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wfjm 1644863a58 pdp11_mmu.vhd: BUGFIX: correct trap and PDR A logic

- tools/asm-11/lib/defs_mmu.mac: rename md.a??, saner names for ACF
- tcode/cpu_mmu.mac: add E1.1, test m0.trp, pdr aia/aiw transitions (verify fix)

Closes #34
Closes #33
Closes #26
Closes #25

2022-09-09 09:01:06 +02:00

1.9 KiB

Raw Blame History

ECO-033: MMU: ACF=1 trap and PDR A fix (2022-09-07)

Scope

was in w11a since 2009
affects: all w11a systems

Symptom summary

part 1: ACF=1 traps on any access
Test 055 of ekbee1 fails with

    MEMORY MANAGEMENT TRAP OR ABORT HAD INCORRECT CONDITION
    EXPECTD ERROR   AUTOI/D VIRTUAL
    CONDITN REGISTR REGISTR ADDRESS TESTNO  PC AT ABORT
    020011  030011  013427  054032  000055  054040

part 2: PDR A bit is set for every access
This was discovered in a code review. The PDR A bit was set for all accesses. The PDR A bit should be set only when "trap condition met by the Access Control Field (ACF)" is fulfilled. Thus for
```
   ACF=001 read-only    trap and A bit on read
   ACF=100 read/write   trap and A bit on read or write
   ACF=101 read/write   trap and A bit on write
```
ekbee1 only checks whether this bit is set when expected, but does not verify that is stays '0' when it should.

Analysis

part 1: ACF=1 traps on any access Caused by a simple mistake in the ACF handling in pdp11_mmu.vhd

    case PARPDR.acf is                -- evaluate accecc control field
    when "001" =>                     -- read-only; trap on read
      if CNTL.wacc='1' or CNTL.macc='1' then
        abo_rdonly := '1';
      end if;
      dotrap := '1';                  -- <== BUG, should be 'not write'

part 2: PDR A bit is set for every access Caused simplistic AIB handling in pdp11_mmu.vhd

      if doabort = '0' then
        AIB_SETA <= '1';              -- <== BUG, should be 'dotrap'
        AIB_SETW <= CNTL.wacc or CNTL.macc;
      end if;

Fixes

part 1: AIB_SETA <= dotrap;
part 2: dotrap := not iswrite;

Hindsight

Took 13 years to fix. The MMU traps and PDR A and W bits are 11/45 and 11/70 specific and not used by any operating system. Only tests like ekbee1 use this functionality.

1.9 KiB Raw Blame History