Saturday, November 24, 2012

Starting the Instruction Decode board

If I'm going to have boards to work on by Christmas I need to get them submitted in the next week or so. It's not the highest thing on my priority list, but I did make a start this week.

I started by beginning the layout of the Instruction Decode board. Here's a snapshot of its current state, with about 80 components yet to be placed and essentially no routing done:

The schematic shows the OPR decoder above the OPA decoder, with the final combinational logic below that. This makes sense if the ALU is the next thing down, but in my implementation the ALU is on another board reached through the smaller connector. This led me to rearrange the groups a bit so the final combinational logic is between the OPR and OPA decoders. The groups of components nearest the larger connector form the OPR and OPA "registers"; since these registers are updated for each instruction cycle, transmission gates capture the state of the data bus on MOSFET gate capacitances rather than using flip-flops.

The structure of the combinational logic doesn't lend itself to tight packing, so I went ahead and repartitioned the design as described in the post A little re-partitioning. This moved the 61 components of the ALU timing circuits off the Instruction Decode board and onto the ALU board where they belong. To avoid making the ALU board more dense, I moved the 60 components of the external chip select logic off the ALU board and onto the Timing and I/O board.

Not bad for a day's work.

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