added, untested, however may have a block when both zero and immediate set.

(In reply to Luke Kenneth Casson Leighton from comment #1)
> added, untested, however may have a block when both zero and immediate set.

it's fine.  unit test modified as follows:

+    rd = 0b00
+    if not imm_ok:
+        rd |= 0b10
+    if not zero_a:
+        rd |= 0b01
+
+    if rd:
+        yield dut.rd.go.eq(rd)
+        yield from wait_for(dut.rd.rel)
+        yield dut.rd.go.eq(0)

cesar this one likewise needs similar tests to the MultiCompUnit, might even
be able to share the same base classes at some point.

again it needs that "multi-process" simulation model:

* one function doing setup, setting oper_i, issue_i, then monitoring busy_o
  before exiting

* another that checks busy_o and, when set, proceeds to monitor a single
  bit of rd.req, sets the corresponding bit of rd.go for one cycle, then
  monitors busy_o before exiting

* likewise for write requests.

this is exactly the same as needed for MultiCompUnit (up to this point).
the additions obviously are to have extra functions that:

* monitor req_adr_o and set go_adr_i
* monitor req_st_o and set go_st_i
* check the address and the data afterwards