Re: Dating a C-64 board

From: William Levak (wlevak_at_cyberspace.org)
Date: 1999-04-19 19:49:40

On Mon, 19 Apr 1999, Andre Fachat wrote:

> William Levak wrote:
> > Read and write to PET video memory is crontrolled by an interrupt
> > generated by the clock on one of the interface chips.  This same signal
> > (among others) is used to controll screen memory read by the display
> > circuitry.  This signal separates read and write of screen memory so that
> > basic can only access screen memory on it's half of each cycle and the
> > display circuitry accesses it the other half (thirty times per second).
> 
> Ok, now I get what you mean. It's the vertical retrace input that says
> the CPU when the video logic does not access video memory. 
> > 
> > It is possible to disable this interrupt and speed up the PET.  However,
> > when this is done, it results in "snow" on the screen.  Simply put,  the
> > cirtuitry is not quite as fast as the numbers say it should be, and it is
> > necessary to resort to this scheme to produce an acceptable display.
> 
> Yes, this method is possible. But all PET boards but the very first one
> have this fixed. The very first PET boards had a slow video memory, so 
> either the CPU or the video could access the memory, but not both
> in one cycle. Accessing video memory by the CPU when the video reads
> its data then sends wrong video data to the screen - "snow". 
> 
> But all but the first PET computers have this fixed (i.e. they ahve
> faster video RAM that allows interlaced - Phi1-video/Phi2-CPU - access).
> 
The problem is much improved in the later models but not entirely 
eliminated.  If the screen is completely filled and you scroll the screen,
you still get a little snow if the interrupt is disabled.

Bill


-
This message was sent through the cbm-hackers mailing list.
To unsubscribe: echo unsubscribe | mail cbm-hackers-request@dot.tcm.hut.fi.

Archive generated by hypermail 2.1.1.