Computer Organization and Architecture Session 2

Study Notes

The University of Pennsylvania proposed to build a general-purpose computer using vacuum tubes for the army's Ballistics Research Laboratory (BRL) in 1943.
The resulting machine was called ENIAC, which was enormous, weighing 30 tons, occupying 1500 square feet of floor space, and containing more than 18,000 vacuum tubes.
ENIAC was completed in 1946, too late to be used in the war effort, and continued to operate under BRL management until 1955.

The task of entering and altering programs for ENIAC was extremely tedious.
The stored-program concept was developed, which allowed a computer to get its instructions by reading them from memory, and a program could be represented in a form suitable for storing in memory alongside the data.

The general structure consists of:
- A main memory, which stores both data and instructions.
- An arithmetic and logic unit (ALU) capable of operating on binary data.
- A control unit, which interprets the instructions in memory and causes them to be executed.
- Input/output (I/O) equipment operated by the control unit.
The control unit operates the computer by fetching instructions from memory and executing them one at a time.
Registers include:
- Memory buffer register (MBR): Contains a word to be stored in memory or sent to the I/O unit, or is used to receive a word from memory or from the I/O unit.
- Memory address register (MAR): Specifies the address of the word to be stored or retrieved.
- Instruction register (IR): Contains the 8-bit opcode instruction being executed.
- Instruction buffer register (IBR): Holds temporarily the right-hand instruction from a word in memory.
- Program counter (PC): Contains the address of the next instruction pair to be fetched from memory.
- Accumulator (AC) and multiplier quotient (MQ): Employed to hold temporarily operands and results of ALU operations.

The integrated circuit exploits the fact that components can be fabricated from a semiconductor such as silicon.
Many transistors can be produced at the same time on a single wafer of silicon.
Initially, only a few gates or memory cells could be reliably manufactured and packaged together (small scale integration, SSI).
As time went on, it became possible to pack more and more components on the same chip, following Moore's Law.

Moore observed that the number of transistors that could be put on a single chip was doubling every year and correctly predicted that this pace would continue into the near future.
The consequences of Moore's Law are:
- The cost of a chip has remained virtually unchanged during this period of rapid growth in density.
- Because logic and memory elements are placed closer together on more densely packed chips, the electrical path length is shortened, increasing operating speed.
- The computer becomes smaller, making it more convenient to place in a variety of environments.
- There is a reduction in power and cooling requirements.
- The interconnections on the integrated circuit are much more reliable than solder connections.

In evaluating processor hardware and setting requirements for new systems, performance is one of the key parameters to consider.
Operations performed by a processor are governed by a system clock, which is typically measured in cycles per second (Hz).
The execution of an instruction involves a number of discrete steps, including fetching the instruction from memory, decoding the various portions of the instruction, loading and storing data, and performing arithmetic and logical operations.
The instruction count for a program is defined as the number of machine instructions executed for that program until it runs to completion or for some defined time interval.
The average cycles per instruction (CPI) for a program is an important parameter for evaluating processor performance.