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.
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