D17133CS datasheet(110/289 Pages) NEC

110 / 289 page

CHAPTER 11

ARITHMETIC AND LOGIC UNIT

11.3.1 Addition and Subtraction When CMP=0 and BCD=0

Addition and subtraction are performed in 4-bit binary and the result is stored in the general register or data

memory.

When the result of the operation is greater than 1111B (carry generated) or less than 0000B (borrow generated),

the CY flag is set (1); otherwise it is reset (0).

When the result of the operation is 0000B, the Z flag is set (1) regardless of whether there is carry or borrow;

otherwise it is reset (0).

11.3.2 Addition and Subtraction When CMP=1 and BCD=0

Addition and subtraction are performed in 4-bit binary.

However, because the CMP flag is set (1), the result of the operation is not stored in either the general register

or data memory.

When there is a carry or borrow in the result of the operation, the CY flag is set (1); otherwise it is reset (0).

When the result of the operation is 0000B, the previous state of the Z flag is maintained; otherwise it is reset

(0).

11.3.3 Addition and Subtraction When CMP=0 and BCD=1

BCD operations are performed.

The result of the operation is stored in the general register or data memory. When the result of the operation

is greater than 1001B (9D) or less than 0000B (0D), the carry flag is set (1), otherwise it is reset (0).

When the result of the operation is 0000B (0D), the Z flag is set (1), otherwise it is reset (0).

Operations in BCD are performed by first computing the result in binary and then by using the decimal conversion

circuit to convert the result to decimal. For information concerning the binary to decimal conversion, refer to Table

11-2.

In order for operations in BCD to be performed properly, note the following:

(1) Result of an addition must be in the range 0D to 19D.

(2) Result of a subtraction must be in the range 0D to 9D, or in the range –10D to –1D.

The following shows which value is considered the CY flag in the range 0D to 19D (shown in 4-bit binary):

0, 0000B to 1, 0011B

The following shows which value is considered the CY flag in the range –10D to –1D (shown in 4-bit binary):

1, 0110B to 1, 1111B

When operations in BCD are performed outside of the limits of (1) and (2) stated above, the CY flag is set

(1) and the result of operation is output as a value greater than or equal to 1010B (0AH).

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