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HARDWARE

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3886 Group User’s Manual

[Processor status register (PS)]

The processor status register is an 8-bit register consisting of 5

flags which indicate the status of the processor after an arithmetic

operation and 3 flags which decide MCU operation. Branch opera-

tions can be performed by testing the Carry (C) flag , Zero (Z) flag,

Overflow (V) flag, or the Negative (N) flag. In decimal mode, the Z,

V, N flags are not valid.

•Bit 0: Carry flag (C)

The C flag contains a carry or borrow generated by the arithmetic

logic unit (ALU) immediately after an arithmetic operation. It can

also be changed by a shift or rotate instruction.

•Bit 1: Zero flag (Z)

The Z flag is set if the result of an immediate arithmetic operation

or a data transfer is “0”, and cleared if the result is anything other

than “0”.

•Bit 2: Interrupt disable flag (I)

The I flag disables all interrupts except for the interrupt

generated by the BRK instruction.

Interrupts are disabled when the I flag is “1”.

•Bit 3: Decimal mode flag (D)

The D flag determines whether additions and subtractions are

executed in binary or decimal. Binary arithmetic is executed when

this flag is “0”; decimal arithmetic is executed when it is “1”.

Decimal correction is automatic in decimal mode. Only the ADC

and SBC instructions can be used for decimal arithmetic.

•Bit 4: Break flag (B)

The B flag is used to indicate that the current interrupt was

generated by the BRK instruction. The BRK flag in the processor

status register is always “0”. When the BRK instruction is used to

generate an interrupt, the processor status register is pushed

onto the stack with the break flag set to “1”.

•Bit 5: Index X mode flag (T)

When the T flag is “0”, arithmetic operations are performed

between accumulator and memory. When the T flag is “1”, direct

arithmetic operations and direct data transfers are enabled

between memory locations.

•Bit 6: Overflow flag (V)

The V flag is used during the addition or subtraction of one byte

of signed data. It is set if the result exceeds +127 to -128. When

the BIT instruction is executed, bit 6 of the memory location

operated on by the BIT instruction is stored in the overflow flag.

•Bit 7: Negative flag (N)

The N flag is set if the result of an arithmetic operation or data

transfer is negative. When the BIT instruction is executed, bit 7 of

the memory location operated on by the BIT instruction is stored

in the negative flag.

Table 5 Set and clear instructions of each bit of processor status register

Set instruction

Clear instruction

C flag

Z flag

I flag

D flag

B flag

T flag

V flag

N flag

SEC

CLC

SEI

CLI

SED

CLD

SET

CLT

CLV

FUNCTIONAL DESCRIPTION

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