UPD17P136ACT datasheet(102/292 Pages) NEC

102 / 292 page

CHAPTER 11

ARITHMETIC AND LOGIC UNIT (ALU)

The ALU is used for performing arithmetic operations, logical operations, bit judgements, comparison judgements,

and rotations on 4-bit data.

11.1 ALU BLOCK CONFIGURATION

Figure 11-1 shows the configuration of the ALU block.

As shown in Figure 11-1, the ALU block consists of the main 4-bit data processor, temporary registers A and B

which are peripheral circuit of the ALU, the status flip-flop for controlling the status of the ALU, and the decimal

correction circuit for use during arithmetic operations in BCD.

As shown in Figure 11-1, the status flip-flop consists of the following flags: Zero flag FF, carry flag FF, compare

flag FF, and the BCD flag FF.

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

7EH, 7FH) in the system register. The flags in the program status word are the following: Zero flag (Z), carry flag

(CY), compare flag (CMP), and the BCD flag (BCD).

11.2 FUNCTIONS OF THE ALU BLOCK

Arithmetic operations, logical operations, bit judgements, comparison judgements, and rotations are performed

using the instructions in the ALU block. Table 11-1 lists each arithmetic/logical instruction, judgement instruction, and

rotation instruction.

By using the instructions listed in Table 11-1, 4-bit arithmetic/logical operations, judgements and rotations can be

performed in a single instruction. Arithmetic operations in decimal can also be performed in a single instruction.

11.2.1 Functions of the ALU

The arithmetic operations consist of addition and subtraction. Arithmetic operations can be performed on the

contents of the general register and data memory or on immediate data and the contents of data memory. Operations

in binary are performed on 4 bits of data and operations in decimal are performed on one place (BCD operation).

Logical operations include ANDing, ORing, and XORing. Their operands can be general register contents and data

memory contents, or data memory contents and immediate data.

Bit judgement is used to determine whether bits in 4-bit data in data memory are 0 or 1.

Comparison judgement is used to compare contents of data memory with immediate data. It is used to determine

whether one value is equal to or greater than the other, less than the other, or if both values are equal or not equal.

Rotation is used to shift 4-bit data in the general register one bit in the direction of its least significant bit (rotation

to the right).

UPD17P136ACT Datasheet(PDF) 102 Page - NEC