memory

1. memory More technically referred to as Primary Storage

2. Computer Storage • Computers have two kinds of storage: • Primary Storage • Secondary Storage • Primary storage is also called memory. • Secondary storage is often called storage. • Individuals often do not distinguish between these two types of computer storage.

3. Primary Storage • Connected to the motherboard and is the place where programs and data are kept while they are being used. • The ENIAC was one of the first computers, built for computing artillery trajectories during WWII. • In order to change its program, the machine would have to be rewired. • John von Neumann, one of the mathematicians on the ENIAC project came up with the idea that a program should be stored in the computer’s memory while it was running. • All modern computers use von Neumann architecture.

4. Secondary Storage • The place where programs and data are kept when not in use. • Secondary storage is typically implemented as hard disks, CD/DVD, SSD, and flash “memory”

5. Memory Implementation • On modern computers (almost any computer from the past 20 years or so) memory is composed of semi-conductor chips. There are several kinds. • RAM (random access memory) • Dynamic RAM • Static RAM • ROM (read only memory) • There are other types which we shall conveniently ignore which combine the two characteristics.

6. RAM • RAM is volatile meaning that when power is lost the contents of RAM are erased. • RAM is therefore not suitable for long term storage of stuff. • RAM can be manipulated while the computer is running. • The contents of a RAM cell can be determined by the computer using a process that is referred to as reading • The contents of a RAM cell can be changed by the computer using a process that is referred to as writing. • When a memory location is written to, its former contents are lost

7. RAM (continued) • When a memory cell is read, that particular cell cannot be read again for a while. • When a memory cell is written, that particular cell cannot be written again for a while. • Both of these “whiles” are very short times in the order of 10/1000000 or 100/1000000 of a second. • We refer to these mandatory rest times as the speed of the memory. • In general if you use memory in a computer that’s too fast, that will not hurt although it might actually slow it down under certain circumstances.

8. Static and Dynamic RAM • Static RAM is much faster than dynamic ram but also much more expensive. • Dynamic RAM is typically much much larger than static ram. • A typical static RAM module is less than 10 million cells in size. • Typical dynamic RAM modules range from about 250 million to 4 billion cells each. • Static RAM is typically built into the processor and cannot be purchased separately. • There are numerous types of dynamic RAM modules and are generally not interchageable

9. ROM • Read Only Memory is similar to RAM except that its contents are not erased when the power is interrupted. • What is in the ROM stays in the ROM. • This is useful for things that will never change. • It is mandatory for stuff that has to be there when the computer is turned on, such as instructions that tell it how to start.