A89: More ASM help please

To: assembly-89@lists.ticalc.org
Subject: A89: More ASM help please
From: BLoWh0Le@aol.com
Date: Tue, 24 Nov 1998 22:55:45 EST
Delivered-To: assembly-89-outgoing@towerguard.unix.edu.sollentuna.se
Reply-To: assembly-89@lists.ticalc.org


Can someone explan the following code excerpt from the hello world program?

   move.w   #2,-(a7)               ; use font #2 (large font)

<<<ok, I think this moves 2 into (a7), but what is the negative sign? is that
like variable-- in C++?>>>

    jsr      tios::FontSetSys       ; set the font

<<< I'm guessing 2 is pushed into the stack and FontSetSys uses 2 as a
parameter, right?  When FontSetSys is called, is the 2 popped out or does it
stay there?>>>

    add.l    #2,a7                  ; clean up the stack

<<< How does adding 2 to a7 clean up the stack? What's going on? >>>

    move.w   #4,-(a7)               ; move the first param to the stack

<<< Ok, here's another parameter, but for what? >>>

    lea      string(pc),a0          ; move adress of string to the stack

<<< What does lea do and why does string have a (pc) next to it? >>>

    move.l   a0,-(a7)

<<< String is being pushed into the stack via a0, but why did we have to use
a0 when it seems logical enough to just push string in directly?>>>

    move.w   #0,-(a7)               ; push position

<<< Pushes 0 into stack as Y-position parameter >>>

    move.w   #0,-(a7)               ; push position

<<< Pushes 0 into stack as X-position parameter >>>

    jsr      tios::DrawStrXY        ; call the DrawStrXY ROM function

<<< Once again, is everything popped outta the stack, or is it still there?
>>>

    add.l    #10,a7                 ; restore the stack pointer

<<< How does this work too?  What does it mean by clearing and restoring the
stack? >>>

Thanks.