ece222: finish datapath

docs/2a/ece222.md

@@ -103,5 +103,11 @@ To execute an instruction, the following steps are observed:
   - decode fields like the op-code, offset
   - read recoded registers
 3. Execute (EX)
-  - ALU shenanigans
+  - ALU calculates ADD, SUB, etc, as well as addresses for LDR/STR, sets zero status for CBZ
+  - branch adder calculates any branch target addresses
+4. Memory (ME)
+  - if memory needs to be reached, either `Write` or `Read` must be asserted to prepare for it
+  - write to memory
+5. Writeback (WB)
+  - write results to registers from memory, the ALU, or another register