Introduction to Computer Systems

Review these notes. For maximum benefit, go through these notes interactively, thinking about and answering the questions at the bottom of each page. These notes are an experiment in applying the "programmed learning" method to web-based computer aided instruction. The subject is Java Programming for beginning programmers. The content is intended to start beginning programmers out on the track to professional-level programming and reinforce learning by providing abundant feedback. Java is a programming language that is used often in professional practice.

Main Memory

Answer:

Which is fastest? Main memory or secondary memory?

Main memory

Which is largest? Main memory or secondary memory?

Secondary memory

There is a design trade-off between access speed and size. A computer would be tremendously expensive if all it had was a huge amount of main memory. It would be very slow if all it had was secondary memory. So modern desktop and laptop have both, and use each for what it does best.

 

Main Memory

Main memory is where programs and data are kept when the processor is actively using them. When programs and data become active, they are copied from secondary memory into main memory where the processor can interact with them. A copy remains in secondary memory.

Main memory is intimately connected to the processor, so moving instructions and data into and out of the processor is very fast.

Main memory is sometimes called RAM. RAM stands for Random Access Memory. "Random" means that the memory cells can be accessed in any order. However, properly speaking, "RAM" means the type of silicon chip used to implement main memory.

When people say that a computer has "2 gigabytes of RAM" they are talking about how big its main memory is. One megabyte of memory is enough to hold approximately one million (106) characters. A mid-priced desktop has about 8 gigabytes or more of main memory in 2018. A gigabyte is approximately one billion (109) characters. (There will be more about bytes and megabytes later on in these notes.)

Nothing permanent is kept in main memory. Sometimes data are placed in main memory for just a few seconds, only as long as they are needed by the processor.

 

Question 6:

Imagine that you are using a computer to write a book report on Tom Sawyer.

  • Where is the word processor program that you are running located?
  • Where are the characters you have typed?