The SPAM-1 instruction is 48 bits wide and is spread across 6 program ROMS that are used in parallel.
The address lines of the program ROM are hooked up directly to the 16 bit program counter output.
The program rom supplies:
- configuration to the control logic
- optionally, direct 16 bit RAM addressing
- optionally, an 8 bit immediate value into the ALU
Connectivity to the ALU or the address bus is via 74HCT245 tristate buffers.