View source for NEC V30MZ registers

= NEC V30MZ registers =

* Four 16-bit general-purpose registers, with their (low, high) components accessible as individual 8-bit sub-registers:
** '''AX'''<sup>AW</sup> ('''AL''', '''AH''') - the ''accumulator'' register,
** '''BX'''<sup>BW</sup> ('''BL''', '''BH''') - the ''base'' register,
** '''CX'''<sup>CW</sup> ('''CL''', '''CH''') - the ''count'' register,
** '''DX'''<sup>DW</sup> ('''DL''', '''DH''') - the ''data'' register,
* Four additional 16-bit registers:
** '''SI'''<sup>IX</sup> - the ''source index'' register,
** '''DI'''<sup>IY</sup> - the ''destination index'' register,
** '''SP''' - the ''stack pointer''; the stack is always addressed in the stack segment (so as <code>SS:SP</code> far addresses),
** '''BP''' - the ''base pointer''.
* Four segment registers:
** '''CS'''<sup>PS</sup> - the ''code segment'',
** '''DS'''<sup>DS0</sup> - the ''data segment'',
** '''ES'''<sup>DS1</sup> - the ''extra segment''; an additional data segment,
** '''SS''' - the ''stack segment''.
* '''IP'''<sup>PC</sup> - the ''instruction pointer''; instructions are always addressed in the code segment (so as <code>CS:IP</code> far addresses),
* '''FLAGS'''<sup>PSW</sup> - the 16-bit processor flag register.

While the eight registers can be used in a general-purpose manner, some opcodes are constrained to only using certain registers:

* '''AX''' benefits from more compact encoding for certain instructions; it's also used as an output and input register in multiplication/division, port access, BCD conversions, and (as AL) in the XCHG opcode.
* '''CX''' is used for loop and repeat instructions as a counter.
* '''DX''' can be used used as the address register for port access, it is also used as an output register for word multiplication/division.
* '''BX''', '''SI''', and '''DI''' can be used as indices for accessing data in tables in the DS segment (or other segments when overridden).
** '''SI''' and '''DI''' are, in addition, used as source and destination pointers by string instructions.
* Similarly, '''BP''' can be used as an index for accessing stack data; this makes it particularly useful as a frame pointer.