IBM 5162  -  POST Code of DD


The Power-On Self Test (POST) within the IBM BIOS of an IBM 5162 (IBM XT Model 286) generates POST codes.  Those POST codes can be viewed via the use of a POST card that monitors port 80h.

For the 5162, the POST code of DD indicates an error in the first 64 KB of motherboard RAM.  Per here, the first 64 KB corresponds to the first 64 KB of motherboard SIMM pair, S1 and S2.

The 'DD' code will be followed by either one or two codes, and be output in an endless repeating loop:
Example: DD,00,08,DD,00,08,DD,00,08,DD,00,08,DD,00,08 ...

Note that this error does not necessarily mean that a SIMM has failed.
For example, it could be that a SIMM has developed poor electrical contact with its socket, and a fix is simply to re-seat the SIMM's (remove then refit the SIMM's).
For example, part of the motherboard's RAM support circuitry has failed.


First
Code		 Second
Code		 Third
Code		   Bit in SIMM at fault
DD 00 none  One (or possibly both) of the two parity bits
DD 00 01  Bit 0.   Per here, bit 0 is provided by SIMM S1.
DD 00 02  Bit 1.   Per here, bit 1 is provided by SIMM S1.
DD 00 04  Bit 2.   Per here, bit 2 is provided by SIMM S1.
DD 00 08  Bit 3.   Per here, bit 3 is provided by SIMM S1.
DD 00 10  Bit 4.   Per here, bit 4 is provided by SIMM S1.
DD 00 20  Bit 5.   Per here, bit 5 is provided by SIMM S1.
DD 00 40  Bit 6.   Per here, bit 6 is provided by SIMM S1.
DD 00 80  Bit 7.   Per here, bit 7 is provided by SIMM S1.
DD 01 00  Bit 8.   Per here, bit 8 is provided by SIMM S2.
DD 02 00  Bit 9.   Per here, bit 9 is provided by SIMM S2.
DD 04 00  Bit 10.   Per here, bit 10 is provided by SIMM S2.
DD 08 00  Bit 11.   Per here, bit 11 is provided by SIMM S2.
DD 10 00  Bit 12.   Per here, bit 12 is provided by SIMM S2.
DD 20 00  Bit 13.   Per here, bit 13 is provided by SIMM S2.
DD 40 00  Bit 14.   Per here, bit 14 is provided by SIMM S2.
DD 80 00  Bit 15.   Per here, bit 15 is provided by SIMM S2.



Multiple bit failure

You will see variations of the above if there are multiple bits in the SIMM that have failed.
Use the diagram at here to calculate the failing bits.