M68HC05 datasheet(39/128 Pages) FREESCALE

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Central Processor Unit (CPU)

Arithmetic/Logic Unit (ALU)

MC68HC05J1A — Rev. 3.0

Technical Data

Central Processor Unit (CPU)

N — Negative Flag

The CPU sets the negative flag when an arithmetic operation, logical

operation, or data manipulation produces a negative result (bit 7 in the

results is a logic 1). Reset has no effect on the negative flag.

Z — Zero Flag

The CPU sets the zero flag when an arithmetic operation, logical

operation, or data manipulation produces a result of $00. Reset has

no effect on the zero flag.

C — Carry/Borrow Flag

The CPU sets the carry/borrow flag when an addition operation

produces a carry out of bit 7 of the accumulator or when a subtraction

operation requires a borrow. Some logical operations and data

manipulation instructions also clear or set the carry/borrow bit. Reset

has no effect on the carry/borrow flag.

3.4 Arithmetic/Logic Unit (ALU)

The arithmetic/logic unit (ALU) performs the arithmetic and logical

operations defined by the instruction set. The binary arithmetic circuits

decode instructions and set up the ALU for the selected operation. Most

binary arithmetic is based on the addition algorithm, carrying out

subtraction as negative addition. Multiplication is not performed as a

discrete operation but as a chain of addition and shift operations within

the ALU. The multiply instruction requires 11 internal clock cycles to

complete this chain of operations.

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