Another response by Jim Butterfield. I will post the earlier response very soon. Enjoy. -Todd Elliott --------- Forwarded message ---------- From: Jim Butterfield <firstname.lastname@example.org> To: Todd S Elliott <email@example.com> Date: Thu, 22 Apr 1999 11:34:29 -0400 (EDT) Subject: Re: Fw: Re: Modern myths Message-ID: <Pine.3.89.9904221104.B7311-0100000@queen> Good for Andre ... OK, have looked into stuff a little more. My original response can be set loose, but I'll offer the following revision... ----------------------- Looking further at the original PET interfaces: Levak's confusion regarding the PIA chip may stem from this: That although the EOI-input bit he identifies (bit 6 at 59408) has NOTHING to do with the screen ... ... the EOI-output bit (controlled by bits 3 to 5 at 59409) was also an OUTPUT control to blank the screen. Early users of disk drives may recall the screen "blinking" when they wrote to the drive. On the same chip (at 59411, bit 7), the retrace-interrupt flag is found; I don't recall this flag being used for anything. Yes: in early 1979, Commodore changed the cicuitry in this area. To quote from an article by Jim Russo in The Transactor, V2, N1, May 1979 (at that time, The Transactor was published by Commodore) dealing with changes between the original 8K PET and the newly released 16/32K PET: " - The signal which blanks the video on the 8K is not connected on the 16/32, so POKE 59409,52 no longer works. The ROM routines still reference this address but the required hardware seems to have been omitted". Note that we're talking about an output bit here, which has nothing to do with detecting whether we're in retrace mode or not. As I previosly noted, that's on the VIA at 59406. I would have thought that a self-styled qualified engineer would have noticed the difference between input and output circuitry. The video-blanking circuitry was popular with game-writers .. you could get a good explosion effect by blasting the screen on and off. It was in no way related to the later C64 screen-blanking feature. --Jim ___________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com/getjuno.html or call Juno at (800) 654-JUNO [654-5866] - This message was sent through the cbm-hackers mailing list. To unsubscribe: echo unsubscribe | mail firstname.lastname@example.org.
Archive generated by hypermail 2.1.1.