As I build the instruction assembler and disassember, I must also provide a way to store instructions. This was already covered in #YGREC16 - YG's 16bits Relay Electric Computer in the log PROM boards.

Things have evolved since 2017 and I now have a whole bunch of DIP switches (Log: I hope that's enough this time) and pretty large proto boards... I have just what is required to build boards with 16×4=64 instructions :-)

But let's look at how these boards must be organised. The YGREC8 has 16-bits instructions so a higher density than the 24 bits of the YGREC16. And there are only 256 addressable instructions. That's 256×2=512 DIP switches, I have more than 700 in stock these days :-)

Then the topology must be optimised to reduce the number of relays : how many lines and columns ?

• 1 column and 256 lines : 1 huge MUX256, or 255 relays. HUGE...
• 2 columns and 128 lines : 1 huge MUX128 and 16 relays to select the column, or 143 relays. Better but still not optimal.
• 4 comumns and 64 lines : 1 large MUX64 and 16×MUX4, or 63+(16×3) = 111 relays. Good.
• 8 columns and 32 lines : 1 MUX32 and 16×MUX8 = 31 + 16×7 = 143 : the column MUX gets too large now.

So be it : the PROM cards are organised with 4 columns.

I'll try to build a 8×4 board to get started... because I also need to build the 2 big MUXes for lines and colums. Where are my boxes of SMD diodes ? :-D