Re: My ColourPET Project

From: Nils Eilers <>
Date: Fri, 12 Apr 2013 08:14:16 +0200
Message-ID: <>
> I was hoping to do an 80 column version eventually, but I was basically
> going to duplicate the existing ram circuits (odd and even) as my colour
> ram and map them into $8800-8FFF. So basically the CRTC controller would
> read all 4  video ram blocks at once.
That's impossible due the lack of bandwidth. You cannot read the 
required number of bytes at a time with the given number of memory 
buses. André described it here:

> I'm not sure I understand how
> dual-port ram would work in this situation.
A dual-port RAM is basically just a normal static RAM: address lines, 
data lines, chip enable, read/write:
What makes a dual-port RAM a dual-port RAM is having two instead of one 
memory bus. You've got two sets of address and data lines that are 
completely independent but share the same amount of memory inside.
This is the solution to the lack of bandwidth: connect one port to the 
CPU and map this 2K to where you'd like to have your color RAM. Connect 
the other port to the CRTC, addressed by the video RAM addresses but 
shift the address lines by one: A0 (CRTC) --> A1 (RAM), A1 (CRTC) --> A2 
(RAM) and so on. Connect the line that selects the even/odd latch as A0 

On the downside, this 80 columns color patch requires a lot more wires 
than the 40 columns version ;-)


       Message was sent through the cbm-hackers mailing list
Received on 2013-04-12 07:00:03

Archive generated by hypermail 2.2.0.