These instructions terminate normally on SS=2 as the B2 instructions. However, the RBC instructions have a word count associated with them. The word count is a guard in case a module sends back more data than buffer space was allowed for.
As with the B2 instructions, an SS=2 response will cause the updated NTA to be stored in DM 4. An SS response other than 0 or 2 will cause an I/O-Error.
If the word count is exhausted before receipt of an SS=2, an
I/O-Error will be generated. The address contained in DM4 will
point to the location where the data would have been stored had
the word-count not been exhausted. The PSW will indicate an
EXCEPTION=3, word count overflow.