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LCORE - Tiny pipelined 16b CPU

Daniel Kasza - daniel@kasza.hu

The CPU uses a 3-stage pipeline:

  1. IF - Instruction Fetch
  2. DE - DEcode
  3. EX - EXecute

Branch prediction is not implemented, so branches are always treated as not taken. When a branch is taken, the EX stage tells the IF stage which instruction to fetch next, and the DE stage invalidates the instruction that was fetched on the previous cycle by replacing it with a no-op. This means that taking a branch has a 1 cycle penalty.

Source registers are fetched during the EX stage.

Differences from LC-3

LCORE is based on the LC-3 ISA with some changes:

  • STI and LDI instructions are not supported because they do not fit in the pipeline.
  • Traps are not supported.
  • Memory mapped I/O is replaced with a dedicated I/O port.
  • NOT instruction is replaced with a more generic XOR instruction. NOT is a special case of this.
  • Reserved opcode is used to implement additional instructions.