Fix IBM's silly memory mapping value engineering and make Tandy 1000 games run without graphical issues on the PCjrDesigned by ZiggyTheHamster's Shop in United States of America
What the TandyMod+ Does It makes Tandy 1000 applications running on the IBM PCjr that look like this: Look like this instead: What the TandyMod+ Does In Detail It's well known that the Tandy 1000...Read More…
It makes Tandy 1000 applications running on the IBM PCjr that look like this:
Look like this instead:
It's well known that the Tandy 1000 is a clone of the IBM PCjr. While cloning the PCjr, Tandy fixed a number of silly bugs that IBM left in (probably to make the PCjr incompatible with the IBM PC). One of those bugs is how the video memory is used. I don't completely understand the bug, so this explanation may be a bit inaccurate, but when in 320x200 16-color mode, you can only access the first 16KB of video memory at BC000 (B8000 is just a repeat of BC000) even though there is at least 32KB of video memory. There is some other way to access the other half of the 32KB video memory, which PCjr applications have to use, but this makes programming much more difficult. On the Tandy 1000, BC000 to BFFFF is accessible as a contiguous 32KB chunk. This mod fixes this. The end result is that Tandy 1000 applications that normally have a Venetian blinds defect (black bars every few lines) will now display correctly. There are only a handful of PCjr applications which don't work with this mod enabled, which is why there is a toggle jumper.
This mod is an iteration on the traditional PCjr Tandy Mod, which has been floating around in newsletters and magazines since the Tandy 1000 became commercially successful. The mod is a simple logic change so that the correct page of video memory is loaded into the CGA chip and was done with some basic 74 series logic chips.
The original creator of the more complicated logic used in the new mod was Alan Hightower, the person who made jrIDE. Alan made logic for a programmable logic device, which is difficult to program without specialized equipment. I took that logic and reformatted it to work on a memory device using a simple Ruby program. The address lines of the ROM are the logic inputs, and the data lines of the ROM are logic outputs. The new mod has a toggle to enable/disable the mod and fixes an issue where the old Tandy Mod would map the wrong bank in certain instances.
The mod comes pre-tested and ready to go. You just need to remove the ZM31 chip and install the supplied socket. Please be aware that these use standard header pins and as a result will not insert into round pin sockets. You can find the ZM31 chip by referencing page B-19 of the PCjr technical reference.
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