NEC V30MZ registers

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