D17133CS datasheet(105/289 Pages) NEC

105 / 289 page

CHAPTER 11

ARITHMETIC AND LOGIC UNIT

11.2.2 Functions of Temporary Registers A and B

Temporary registers A and B are needed for processing of 4-bit data. These registers are used for temporary

storage of the first and second data operands of an instruction.

11.2.3 Functions of the Status Flip-flop

The status flip-flop is used for controlling operation of the ALU and for storing data which has been processed.

Each flag in the status flip-flop corresponds directly to a flag in the program status word (PSWORD) located in the

system register. This means that when a flag in the system register is manipulated it is the same as manipulating

a flag in the status flip-flop. Each flag in the program status word is described below.

(1) Z flag

This flag is set (1) when the result of an arithmetic operation is 0000B, otherwise it is reset (0). However,

as described below, depending on the status of the CMP flag, the conditions which cause this flag to be set

(1) can be changed.

(i)

When CMP=0

Z flag is set (1) when the result of an arithmetic operation is 0000B, otherwise it is reset (0).

(ii) When CMP=1

The previous state of the Z flag is maintained when the result of an arithmetic operation is 0000B,

otherwise it is reset (0). Only affected by arithmetic operations.

(2) CY flag

This flag is set (1) when a carry or borrow is generated in the result of an arithmetic operation, otherwise

it is reset (0).

When an arithmetic operation is being performed using a carry or borrow, the operation is performed using

the CY flag as the least significant bit. When a rotation (RORC instruction) is performed, the contents of the

CY flag becomes the most significant bit (bit b3) of the general register and the least significant bit of the

general register is stored in the CY flag.

Only affected by arithmetic operations and rotations.

D17133CS Datasheet(PDF) 105 Page - NEC