[Coco] Next arcade port to CoCo 3
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.
No virus found in this outgoing message.
Checked by AVG Free Edition.
Version: 7.5.446 / Virus Database: 269.0.0/752 - Release Date: 4/8/2007 8:34 PM
More information about the Coco