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A project log for The 74's called ... (SUBLEQ)

Yetanother simple CPU of TTL chips.

Michael MöllerMichael Möller 06/06/2020 at 20:180 Comments


I now unconditionally copy Register A into Result (what 2nd argument points to) to test my write back.

The "bus" in the middle will have LEDs for all 24 lines, when I get to a CPU project with 8 data 16 address. At the moment 4 bit databus.

I have reaaranged a little (as described in previous log) but I am NOT happy with the layout as a whole. Implementation is "organic growth", but I do not want to rearrange everything or every step. The jumperwires are for temporary fixes (like LEQ tied low, as there is no SUB to generate the signal yet, but I can still test how it would work on the µ-sequencer)

Ooops ?

At this point I discover a few (more) errors in the design <sigh>

  1. µstep5 needs the SXP else it loads random data. 
  2. Total misunderstanding - I have indirection on the jump, too, ie it jumps to the location of the content of what the 3rd arguments point to. Great for subroutines and such like, but very difficult when you do not want a jump, even if the result is negative (which is quite often).
  3. Using "0" as the special location required a multi input NOR, which required too many chips, so that changes to "-1". (I afterwards noticed this was assumed by some other emulator implementations, too). 
  4. There was no HALT condition in the diagrams, only loosly planned in my head
  5. The INP is LSB on left, LEDs are LSB on right.
  6. POTENTIAL - Unsafe conditions like DOD & WRE ( & DATA ) changing simultaneously  - ignoring  it for the while being

Updated design

Block schematic 

RAMMemoryADDRAddress to memory
SELSelect address sourceDATAbidirectional data (a bus)
INPInput, currently DIP switchSXPSelect XFR or PC as source for ADDR
OUTOutput Buffer (goes to display)LDPLoad PC with new value
XFRTransfer Buffer, for the indirect argumentsINC'Increment PC
PCProgram counterIODInput/Output-Detect, ie address is -1
AregA registerLDX'Load the XFR register
BregB registerDODDataOutputDeny (=not RAM on Databus)
SUBSubtraction unitWREWrite Enable
In these schematic the signals are all positive. (In the circuit half are negative=true)

The '-mark is for edge triggered (otherwise they are static levels)
ENIEnable Input (INP on databus)
ENREnable Result (SUB on databus)
LDA' & LDB'Load A and B register, respectivly
LDOLoad Output Register
LEQLess-or-Equal result in SUB


Signals activation by step above the box, with hollow arrows show the alternative on a condition being true. The listing shows data flow, and the last is the special event when pressing the "load"-button (two pulses generated, not the clock)


This shows the complex truth with negative/positive and static/edge logic mix. The output suffixes mean "~"inverted, "+"ve and "-"ve edge. The LD1 and LD2 are the two pulses from the manual Load button. The counter reset takes away the 8th redundant µstep, and I should do it for the nonbranch 7th step, too,  but I've run out of room for more gates.

Next Planned Step(s)

in the probable order

And when it runs, extend to 8 bit data/addr and loading of programs from Arduino