[Coco] Next arcade port to CoCo 3

Steve Bjork 6809er at bjork-huffman.net
Mon Apr 9 22:23:21 EDT 2007

The type of up/down scrolling found in Elevator 
Action can be done on 512k color computer 3 
without take any extra CPU time to scroll the screen.

Each of the screens (flipping between the work 
and display) would take 130k of the 512k.  Costly 
on ram but well worth the speed that you would 
pickup without having to  update the whole screen 
every time.  (Note: Marty's Nightmare used this 
trick to speed up the frame rate of the 
game.)  As the screen scrolls down, the game 
would tell the computer to display the memory just that much farther down.

Because the size of the buildings in Elevator 
Action we could not put the whole building screen 
in ram like I did in Marty's Nightmare. (It would 
take more ram then there is on a  512k 
coco.)  But there is at way to do this using ram 
that's only about twice the size of the 
screen.  As the screen scrolls down, you build 
not only the part of the building below the 
screen but also draw the same thing in the top 
part about the screen memory that is no longer 
being used.  (As you are drawing the new parts of 
the screen, a copy of the screen is also being 
drawn first part of screen memory.  Once the 
scrolling takes the screen to the end of screen 
memory, the game would reset the screen display 
pointer back to the beginning of screen 
memory.  This let's the coco keep scrolling and 
scrolling without rebuilding the full background 
image every game frame (like in Super Pitfall), there by speeding up the game.

While I have only talked about scroll down, this 
type system also works for scrolling up too.  (Like in Elevator Action.)

Also, the speed of a 3 MHZ Z-80 is less than 1/2 
the speed of the 1.79 MHZ 6809 on the CoCo 3.

The Z-80 takes about 3 or 4 clocks to fetch a 
byte of memory, but the 6809 only needs one clock to do the same work.

Not only can Elevator Action be done, but it 
could have a faster fame rate that Sock's Donkey 
Kong, that runs at 15 to 19 frames per 
second.  (if I remember right from what Sock said.)

Remember, almost all Arcade Video Games run at 60 
frames per second.  (most CoCo games run at 12 to 30 frames per second.)

Oh yes, any sound will also slow down a game's 
frame rate.  Super-Pit sound and music system 
took about 25% of the CPU time.  (and slowed the 
game down by 25%.) That's why I included an 
option to turn off the Music in Super Pitfall so 
it could run at a more playable frame rate.

I'know that in one the chat rooms I said that 
it's too much work for the coco to scroll a 
screen.  I was talking about full scrolling, not 
limited scrolling along one axis.  A game with 
full scrolling like Super Pitfall, it is very 
hard to scroll.  But in the case of limited 
scrolling like Elevator Action, it can be done easily!


At 05:43 PM 4/9/2007, you wrote:

>Marcus Vinicius Garrett Chiado wrote:
>>The very next arcade port I´d love to see is ELEVATOR ACTION.
>>Do you think our CoCo can handle it?
>IMHO - and I'd love to be proven wrong - I'd have to say no.
>The main CPU is a 4MHz Z80. Sound involves a 
>3MHz Z80, another MCU and no less than four (4) 
>AY8910s - so definitely no sound except for 
>samples I suppose. I bit much to ask of a 1.79MHz 6809...
>The biggest problem I see is scrolling large 
>areas of the screen as the player moves up and 
>down in the elevator - something done in hardware on the machine itself.
>Regards,Mark McDougall

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