Re: [stella] How's my arithmetic?

Subject: Re: [stella] How's my arithmetic?
From: emooney@xxxxxxxxxxxxxxxx (Erik Mooney)
Date: Fri, 14 Aug 1998 20:37:47 GMT
>After VBLANK and before beginning to draw the screen in Nick's HtDaP, I 
>_think_ I have #44 times 64 clock counts, or 2816 total clocks for game 
>calcs, etc.  That would be about 938.7 machine cycles if I believe page 2 
>of my fancy Stella Programmer's Guide where it says "3 CLOCK COUNTS PER 
>MACHINE CYCLE".  Now that only comes to 12 lines and change (938.7 div 
>76) worth of time for game calculations...  Since there are 37 lines of 
>VBLANK, that seems a bit small.  What did I do wrong?

There's two kinds of clock counts.  The 6507 CPU clock runs at 1.19 MHz;
the graphics clock runs at 3.57 MHz, or three times the CPU speed.  In
other words, three pixels get drawn on the screen during one CPU cycle.
Your #44 times 64 clock counts is correct... the timer uses CPU clocks, not
graphics clocks, so it's 2816 machine cycles which is (div 76) 37-some-odd

>Secondly, there isn't anything about the overscan that's tricky, is 
>there?  Meaning I can treat that exactly like it's 30 lines of normal 
>screen scan, correct?  Is the gun still shooting the screen?  Could I put 

The gun is always shooting as long as VBLANK holds a nonzero value.  You
can treat it as normal screen scan, yes.  The only catch there is that
after 5 to 25 lines of overscan (the exact number depends on the TV set),
the electron gun is no longer pointing at the phosphored screen - it's
aiming at the plastic below there (or wood if you have a really old set),
so anything you draw won't show up.

Same holds for vertical blank - you can start drawing before 37 lines have
passed and it'll show up higher on the screen, but too high and it'll be
off the top of the screen.

>two 1702 monitors together and get a really nice long screen and make 
>Millipede the right size?  (Just kidding about the last question, of 

I don't think so :)

Archives (includes files) at
Unsub & more at

Current Thread