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Status Register - STATUS

This 8-bit register (0AH) contains the zero flag (Z), carry flag (C), auxiliary carry flag (AC), overflow

flag (OV), power down flag (PDF), and watchdog time-out flag (TO). It also records the status infor

mation and controls the operation sequence.

With the exception of the TO and PDF flags, bits in the status register can be altered by instruc

tions like most other registers. Any data written into the status register will not change the TO or

PDF flag. In addition, operations related to the status register may give different results due to the

different instruction operations. The TO flag can be affected only by a system power-up, a WDT

time-out or by executing the ²CLR WDT² or ²HALT² instruction. The PDF flag is affected only by ex

ecuting the ²HALT² or ²CLR WDT² instruction or during a system power-up.

The Z, OV, AC and C flags generally reflect the status of the latest operations.

C is set if an operation results in a carry during an addition operation or if a borrow does not take

place during a subtraction operation; otherwise C is cleared. C is also affected by a rotate

through carry instruction.

AC is set if an operation results in a carry out of the low nibbles in addition, or no borrow from the

high nibble into the low nibble in subtraction; otherwise AC is cleared.

Z is set if the result of an arithmetic or logical operation is zero; otherwise Z is cleared.

OV is set if an operation results in a carry into the highest-order bit but not a carry out of the high

est-order bit, or vice versa; otherwise OV is cleared.

PDF is cleared by a system power-up or executing the ²CLR WDT² instruction. PDF is set by ex

ecuting the ²HALT² instruction.

TO is cleared by a system power-up or executing the ²CLR WDT² or ²HALT² instruction. TO is

set by a WDT time-out.

In addition, on entering an interrupt sequence or executing a subroutine call, the status register

will not be pushed onto the stack automatically. If the contents of the status registers are important

and if the subroutine can corrupt the status register, precautions must be taken to correctly save it.

I/O Type MCU

T O P D F O V Z A C C

S T A T U S R e g i s t e r

A r i t h m e t i c / l o g i c o p e r a t i o n f l a g s :

C a r r y f l a g

A u x i l i a r y c a r r y f l a g

Z e r o f l a g

O v e r f l o w f l a g

S y s t e m m a n a g e m e n t f l a g s :

P o w e r d o w n f l a g

W a t c h d o g t i m e - o u t f l a g

N o t i m p l e m e n t e d , r e a d a s " 0 "

b 7 b 0

Holtek HT48R30A-1 User Manual

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