Well, I did it again - build another 8-bit home computer. The first one was Ben Eater's now rather famous design based on the SAP-1 created by Albert Malvino and Jerald Brown and captured in their book "Digital Computer Electronics." The idea being, use DIP logic chips as basic building blocks and build all the functional components of a Von-Neumann styled computer. The purpose was to have a upward compatible design that any more elaborate computer design would still have to have to work. Ben Eater built a SAP with a bunch of breadboards as an educational device. The PC only has 16 counts so only 16 memory spaces can be accessed and the only functionality is to add or subtract two 8-bit numbers (minus numbers, only 7-bits). But all the steps a real computer goes through to do that is represented in the design including a niffty output display.

The construction was a challenge but guided by excellent videos how to build each subsystem on a particular breadboard was presented by Ben only as a talented teacher could do. Within a year, many people posted their attempts to reconstruct this project.

There have been two significant advances in the Ben Eater baseline. Carsten Herting created the Minimallistic CPU, which really advanced what can be done on the software side by compressing the number of control lines to stay within 16 while expanding the baseline SAP to 16 bit PC and memory addresses and still just have an 8-bit main bus going between all the components. That means sending out LSB and then MSB (Least Signigicant Byte and Most Significant Byte) to form a 16-bit address or data word. Carsten really went beyond the call of duty by creating an OS for this little computer and providing several apps to demo its capabilties including a version of Pong and Tetris.

So what could be added to the SAP at this point? Kaveh Majidi has created a design as well as a kit to bring the design to the hands of interested hobbyists that has some really nice additions to the SAP concept by modularizing the components to separate PCBs and interconnecting them via edge connectors so the boards are stacked and the address, data, and control signals are available on each board. Also, the platform board on the bottom also has pinout for an Arduino Mega2560. This is used to exercise each board separatly as you solder them and test them. The Arduino sketches are a real insight to how the system works. The clocking can be slowed down to see all the bus activity on the top PCB that is full of LEDs for the A and B registers, main bus, address bus (now 16 bits), the flags, and the control signals. The kit is available at www.8bitkits.com.

What I've done here at Wolf Den Electronics is bought a kit and present the results in the Projects section. Check it out.