UPD17P136ACT datasheet(111/292 Pages) NEC

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CHAPTER 11 ARITHMETIC AND LOGIC UNIT (ALU)

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 correction

circuit to convert the result to decimal. For information concerning the binary to decimal conversion, see 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).

11.3.4 Addition and Subtraction When CMP = 1 and BCD = 1

BCD operations are performed.

The result is not stored in either the general register or data memory.

In other words, the operations specified by CMP = 1 and BCD = 1 are both performed at the same time.

Example

MOV

RPL,

#0001B

; Sets the BCD flag (BCD = 1).

MOV

PSW,

#1010B

; Sets the CMP and Z flag (CMP = 1, Z = 1) and resets the CY flag

; (CY = 0).

SUB

M1,

#0001B

; (1)

SUBC

M2,

#0010B

; (2)

SUBC

M3,

#0011B

; (3)

By executing the instructions in steps numbered (1), (2), and (3), the 12 bits in memory locations M1,

M2, and M3 and the immediate data (321) can be compared in decimal.

11.3.5 Notes Concerning Use of Arithmetic Operations

When performing arithmetic operations with the program status word (PSWORD), caution should be taken with

regard to the result of the operation being stored in the program status word.

Normally, the CY and Z flags in the program status word are set (1) or reset (0) according to the result of the

arithmetic operation being executed. However, when an arithmetic operation is performed on the program status word

itself, the result is stored in the program status word. This means that there is no way to determine if there is a carry

or borrow in the result of the operation nor if the result of the operation is zero.

However, when the CMP flag is set (1), results of arithmetic operations are not stored. Therefore, even in the above

case, the CY and Z flags will be properly set (1) or reset (0) according to the result of the operation.

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