From: Nicolas Welte (welte_at_chemie.uni-konstanz.de)
Date: 2002-04-16 15:46:09
Marko Mäkelä wrote: > > On Tue, 16 Apr 2002, Nicolas Welte wrote: > > > It seems the VSYNC signal is also needed for the computer part to > > start up (IRQ source, IIRC), so this machine also showed the dead > > machine symptoms. > > True, VSYNC is the source of the periodic system interrupt that reads the > keyboard and updates TI. I don't know what other things it could do. But > shouldn't the PET display the start-up screen when this IRQ source is > disabled? It just wouldn't respond to the keyboard. I think it didn't show any start-up screen, because I had a multisync monitor attached, via my userport-to-DSUB adaptor. There I should've seen a rolling start-up screen, but I only got an empty raster. Doesn't the same happen to the C64 as well, if you remove the IRQ-CIA, it starts up with an empty screen, with the correct colors? I know for sure it happens to the CBM 710 if you don't supply the 50Hz signal from the powersupply. > I think that there must have been other problems as well in that machine. > Maybe the ROM code hung in some place where it polled the state of the > video circuitry to see when data can be safely written on the screen > (remember the "smoke poke" discussions a few years ago?). Yes, the screen output routine waits for the vertical refresh so it doesn't interfere with the video output. Could explain the behaviour very well. But why does it also happen on the C64 and the 710? I don't think those wait for VSYNC. Nicolas Message was sent through the cbm-hackers mailing list
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