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Exidy Experiment

The Sorcerer has 128 user-defined characters in 1K of SRAM. That's great for most games which use tiled displays but it's not enough to cover the whole screen with arbitrary graphics.

I have some 16Kx4 SRAMs which are mostly pin-compatible with the original 1Kx4 chips, so I plugged them in along with a mux chip to drive the 4 extra address lines. During video access, those lines come from the video line counters. During CPU access, they come from a parallel output port. In this prototype, it's the keyboard scan port. So the bitmap display consists of 15 1K banks at the normal addresses FC00 - FFFF. The software paints the screen with 15 copies of the user character set and then it switches banks and fills each with a slice of the image. This test image is a monochrome BMP of Wikipedia's photo of a Sorcerer.
 
The Z80 runs at 2.1 MHz. Loading a 12K .BMP file into main RAM takes 7 seconds at 20 kbaud with my TapeSim. Then the 36-byte loader program uses 12 LDIRs to render it in less than 0.2 second.

I have not written any line drawing or fill routines yet.
 
Imagine what such an upgrade might have been like in 1979. 30 1Kx4 SRAM chips plus a few logic chips. Maybe 2/3 the size of an S100 board.
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It could have plugged into the rear expansion port. And some ribbon cables with dip headers to tap into the display SRAM sockets. It might have cost $400, half the price of a Sorcerer.

By 1982 it could use 2Kx8 SRAMs and be small enough to fit inside the case as a daughter board. Maybe $200 then.

Now it could use a single 32K or 128K SRAM and have multiple display pages. Or it could use a modern microcontroller to simulate RAM and include a high speed blitter.
 
Neat.

Hijacking the character generation system to wedge graphics into a text system like this was a pretty common approach, but this is certainly a unique take on it, in how the Sorcerer already *had* definable graphics characters, you just needed to add more memory and an automatic line-based bank switcher.
 
A plot using the algorithm published in a BYTE article from August 1977 called Serendipitous Circles:
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