wfjm.w11/doc/ECO-037-mmu-instruction_complete.md

ECO-037: implement MMR0,MMR2 instruction complete (2022-12-16)

Scope

• was in w11a since 2008
• affects: all w11a systems

Symptom summary

Test 067 of ekbee1 ends in an infinite loop.

Analysis

The 11/70 supports the recovery of MMU aborts in the push phase of a vector flow. The documentation of this feature is scarce and in some places misleading or just plain wrong. What is actually done is quite simple:

• an MMU abort during a push to a non-kernel stack during a vector flow sets bit MMR0(7) to '1' and MMR2 to the vector address.
• a vector push abort to the kernel stack is converted into a fatal stack error.
• all other MMU aborts set MMR0(7) to '0' and MMR2 to the address of the aborted instruction.

The bit MMR0(7) is named instruction complete in the DEC documentation. That's technically correct but highly misleading. A much better name would have been vector push abort, because that's what this flag signifies.

The 11/70 sets MMR0(7) only for aborts, traps and interrupt flows, but not in vector flows started via the trap instructions BPT, IOT, EMT and TRAP.

Changes

To simplify things, the w11 handles all vector flows uniformly, trap instructions have no special treatment as in the 11/70. With this, the implementation of the instruction complete feature is quite simple.

The MMR0(7) bit is loaded in each cycle from the cpu_status state flag in_vecfow if the MMR0 abort status bits are not locked. This ensures that the bit is '1' after a vector push abort and '0' in all other cases.

The MMR2 register is loaded from the virtual address

• at the beginning of vector flow in state s_vec_getpc (VA = vector address)
• at the beginning of an instruction fetch (VA = instruction address)

Hindsight

This is the most mysterious feature of an 11/70. It was only exercised in test 067 of the xxdp program ekbee1 and never used. Because of poor documentation, it took a long time to understand what it really does and what it can be used for. Nevertheless, the goal of the w11 is to be an as precise as feasible replica of the 11/70, and it was time to finally implement this esoteric feature.