A28

Explanation
The DAT-off machine check handler (MCH) routine
(DAT is Dynamic Address Translation) cannot reach the DAT-on MCH routine.
This situation can arise when a second machine check occurs when the
load program status word (LPSW) assembler instruction gives control
to the DAT-on MCH routine.

System action
The system enters wait state X'A28'.

Operator response
To recover possible unrecorded machine check
error records, request a stand-alone dump. Retain the dump. Contact
hardware support.

Source
Machine check handler (MCH)