Somewhere here in my stuff I have a manual and schematics for a teaching box designed and sold by my former instructor around 81/82. It was meant to replace the ET3400 we used at the time. If you're interested I can dig it up and loan you it in exchange for a scan. I believe it was also based on the 6800 like the Heathkit. ciao larry Reply to: lgwalker(a)look.ca

Neat. I occasionally spend a little time working toward building myself a simple processor. My priorities are rather different, though; I want to make the 'moving parts' visible as far down as is practical and affordable (which for me will probably mean SSI TTL, except for the memory), rather than to make the programming model simple. Could one build "nixie tube emulators", for instance using seven-segment LEDs, and then borrow real ones from some other device for special occasions? I've tentatively decided that I'll go for a store-and-clear operation, ? la EDSAC and PDP8, since, in the few sample programs I tried, the value in the accumulator was still needed after fewer than half the stores. This might conflict with your goal of simplicity, but on the other hand, the question of how to retain (regain) the accumulator after a store might make a good 'Exercise 1'. Alternatively you could clear with a STORE,SUB pair, at the cost of a scratch location. After several partial designs than included IN and OUT operations, I decided to punt I/O almost completely. In my current plans, one operation can take the switch register as source, and the accumulator lights (and/or other lights) are the only output. I have provisionally punted the CALL, too, as a separate operation; instead, the same operation that can load the switch register into the accumulator can similarly load the PC. Unfortunately my current design requires the PC to then be adjusted explicitly in the subroutine. I'm still at the stage of designing and "building" fragments in a simulator, since I have no real experience; I keep making things too complicated, and then backtracking to simply again. (And I don't work on it very often.) My current plan is roughly an 8-bit, stripped-down-PDP8-ish architecture: [0|0|0| | | | | ] halt [0|0|1|u|_|_|n|z] skip if u = U then PC := PC + 1 fi where U := (n = sgn(AC)) and (z = (AC=0)) [0|1|0|_|src|c|s] lac AC := if s then rshift(T) else T fi where T := if c then ~S else S fi where S := {1,AC,PC,SW}[src] [0|1|1| | | | | ] - [1|0|0|i|p| m] and AC := AC & M[MA] [1|0|1|i|p| m] add AC := AC + M[MA] [1|1|0|i|p| m] dca M[MA] := AC, AC := 0 [1|1|1|i|p| m] jmp PC := MA where MA := if i then M[TA] else TA fi where TA := if p then PC+signext(m) else zeroext(m) fi Any bits in the instruction may of course end up complemented depending on the nature of available gates :-) I want to hard-wire this one; if I eventually finish it, next will come a 16-bit (or perhaps 12- or 18- or 24- bit) device with a more sophisticated instruction set and wide dumb microcode in EEPROMs. I do have a couple of devices in a similar spirit, both refugees from the UW EE dept. One is an 8-bit processor, built from TTL on an 8"x14" prototyping board, with point-to-point soldered wire. The ALU is a pair of 74181s, and seems to use 7475s for the registers. There are five sets of 8 LEDs -- memory address register, program counter, instruction register, memory, and accumulator -- plus seven state LEDs, all labelled with pen on masking tape. There are [missing] eight data switches and eight control switches: halt/run, halt/step, halt/step cycle, deposit, examine, "pcl" (pc load, presumably), "acl" (accumulator load), and continue. There's also a jumper area. Unfortunately the device is damaged; besides the missing switches (and no sign of where they were wired in), some of the LEDs are broken or torn off, and a few wires are hanging loose. The wiring is mostly tied in tight bundles between the rows and columns of ICs, so it won't be trivial for me to trace it out fully to repair it (and determine the instruction set). I don't have any documentation. I do have a schematic for (and not the device) an even lower-level 8-bit processor in TTL, pushing everything through a bit-serial adder. A somewhat related 'thing' I picked up last week is some sort of logic trainer. In a clear plastic frame about 12" x 20" x 6" thick, it has six *huge* flip-flops (built with power transistors) each with a pair of output lamps, a jumper area for "programming", and input in the form of a telephone-style dial. (I assume people here know what a telephone dial is :-) Haven't traced this one out yet either, but at its scale it won't be hard. -- Kevin Schoedel schoedel(a)kw.igs.net