Determining actual IBM 1401 core size

When a one- or four-character clear storage instruction (/) is executed, if the value of the B-address register before execution is greater than 99 its value after execution is x99, where x is one less than the hundreds-thousands part of the value of the B-address register when the instruction began execution. For example, after CS 345 the B-address register contains 299. After CS 345 followed by CS, the B-address register contains 199.

If the value of the B-address register when execution of the instruction begins is less than 100, its value after execution is the address of the last installed character. This value will be 1399, 3999, 7999, 11999 or 15999, depending on the amount of core installed.

If the indexing and store address feature is installed, the B-address register can be stored after a clear storage instruction. If it is stored after one clears starting from any address in the range 0-99, it contains the address of the highest-addressed character.