Day 22

Port I/O

	There are many things to do with computers that are called "ports". However, the ones we are
talking about today are internal I/O controll ports. Things described here will most likely work on
a DOS or Win9x computer, but NOT on a WinXP or NT based computer. I take no responsibility if you
crash your computer with these instructions (or any others for that matter.). I also give no 
guarantee that any code will work on your computer and/or specific hardware. Now that the disclaimer
is out of the way, let's see some code!!

The IN instruction

The IN instruction gets a byte of data from a port. It works like this: in al,dx ; will get a byte of data from port DX and put it in AL. I find that the OUT instruction is used (somewhat) more often:

The OUT instruction

The OUT instruction works like IN, except AL is written to the port in DX. out dx,al ; the reverse of the IN instruction Note that you MUST use AL and DX for this instruction. That probably isn't true, but it's easiest to remember. (for me anyway). Now for a little something! This should turn on some lights on the keyboard: ;;---CODE START---;; mov dx,60h ; keyboard control port 60h mov al,0EDh ; set LED status out dx,al ; write to port mov al,3 ; experiment with 0,1,2,3 to see what lights go on. out dx,al ; write to port mov ah,4ch int 21h ; 4Ch (Exit Program) ;;---CODE START---;; Here's an interesting note: A friend and I tried to write a program that turns on CAPS Lock, but turns off the LED (the light), the BIOS of the computer constantly checks and corrects things so that the actual status and the LED were in agreement. To make a story short, the program didn't work.

This Day In Review

There are many ports that do many things, BE CAREFULL! Have fun!, - Mike
Intro - Day 23

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