Re: DX needed for printing charcter?

Ed Casas (edc@ece.ubc.ca) Wed, 18 Oct 2000 21:29:11 -0700 

Date: Wed, 18 Oct 2000 21:29:11 -0700
From: Ed Casas <edc@ece.ubc.ca>
Subject: Re: DX needed for printing charcter?



> in the example on page 9 of our notes on 80386:
> 
> push dx
> mov  dl, al
> mov  ah, 02H
> int  21H
> 
> is the DX register needed for outputing a character on screen?  and if so, 
> why?

The second line changes the register DL.  Before this we save DL
to avoid modifying any values that may have been stored there by
the calling routine.

Since we can only push 16-bit registers we push DX instead of
just DL.

> if we have a 2-bit mode signal (AB) encoded from a 4-bit
> control signal (CDEF), are we allowed to use something like
> this
>   if CDEF = 0101 then AB = 10

Yes (although your example is not in VHDL syntax and you have to
specify what will be assigned to AB for all possible cases.

> or do we need to simplify the truth table ourselves and control
> the A and B bits with combinational logic?

No, don't try to design using truth tables!  It will take you too
long and you won't have time to finish the questions on the
exams.

-- 
Ed Casas   edc@ece.ubc.ca   http://casas.ece.ubc.ca   +1 604 822-2592