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Although core memory is famous for its ability to retain data when the power is removed, another property is that the read-out of the stored data is destructive. This means that every read operation must be followed by a write operation to restore the information that was just read. This process is called a read-write cycle.
If the power fails while the computer is running, the contents of the memory word being accessed is not restored. Since most memory accesses are instruction fetches, an instruction in the running program is usually corrupted. This is not serious because most programs must be reloaded and restarted after a power failure anyway.
If, however, the loader was running when the power failed, one of its 64 words is erased and the loader no longer functions.
When this occurs, the entire 64 word loader must be checked from the front panel to find and repair the corrupted location. Fortunately, the corrupted location is almost always one of the two instructions in the loop that waits for the next character, so these two locations should be checked first.
Computers with the power-fail interrupt option (standard on 2100A) do not have this problem, because loss of power causes an interrupt to a location which contains a halt instruction.
Allowed the DMA to read or write from devices with an 8 bit character and pack or unpack the two characters into a 16 bit word.
Was only available on the 2116 and was dropped from all later models.
This forced some customers who relied on the feature to continue using the 2116 long after it was otherwise obsolete.