To find out, I opened up a ALU chip, took high-resolution die photos, and reverse-engineered the chip. The was a popular chip in the s used to perform calculations in the arithmetic-logic unit ALU of minicomputers. It is a moderately complex chip containing about 67 gates and transistors 3 , implemented using fast and popular TTL transistor-transistor logic circuitry. The die photo is below. Click the image for a high-resolution version.
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This is in contrast to a floating-point unit FPU , which operates on floating point numbers. In many designs, the ALU also has status inputs or outputs, or both, which convey information about a previous operation or the current operation, respectively, between the ALU and external status registers.
Significance : Although the is only an ALU and not a complete microprocessor, it greatly simplified the development and manufacture of computers and other devices that required high speed computation during the late s through the early s, and is still referenced as a "classic" ALU design. Prior to the introduction of the , computer CPUs occupied multiple circuit boards and even very simple computers could fill multiple cabinets.
The allowed an entire CPU and in some cases, an entire computer to be constructed on a single large printed circuit board. The occupies a historically significant stage between older CPUs based on discrete logic functions spread over multiple circuit boards and modern microprocessors that incorporate all CPU functions in a single component.
The was used in various minicomputers and other devices beginning in the s, but as microprocessors became more powerful the practice of building a CPU from discrete components fell out of favor and the was not used in any new designs.
It takes 4-bit operands and a carry input and calculates one of 16 logical or 16 arithmetic functions. In each mode, the four S S0 opcode inputs select one of the available operations; check the table in the applet schematics for the specific funtions selected by each opcode. It is because of fabrication of various circuit elements in a single chip of semiconductor material.
Owing to mass production of an IC costs as much as an individual transistor. Related Papers.
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But if you look at the chip more closely, there are a few mysteries. And if you look at the circuit diagram below , why does it look like a random pile of gates rather than being built from standard full adder circuits. And I show how the implements carry lookahead for high speed, resulting in its complex gate structure. The internal structure of the chip is surprisingly complex and difficult to understand at first. The chip is important because of its key role in minicomputer history.