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Re: Apple II VGA Video Generator Card Project

aiiadict@gmail.com wrote:

the dual ported ram ?




8k RAM with two data/address busses, a select
line chooses which of these two to actually control
the RAM



$400 to $800 =    text OR lo-res screen   1kilobyte

           |===> $7FF

$800 to $1600 =  text OR lo-res screen 2 1 kilobyte

           |===> $BFF

$2000 - $3FFF = hires screen 1    8 kilobytes
$4000 - $4FFF = hires screen 2    8kilobytes

           |===> $5FFF

AUX RAM copies are also needed for 80-column and DHR.


18k dual port RAM?  It only has to be READ
by the card, not written to (unless we make
some hardware based sprite generator that
stores into RAM as well as being displayed)

Alex Freed wrote:



BTW there is no need for dual ported RAM: 500 nS out of every 1 mS 6502
does not acess the memory. That's how the normal Apple video works.
Using modern SRAM it is trivial to read 4 bytes in 500 nS (double scan
rate AND double hi-res or 80 columns).



I understand the scheme, but will it work?

Grabbing the video data off the bus (according to which address
is being accessed by the CPU), and directly sending it to the
VGA screen...


He's still talking about buffering it (as it must be) but
using a fast single ported SRAM to synthesize two (or more)
slower ports.


The apple II screen is refreshed at 60hz.  What does a VGA
refresh at?


60Hz is fine.


loop
wait for video address on address bus
get byte from databus
send to VGA
goto loop

Won't the VGA screen refresh several times while the
video generator in the apple II is accessing 1 frame?


Typically, at 60Hz and 2x15kHz, each line is displayed
twice, but any other asynchrony is optional.

-michael

Music synthesis for 8-bit Apple II's!
Home page:  http://members.aol.com/MJMahon/

"The wastebasket is our most important design
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