I managed to test the version of MCMON for Digital Group Video card on real hardware, but there was a bit of drama involved. I started seeing corrupted characters when running Hangman with this card. From the beginning it looked like a memory issue.

First, I ruled out the problem being in the SCLEBI system memory by stopping the program and examining the local copy of the video buffer which looked correct. Then I took some time to isolate exactly what was wrong with the Digital Group Video Card memory. After closely examining the symptoms and running some simple tests, I determined that the most significant bit of video memory had a problem with adjacent locations. In other words, if I wrote MSB of address 0 to zero or one, then MSB of address 1 would also change to the same value. Same thing happened when writing to address 1. This was true of all pairs of memory locations where the upper six address bits (of seven) were the same.

The problem was finally tracked down to a pin that wasn’t soldered which left this address line floating. This turned the 256 bit SRAM chip in that location into a 128 bit chip. Adjacent memory locations were sharing the same data in this chip, accounting for the error.

I didn’t see this problem when testing a full array of characters displayed on the screen or when running MCMON because in those cases all adjacent characters share the same value of the MSB.

With this problem resolved, I was able to verify that Hangman with Digital Group video drivers worked correctly. This software can now be downloaded from my 8008 application page.

My MCMON (8008 mini-monitor) with Digital Group Video support can be downloaded from my MCMON page.