Computer Organization & Architecture
Evolution of Computer Systems
First generation computers like the ENIAC and the IAS used vacuum tubes and were much larger than modern day computers and more expensive to run.
The ENIAC was the first general purpose digital computer. The project was a response to US wartime needs and was developed under the Army’s Ballistics Research Laboratory. The machine was enormous, weighing 30 tonnes and occupying 1500 square feet of floor space. It contained over 18,000 vacuum tubes and consumed 140 kilowatts of power. It was a decimal machine and its main drawback was that it had to be programmed manually, by setting switches and plugging and unplugging cables.
The ENIAC was completed in 1946, too late to be used in the war effort. Its first task was to perform a series of complex calculations which were used to determine the feasibility of the hydrogen bomb. This use, for other than its purpose, demonstrated its general purpose nature.
Most machines today are considered von Neumann machines as they have the same general structure:
- An arithmetic logic unit.
- A central control unit (these two are contained in our modern day CPUs).
- Main memory.
- Input/output equipment.
The IAS computer was built by von Neumann and his colleagues at the Princeton Institute for Advanced Studies in 1946. The memory of this computer consists of 1000 storage locations for binary information. These groups of bits are called words. A word is an entity of bits that moves in and out of memory as a unit. Both data and instructions are stored in there.
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In the IAS computer, a data word was represented by a sign bit and a 39 bit value and an instruction word could contain two 20 bit instructions. Each instruction was made up of an 8 bit operation code, also called an opcode, and a 12 bit address.
The opcode is used to specify the operation to be performed and the address is a pointer to one of the words in memory (from 0 to 999).
