D17133CS datasheet(79/289 Pages) NEC

79 / 289 page

CHAPTER 7

SYSTEM REGISTER (SYSREG)

7.7.5 Carry Flag (CY)

The CY flag shows whether there is a carry in the result of an addition operation or a borrow in the result of a

subtraction operation.

The CY flag is set (CY=1) when there is a carry or borrow in the result and reset (CY=0) when there is no carry

or borrow in the result.

When the RORC r instruction (contents in the general register pointed to by r is shifted right one bit) is executed,

the following occurs: the value in the CY flag just before execution of the instruction is shifted to the most significant

bit of the general register and the least significant bit is shifted to the CY flag.

The CY flag is also useful for when the user wants to skip the next instruction when there is a carry or borrow

in the result of an operation.

The CY flag is only affected by arithmetic operations and rotations. Also, it is not affected by CMP flag.

7.7.6 Binary-Coded Decimal Flag (BCD)

The BCD flag is used to specify BCD operations.

When the BCD flag is set (BCD=1), all arithmetic operations will be performed in BCD. When the BCD flag is

reset (BCD=0), arithmetic operations are performed in 4-bit binary.

The BCD flag does not affect logical operations, bit evaluation, comparison evaluations or rotations.

7.7.7 Caution on Use of Arithmetic Operations on the Program Status Word

When performing arithmetic operations (addition and subtraction) on the program status word (PSWORD), the

following point should be kept in mind.

When an arithmetic operation is performed on the program status word and the result is stored in the program

status word.

Below is an example.

Example

MOV

PSW,

#0001B

ADD

PSW,

#1111B

When the above instructions are executed, a carry is generated which should cause bit 2 (CY flag)

of PSW to be set. However, the result of the operation (0000B) is stored in PSW, meaning that

CY does not get set.

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