RE: 2Mhz Geos64??

From: COPLIN, Nicholas. (
Date: 1999-11-12 02:25:31

>From the examples I've seen /used you can get 50 raster lines of 2MHz
operation on NTSC and 100 raster lines on PAL.  The trial I did gives 40%
speed up on PAL in BASIC, I'm sure that this would help greatly when shift
lots of data during screen scrolls in paint/write/calc....

Having thought conceptually a little more about it:
1) you need to find room to install three routines
a) modify vector tables to new IRQ (turn on)
b) modify vector tables to old IRQ (turn off)
c) actual IRQ handler

2) disk I/O vectors need to point to new routines so as to change to 1MHz
during disk ops and to ~2Mhz when finished, ie three JRSs and RTS

in theory it should be possible for the one program to work on both PAL and
NTSC systems (just sets different raster values on startup)

question remains where to find the code space for this....

- nick

> -----Original Message-----
> From:	Larry Anderson []
> Sent:	Friday, 12 November 1999 2:07
> To:
> Subject:	Re: 2Mhz Geos64??
> Cameron Kaiser wrote:
> > 
> > >>Does Geos128 run at 2Mhz when in 40col screen mode or only in 80col?
> > >
> > >Screen mode doesn't matter.  Anything runnning on the C128, in C128
> mode,
> > >is running at 2Mhz.
> > 
> > No, no. The VIC-IIe can't do 2MHz and keep the 40-column screen updated.
> > Unless the screen is blanked, if you're in 40-columns, you're at 1MHz
> (even
> > in 128 mode). Only the VDC can continue to do screen updates at 2MHz.
> > 
> But with the application of raster interrupts you could achieve a speed
> increase if you can get the processor to do 2 mghz when the video scan is
> not
> on the visible screen.  I recall an article stating it would be about a
> 25%
> increase...  I recall there is a 128 version of Mah Jongg that employs
> this technique...
> -- 
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