The final change to OPC-1: we removed SEC, because we had to fix a bug in the verilog, and then the machine didn't fit the CPLD, so something had to go. No great problem though, because with our macro assembler we can have an SEC like this:
MACRO SEC() lda.i 0x01 lxa ENDMACRO
You can run an emulation of OPC-1 in your browser: see here for a trivial program, or here or here for longer programs. (It's an emulator without many features, because, of course, it fits on one page.)
Our next mini-adventure was to see about an OPC-2 - instead of an accumulator machine, how about a load-store machine? More registers, operations on registers, and the only memory accesses to be loads and stores.
Unfortunately, although we did manage to make a working machine, fitting in the CPLD and again with all sources on one page, it didn't turn out very satisfactory. The CPLD is so small we could only have two registers, and the address space shrunk again to just 10 bits. We could only afford load and store from one of the registers. We managed a conditional jump and a jump-and-link, so again one can manage subroutines. But there was so little room JAL takes two bytes even though the second byte contains no information.
At this point we've run out of room in the CPLD, and it's time for a new project. Although we might come back to OPC-1 and get it running in a breadboard - so far we've been exploring with synthesis, simulation and emulation.
For one last gasp, we wrote an OPC-3, which is a simple-minded expansion of OPC-1 into a machine with 16 address and 16 bit data. We quite like word-addressed machines, so this is one of those: addresses are not bytes! There's no simple way to access bytes although of course you can always shift and mask. But a benefit of this size of machine is that a value, whether in register or memory or as an operand byte, is always big enough to specify a full address.
OPC-3 is a bit too big for our CPLD and a bit too simple to be impressive. The instructions have lots of unused bits. But it leads us to think of interesting directions for the next project.