View source for NEC V30MZ flags


= NEC V30MZ flags =

== Layout ==

The V30MZ processor features a 16-bit flag register:

 15  bit  8  7  bit  0
  ---- ----  ---- ----
  m111 odit  sz0a 0p1c
  |    ||||  || |  | |
  |    ||||  || |  | +- Carry (CF<sup>CY</sup>)
  |    ||||  || |  +--- Parity (PF<sup>P</sup>)
  |    ||||  || +------ Auxillary Carry (AF<sup>AC</sup>)
  |    ||||  |+-------- Zero (ZF<sup>Z</sup>)
  |    ||||  +--------- Sign (SF<sup>S</sup>)
  |    |||+------------ Single Step<sup>Break</sup> (TF<sup>BRK</sup>)
  |    ||+------------- Interrupt Enable (IF<sup>IE</sup>)
  |    |+-------------- Direction (DF<sup>DIR</sup>)
  |    +--------------- Overflow (OF<sup>V</sup>)
  +-------------------- Mode (MD)

== Status flags ==

The following flags are typically modified by instructions:

{{Anchor|CF}}
=== Carry ===

Stores the carry/borrow state of the last arithmetic operation, or the bit shifted to it for shift/rotate operations.

{{Anchor|PF}}
=== Parity ===

Set to <tt>1</tt> if, after arithmetic and logical operations, the lower 8 bits of the result are even.

{{Anchor|AF}}
=== Auxillary Carry ===

Stores the carry state from the lower 4-bit nibble (bits 0-3) to the higher 4-bit nibble (bits 4-7), or the borrow state from the higher nibble to the lower nibble.

{{Anchor|ZF}}
=== Zero ===

Set to <tt>1</tt> if, after arithmetic and logical operations, the result is equal to zero.

{{Anchor|SF}}
=== Sign ===

Set to <tt>1</tt> if, after arithmetic and logical operations, the highest bit of the result is set.

{{Anchor|OF}}
=== Overflow ===

Set to <tt>1</tt> if an overflow occured as part of the arithmetic operation.

== Control flags ==

The following flags are typically modified by the developer:

{{Anchor|TF}}
=== Single step ===

If set to <tt>1</tt>, after every instruction, a software interrupt (vector 1) is generated.

Alternatively referred to as the ''trap flag''.

{{Anchor|IF}}
=== Interrupt enable ===

If set to <tt>1</tt>, enables maskable interrupt handling via the interrupt vector table; cleared to <tt>0</tt> as part of interrupt handling and restored by the <tt>IRET</tt> opcode.

{{Anchor|DF}}
=== Direction ===

If set to <tt>1</tt>, string instructions decrement pointers as part of their operation; if set to <tt>0</tt>, the pointers are to be incremented.

{{Anchor|MD}}
=== Mode ===

This flag does nothing on the NEC V30MZ. It was used to implement 8080 emulation mode in other V20/V30-family chips, but this functionality has been removed in the V30MZ.