NSC800D datasheet(13/76 Pages) NSC

13 / 76 page

80 Functional Description (Continued)

843 Carry (C)

A carry from the highest order bit of the accumulator during

an add instruction or a borrow generated during a subtrac-

tion instruction sets the carry flag Specific shift and rotate

instructions also affect this bit

Two specific instructions in the NSC800 instruction reper-

toire set (SCF) or complement (CCF) the carry flag

Other operations that affect the C flag are as follows

Adds

Subtracts

Logic Operations (always resets C flag)

Rotate Accumulator

Rotate and Shifts

Decimal Adjust

Negation of Accumulator

Other operations do not affect the C flag

844 AddsSubtract (N)

This flag is used in conjunction with the H flag to ensure that

the proper BCD correction algorithm is used during the deci-

mal adjust instruction (DAA) The correction algorithm de-

pends on whether an add or subtract was previously done

with BCD operands

The operations that set the N flag are

Subtractions

Decrements (8-bit)

Complementing of the Accumulator

Block IO

Block Searches

Negation of the Accumulator

The operations that reset the N flag are

Adds

Increments

Logic Operations

Rotates

Set and Complement Carry

Input Register Indirect

Block Transfers

Load of the I or R Registers

Bit Tests

Other operations do not affect the N flag

845 ParityOverflow (PV)

The ParityOverflow flag is a dual-purpose flag that indi-

cates results of logic and arithmetic operations In logic op-

erations the PV flag indicates the parity of the result the

flag is set (high) if the result is even reset (low) if the result

is odd In arithmetic operations it represents an overflow

condition when the result interpreted as signed two’s com-

plement arithmetic is out of range for the eight-bit accumu-

lator (ie b128 to a127)

The following operations affect the PV flag according to

the parity of the result of the operation

Logic Operations

Rotate and Shift

Rotate Digits

Decimal Adjust

Input Register Indirect

The following operations affect the PV flag according to

the overflow result of the operation

Adds (16 bit with carry 8-bit withwithout carry)

Subtracts (16 bit with carry 8-bit withwithout carry)

Increments and Decrements

Negation of Accumulator

The PV flag has no significance immediately after the fol-

lowing operations

Block IO

Bit Tests

In block transfers and compares the PV flag indicates the

status of the BC register always ending in the reset state

after an auto repeat of a block move Other operations do

not affect the PV flag

846 Half Carry (H)

This flag indicates a BCD carry or borrow result from the

low-order four bits of operation It can be used to correct the

results of a previously packed decimal add or subtract op-

eration by use of the Decimal Adjust Instruction (DAA)

The following operations affect the H flag

Adds (8-bit)

Subtracts (8-bit)

Increments and Decrements

Decimal Adjust

Negation of Accumulator

Always Set by

Logic AND

Complement Accumulator

Bit Testing

Always Reset By

Logic OR’s and XOR’s

Rotates and Shifts

Set Carry

Input Register Indirect

Block Transfers

Loads of I and R Registers

The H flag has no significance immediately after the follow-

ing operations

16-bit Adds withwithout carry

16-Bit Subtracts with carry

Complement of the carry

Block IO

Block Searches

Other operations do not affect the H flag

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