10
38C2 Group
SINGLE-CHIP 8-BIT CMOS MICROCOMPUTER
MITSUBISHI MICROCOMPUTERS
PRELIMINAR
Y
Notice:
This
is not
a final
specification.
Some
parametric
limits
are
subject
to change.
[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 opera-
tion and 3 flags which decide MCU operation. Branch operations
can be performed by testing the Carry (C) flag , Zero (Z) flag, Over-
flow (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 ex-
ecuted 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 gener-
ated by the BRK instruction. The BRK flag in the processor status
register is always “0”. When the BRK instruction is used to gener-
ate 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 be-
tween accumulator and memory. When the T flag is “1”, direct arith-
metic 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
SEC
CLC
Z flag
–
I flag
SEI
CLI
D flag
SED
CLD
B flag
–
T flag
SET
CLT
V flag
–
CLV
N flag
–