[fpga] microcore POC that just increment the program counter

[sporniket]

Let a fictitious CPU having :

• a simulated Bus driver to communicate with external peripheral (data, address, controls). Job completion takes at least N microcore cycles to complete, N ≥ 4.
• a decoder, for now everything decodes into NOP.
• a register file, containing only the Program Counter (PC)
• a microcore ROM that contains the microcore program
• a microcore with an internal PC
• an address ALU ADALU

steps

Power on

The register’s PC is zeroed, as well as the internal PC.

Main loop

• the register file expose the PC towards the Bus driver and the address ALU as first operand
• the immediate value 2 is exposed to the address ALU as second operand
• the operation of the address ALU is set to ADD
• the address ALU output is exposed toward the register file into the PC
• the microcore wait for the Bus driver to be ready
• the bus driver expose data fetched toward the decoder
• the decoder expose the new internal PC toward the microcore

Pseudo micro code

start: 
  move.l reg(PC),Bus(1)|ADALU(1) ;Bus(1) = address + execute on write ; ADALU(1) = first operand
  move.l #2,ADALU(2) ; ADALU(2) = second operand
  move.l #ADD,ADALU(3) ; ADALU(3) = operation + execute on write
  move.l ADALU(0),reg(PC) ; ADALU(0) = result
  wait Bus
  move.l Bus(0), Decoder(1) ; Bus(0) = data fetched ; Decoder(1) = opcode
  wait Decoder
  jmp Decoder(0) ; Decoder(0) = micro-rom address, returns 0