An address has 17 normal value bits. There are 18 bits for the
operand base address in the instruction word, but the topmost bit
signals a deferred (i.e. indirect) access, so we should never see
a memory access to an address with the 0o400_000 bit set.
Placeholders (saved sequence instruction pointers in the index
registers) use bit 2.9 to indicate that sequence switches to
marked sequences should trap to sequence 42. This means that the
mark bit needs to be retained in the program counter (P register)
so that the sequence is still “marked” after it has run.