Signals Arithmetic logic unit

1 signals

1.1 data
1.2 opcode
1.3 status

1.3.1 outputs
1.3.2 inputs

signals

an alu has variety of input , output nets, electrical conductors used convey digital signals between alu , external circuitry. when alu operating, external circuits apply signals alu inputs and, in response, alu produces , conveys signals external circuitry via outputs.

data

a basic alu has 3 parallel data buses consisting of 2 input operands (a , b) , result output (y). each data bus group of signals conveys 1 binary integer number. typically, a, b , y bus widths (the number of signals comprising each bus) identical , match native word size of external circuitry (e.g., encapsulating cpu or other processor).

opcode

the opcode input parallel bus conveys alu operation selection code, enumerated value specifies desired arithmetic or logic operation performed alu. opcode size (its bus width) determines maximum number of different operations alu can perform; example, four-bit opcode can specify sixteen different alu operations. generally, alu opcode not same machine language opcode, though in cases may directly encoded bit field within machine language opcode.

status
outputs

the status outputs various individual signals convey supplemental information result of current alu operation. general-purpose alus commonly have status signals such as:

carry-out, conveys carry resulting addition operation, borrow resulting subtraction operation, or overflow bit resulting binary shift operation.
zero, indicates bits of y logic zero.
negative, indicates result of arithmetic operation negative.
overflow, indicates result of arithmetic operation has exceeded numeric range of y.
parity, indicates whether or odd number of bits in y logic one.

at end of each alu operation, status output signals stored in external registers make them available future alu operations (e.g., implement multiple-precision arithmetic) or controlling conditional branching. collection of bit registers store status outputs treated single, multi-bit register, referred status register or condition code register .

inputs

the status inputs allow additional information made available alu when performing operation. typically, single carry-in bit stored carry-out previous alu operation.