Freescale Semiconductor, Inc.
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.
Technical Data
48
MC68HC705C8A — Rev. 3
Central Processor Unit (CPU)
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