x7c00] ; add to offsets
jmp start
%include "print.inc"
start: xor ax, ax ; make it zero
mov ds, ax ; DS=0
mov ss, ax ; stack starts at 0
mov sp, 0x9c00 ; 200h past code start
mov ax, 0xb800 ; text video memory
mov es, ax
cli ;no interruptions
mov bx, 0x09 ;hardware interrupt #
shl bx, 2 ;multiply by 4
xor ax, ax
mov gs, ax ;start of memory
mov [gs:bx], word keyhandler
mov [gs:bx+2], ds ; segment
sti
jmp $ ; loop forever
keyhandler:
in al, 0x60 ; get key data
mov bl, al ; save it
mov byte [port60], al
in al, 0x61 ; keybrd control
mov ah, al
or al, 0x80 ; disable bit 7
out 0x61, al ; send it back
xchg ah, al ; get original
out 0x61, al ; send that back
mov al, 0x20 ; End of Interrupt
out 0x20, al ;
and bl, 0x80 ; key released
jnz done ; don't repeat
mov ax, [port60]
mov word [reg16], ax
call printreg16
done:
iret
port60 dw 0
times 510-($-$$) db 0 ; fill sector w/ 0's
dw 0xAA55 ; req'd by some BIOSes
;==========================================
print.inc
Just in case it wasn't clear, we have moved the printing routines from part 4 (dochar, sprint, cprint, and printreg16) into an include file, which we will name print.inc.
;==========================================
; print.inc
;----------------------
dochar: call cprint ; print one character
sprint: lodsb ; string char to AL
cmp al, 0
jne dochar ; else, we're done
add byte [ypos], 1 ;down one row
mov byte [xpos], 0 ;back to left
ret
cprint: mov ah, 0x0F ; attrib = white on black
mov cx, ax ; save char/attribute
movzx ax, byte [ypos]
mov dx, 160 ; 2 bytes (char/attrib)
mul dx ; for 80 columns
movzx bx, byte [xpos]
shl bx, 1 ; times 2 to skip attrib
mov di, 0 ; start of video memory
add di, ax ; add y offset
add di, bx ; add x offset
mov ax, cx ; restore char/attribute
stosw ; write char/attribute
add byte [xpos], 1 ; advance to right
ret
;------------------------------------
printreg16:
mov di, outstr16
mov ax, [reg16]
mov si, hexstr
mov cx, 4 ;four places
hexloop:
rol ax, 4 ;leftmost will
mov bx, ax ; become
and bx, 0x0f ; rightmost
mov bl, [si + bx];index into hexstr
mov [di], bl
inc di
dec cx
jnz hexloop
mov si, outstr16
call sprint
ret
;------------------------------------
xpos db 0
ypos db 0
hexstr db '0123456789ABCDEF'
outstr16 db '0000', 0 ;register value string
reg16 dw 0 ; pass values to printreg16
See Also
External Links
- hardware fun http://chip.ms.mff.cuni.cz/pcguts/ (https://web.archive.org/web/20020801235709/http://chip.ms.mff.cuni.cz/~pcguts/)
- Intel's Summer Reading List http://developer.intel.com/vtune/cbts/refman.htm
- John Fine links to hardware programming http://www.geocities.com/SiliconValley/Peaks/8600/device.html (http://web.archive.org/web/20090723073639/http://www.geocities.com/SiliconValley/Peaks/8600/device.html)