Re: Duplicate Instructions?

Ed Casas (edc@ece.ubc.ca) Sun, 27 Feb 2000 19:44:02 -0800 

Date: Sun, 27 Feb 2000 19:44:02 -0800
From: Ed Casas <edc@ece.ubc.ca>
Subject: Re: Duplicate Instructions?



On Sun, Feb 27, 2000 at 06:24:23PM -0800, ddowler wrote:

> Are the instructions
> 
> PUSH    0
> PUSH    START
> 
> the same in machine code?

The label "START" has the value 0, so yes, they are the same
instruction.

> If so, could you explain how this will work (i.e. how PUSH 0
> will "turn the output off" and PUSH START will jump to address
> zero)?

PUSH 0 pushes the stack down and loads the value 0 into the X
register.  It doesn't do anything else.  It's the following
instruction, OUTP, that transfers the contents of X to the O
register and turns the output off.

Similarly, PUSH START pushes the stack down and loads 0 into X.
It doesn't do anything else.  The following instruction, RET,
transfers X to PC thus causing the processor to fetch the next
instruction from address 0.

The comments "turn the output off" and "repeat forever" refer to
the effects of a block of instructions (2 instructions in these
cases), not to a single instruction.  Look at the course
documentation guidelines, the solutions to Assignment 2 and the
assembly language examples for more examples of this commenting
convention.
 
-- 
Ed Casas   edc@ece.ubc.ca   http://casas.ece.ubc.ca   +1 604 822-2592