12
SINGLE-CHIP 8-BIT CMOS MICROCOMPUTER
MITSUBISHI MICROCOMPUTERS
3850 Group (Spec. H/A)
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 7 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
[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.