Arithmetic Logic Unit Charles Henry

A.L.U Behaves like the “calculator” of the C.P.U

John von Neumann proposes A.L.U concept in 1945

F is input command (say + -) A & B are registers used during the operation at hand.

Registers You could imagine the registers as a couple of post-its. Each time you perform some intermediate step in a calculation, you could erase the data on the post-its, and overwrite them with the data resulting from the intermediate steps.

R is the final output data D is a “flag”

Flags In assembly language we find various flags, which notify us when certain conditions have occurred during computation. For example, if the ALU has added two integers, the sum of which exceeds the register capabilities, an overflow flag would be set.

The ENIAC, completed in 1946, used the great Grandfather of the A.L.U: The accumulator.

ENIAC Accumulators Composed of 20 modular sections of the ENIAC. Able to add & subtract 10-digit decimal numbers Able to store 10-digit decimal numbers in memory

ENIAC Accumulators The ENIAC Accumulator could perform about 5000 basic addition or subtraction operations in a single second. However...

ENIAC Accumulators It took four individual accumulator panels, controlled by a multiplier unit, in order to perform multiplication operations. The multiplication operations were significantly slower: 385 ops per second.

ENIAC Accumulators It took 5 accumulator panels, similarly controlled, to perform division operations. This arrangement could perform division at a rate of 40 operations per second. This arrangement could also perform square- root calculations at a rate of 3 square-roots per second.

Do these Accumulators make my ENIAC look big? Taking into consideration the fact that the Accumulators were components of a larger system, and the fact that that system weighed Approximately 27 tons, it is truly amazing that.....

We can replicate, and exceed the performance of the ENIAC Accumulators (and the ENIAC itself) with a chip just this size. Many of the interconnecting wires within, are far slimmer than a human hair!!!

Modern A.L.U. handles more load The A.L.U now handles addition, subtraction, multiplication & division of integers. The A.L.U leaves operations involving decimal precision, such as non-integer division, to the Floating Point Unit or F.L.U. (another presentation) The A.L.U can also perform bitwise AND, NOT, OR, XOR operations.

Basic 2-bit ALU Multiplexers on right side of diagram choose which Boolean-logic circuit is employed.

Interesting similarity Notable, that the ENIAC and the modern A.L.U increase their performance speed by way of pipelining.

Pipelining Imagine an instruction as the teller line at a bank. One operation instruction is a single person in line. But if the bank had to close shop during every customer transaction... Only the first customer in line would be assisted efficiently.

By pipelining, both the ENIAC and the modern ALU are able to perform Intermediate steps on an initial operation, while accepting new instructions Into the system....keeping everything in a nice orderly line.