DECSYSTEM 2020 RH11-TM02/03-TU16/TU45/TU77 RELIABILITY DIAGNOSTIC(DSTUB) ACCL failure at Write Forward startup

[ks10fpga]

DECSYSTEM 2020 RH11-TM02/03-TU16/TU45/TU77 RELIABILITY DIAGNOSTIC(DSTUB) ACCL failure at Write Forward startup

[jfcl]

The error message in mentioned above is completely incorrect and misleading due to a latent software bug in the DSTUB diagnostic.

The actual failure is that an error condition was raised during the Write Forward command. The specific failure is an implementation error in the IDB logic which raises a Non-Executable Function Error in the Error Register (MTER[NEF]). The NEF Error causes the drive to assert Attention Active bit in the Drive Status Register (MTDS[ATA]),

Note: The software is actually checks the status of the MSB (ATA bit) of the Drive Status Register (MTDS). The software is reporting the error as if the MSB (ACCL bit) of the Tape Control Register (MTTC) is asserted.

The code is question is a little obfuscated. The code copies a tiny stub of timing software into the fast ACs and executes it.
The instruction at 040676 clearly copies a RDIO D1, @MTDS instruction into AC1. The other code is copied by a BLT instruction.

[jfcl]

Fixed by 129db73