NSC800N datasheet(14/76 Pages) NSC

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80 Functional Description (Continued)

847 Zero Flag (Z)

Loading a zero in the accumulator or when a zero results

from an operation sets the zero flag

The following operations affect the zero flag

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

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

Logic Operations

Increments and Decrements

Rotate and Shifts

Rotate Digits

Decimal Adjust

Input Register Indirect

Block IO (always set after auto repeat block IO)

Block Searches

Load of I and R Registers

Bit Tests

Negation of Accumulator

The Z flag has no signficance immediately after the follow-

ing operations

Block Transfers

Other operations do not affect the zero flag

848 Sign Flag (S)

The sign flag stores the state of bit 7 (the most-signifi-

cant bit and sign bit) of the accumulator following an arith-

metic operation This flag is of use when dealing with signed

numbers

The sign flag is affected by the following operation accord-

ing to the result

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

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

Logic Operations

Increments and Decrements

Rotate and Shifts

Rotate Digits

Decimal Adjust

Input Register Indirect

Block Search

Load of I and R Registers

Negation of Accumulator

The S flag has no significance immediately after the follow-

ing operations

Block IO

Block Transfers

Bit Tests

Other operations do not affect the sign bit

849 Additional General-Purpose Registers

The other general-purpose registers are the B C D E H

and L registers and their alternate register set B’ C’ D’ E’

H’ and L’ The general-purpose registers can be used inter-

changeably

In addition the B and C registers perform special functions

in the NSC800 expanded IO capabilities particularly block

IO operations In these functions the C register can ad-

dress IO ports the B register provides a counter function

when used in the register indirect address mode

When used with the special condition jump instruction

(DJNZ) the B register again provides the counter function

8410 Alternate Configurations

The six 8-bit general purpose registers (BCDEHL) will

combine to form three 16-bit registers This occurs by con-

catenating the B and C registers to form the BC register the

D and E registers form the DE register and the H and L

registers form the HL register

Having these 16-bit registers allows 16-bit data handling

thereby expanding the number of 16-bit registers available

for memory addressing modes The HL register typically

provides the pointer address for use in register indirect ad-

dressing of the memory

The DE register provides a second memory pointer register

for the NSC800’s powerful block transfer operations The

BC register also provides an assist to the block transfer

operations by acting as a byte-counter for these operations

85 ARITHMETIC-LOGIC UNIT (ALU)

The arithmetic logic and rotate instructions are performed

by the ALU The ALU internally communicates with the reg-

isters and data buffer on the 8-bit internal data bus

86 INSTRUCTION REGISTER AND DECODER

During an opcode fetch the first byte of an instruction is

transferred from the data buffer (ie its on the internal data

bus) to the instruction register The instruction register feeds

the instruction decoder which gated by timing signals gen-

erates the control signals that read or write data from or to

the registers control the ALU and provide all required exter-

nal control signals

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