>Hello there, everybody, from a newbie. I have shaved my head and put
>on my Shoalin robes. I have a few questions, and one comment to make.
> (I know these may seem trivial to those of you who know what you're
>doing, but I gotta start somewhere.) Please red pencil where I am
>wrong.
>
Welcome aboard.
>2) I was reading a book on programming the Apple IIe (6502 processor,
>doncha know), and it mentions that the branch commands only go 127 (or
>128, depending on direction) bytes at the most. Has anybody ever run
>into a situation where branches are required further than this?
>
I have before. This happens a lot when you have no idea where to place all
the subroutines in code. Basically, I reorganized my code to fit the
branch (move some subroutines around), or I branched to a small subroutine
as in this case:
Looper
;do stuff
BEQ Next
;do more stuff
Next
JMP Destination
;more subroutines
Destination
Yes, it may seem a bit sloppy, but it's an easy way to get more distance
out of a branching command. JMP has a heck of a lot more range than a
branch.
>3) Please explain the Stack Pointer with regards to the subroutines.
>What location in memory is it? How big is it?
The stack pointer is generally set to $FF. It counts down, all the way to
$80. $80 is also where you set up your memory variables though. This is
generally not a problem unless you have a lot of memory variables or some
form of recursion in your program.
You have to set the stack pointer to $FF yourself.
>4) Supposing I JSR to a JMP command. Will a RTS send the program
>back to the command after JSR, or will I screw up the program?
Yes indeed. Works fine. (If your asking if one can RTS from a JMP). It
may be a bit ugly, but it still works.
>5) When the Atari powers up, are all the memory locations, flags,
>etc. reset automatically, or in some random state so the programmer
>needs to reset them himself?
All random. It is good practice to set them all to zero. Oherwise WEIRD
things can happen.
>6) Is there a way to produce text for a few words on the Atari, or do
>any words have to be drawn out by the graphics engine (is that the
>right term?)?
Nope. Graphics all have to be drawn by yourself and set in sprites. No
ASCII here.
>Whew! Sorry if I've gone over already trod ground, and sorry if I ask
>so many questions. But any help will be appreciated. (P.S. I will no
>doubt have more questions as I learn, so this is your only chance to
>not encourage me.)
Not a problem at all. Glad to have ya aboard.
If you're just starting out (which I did recently), the first things I did
was find the Stella Manual online, and the "How to Draw a Playfield" info
by Nick Bensema. The best place to get these is at Nick's website.
http://www.primenet.com/~nickb/atariprg.htm
Also, you probably want a copy of vcs.h. It's on my website:
http://www.cae.wisc.edu/~harveyj/atari2.htm
Adios.
-John K. Harvey
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