78e033cda7
- rtl/w11a - pdp11_mmu.vhd: some logic cleanup - pdp11_vmbox.vhd: BUGFIX: request mmu trap also on ib accesses - tools/tcode/cpu_mmu.mac: add D2.1 (vector abort recovery demo) and E1.5
1.1 KiB
ECO-038: fix MMU trap after IB access; MMU logic cleanup (2022-12-18)
Scope
- was in w11a since 2008
- affects: all w11a systems
Symptom summary
When the IO page, in kernel mode by convention page 7, was mapped with MMU traps
enabled in PDR and MMR0, the AI bits on the PDR and the trap_mmu bit
in MMR0 were set, but no MMU trap was taken.
Analysis
The error was in pdp11_vmbox, the VM_STAT flag trap_mmu was only set in
the state that handles memory access, but not in the states that handle ibus
access.
Further code inspection revealed that the pdp11_mmu code handling traps was
functionally correct but not well structured.
Changes
The trap_mmu flag is only inspected in pdp11_sequencer when ack is set.
It is therefore safe to set trap_mmu in all states of the pdp11_vmbox FSM.
Some logic in pdp11_mmu was restructured and is now more compact, but
stayed functionally equivalent.
Hindsight
The MMU traps are not used by any OS, and therefore were not tested very thoroughly. This bug was only discovered a few months ago and resurfaced when the 'vector push abort recovery' demonstrator was written.