I had often run D.E. WREGE's version of Spacewar! on my PDP-12. But when I tried to run it on my PDP-8/E with a VC8/E and DEC VT01 (Tek 611), I would get flashing artifacts on the screen in addition to the normal display. I initially thought I had a hardware problem - but I could only get my VC8/E and VT01 combination to fail while running DEC's VC8/E diagnostics by setting the timing switch on the VC8/E incorrectly. When I initially looked over the VC8/E code in SPCWAR, it looked O.K. and I thought the problem was likely in the clock or interrupt code. After reviewing it extensively and not finding an issue, I tried multiple hardware clocks (EP, EC) and various VC8/E settings without success. Finally, I revisited the VC8/E code in SPCWAR - and the problem turned out to be a subtle bug in the VC8/E code. According to DEC's VC8/E writeup in the LAB 8/E manual, a "wait for done" should be given AFTER the last LOAD command (X or Y) is given and BEFORE the intensify command. In other words the code should be: TAD X /Get X coordinate DILX /Send to VC8/E TAD Y /Get Y coordinate DILY /Send to VC8/E DISD /Skip on done JMP .-1 /Wait until done DIXY /Display (Intensify) XY point ... The original SPCWAR code it was: Get X DILX /Send to VC8/E Get Y DILY /Send to VC8/E DIXY /Display (Intensify) XY point DISD /Skip on done JMP .-1 /Wait until done Because of the above coding error, the VC8/E analog output would not "settle" the correct amount of time (which for a VT01 is about 80 microseconds) because a DIXY command was given immediately after the DILY. This error created artifacts on the screen because the intensify command was in effect while the VT01 was still in the process of moving it's "beam" to the correct location. BTW: Slower displays, such as the VT01/Tek611/Tek613, do NOT need a clock to slow down Spacewar to an appropriate speed. If you would like a copy of the corrected SPCWAR.PA or a listing (SPCWAR.LS), you can get them via anonymous FTP to bickleywest.com or via your browser at ftp://bickleywest.com/pdp8_spcwar/ While I was at it, I also added an assembly option to start SPCWAR in Spaceship mode (per the original Spacewar!) as opposed to UFO mode. Cheers, Lyle -- Bickley Consulting West Inc. http://bickleywest.com "Black holes are where God is dividing by zero"