S390 offers a few different formats how memory addresses are encoded in an instruction. In general, a memory address is composed of three components:
 b - a register containing the base address.
 x - a register containing a byte index.
 d - a displacement which is a constant byte offset.

All three components are added to form the effective address. Some instructions don't provide space to encode the index register.

During an unrelated investigation it was found that code readability and maintainability could be significantly improved by unifying and modularising the encoding code in the instruction emitters.