3.2.5 Outline the
         characteristics of
         primary memory.

3.2.6 Outline the
         characteristics of
         secondary memory.

Random access because all of it can be accessed by loading its address into the CIR (see architecture page).

"boot" originates from bootstraps and the expression "to pull oneself up by ones bootstraps".

The cache on the microprocessor itself is referred to as Level 1 cache and that between main memory and the processor as Level 2 .

These things tend to change quite fast; 5 years ago a typical personal computer would probably have had 128 or even 64 Mbytes of RAM and the cache measured in Kbytes.

Virtual memory is a complex subject and beyond the scope of the syllabus. Download this powerpoint presentation only if you are really interested.

Floppy discs are still with us aren't they? Not for long, I guess.

A supercomputer used to model weather patterns over the continental USA has a secondary memory of 4 Terabytes. (1 Tbyte = 1024 Gbyte).

Details of the block structure
(sectors, tracks and cylinders)
are part of HL Topic 6 ).

Often the term FAT or File Address Table will be used for the index.

It would be useful to make a revison table on a large sheet.

On this page:[ primary memory | RAM | ROM | cache | virtual memory ]

              [ secondary memory | flash memory | disc | optical storage | tape ]

Primary Memory

The primary memory stores the program instructions and the data in binary machine code. The Control Unit deals with the instructions and the Arithmetic and Logic unit handles calculations and comparisons with the data. Data and instructions are moved by buses.

There are two types of memory in primary memory (also known as the Immediate Access Store ) of the computer, RAM and ROM:

RAM is Random Access Memory which loses its contents when the computer is switched off (it is volatile ). This memory can be written to, instructions and data can be loaded into it.

ROM , or Read Only Memory is non-volatile and is used to store programs permanently (the start-up or " boot " instructions, for example), the computer cannot store anything in this type of memory.

When the programs and data files (known as the software ) are not in RAM, they are stored on secondary memory (also known as backing store ) such as tapes or discs. The tape or disc drives and any input and output devices connected to the CPU are known collectively as peripherals .

Flash memory is discussed below under secondary storage.

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Cache Memory
There are two main types of RAM, fast dynamic RAM ( DRAM ) or even faster static RAM ( SRAM ), naturally SRAM is more expensive and is thus not used for all of the IAS. However, some SRAM can be placed between main RAM and the processor as a temporary store for blocks of program instructions. When the processor has to look for the next instruction, chances are it can be found in the cache.

Modern processors also contain a small amount of fast memory incorporated into the processor itself, internal cache which is speedier still because of the very short distance travelled.

whereas the primary memory may be anything from 512 Mbytes to 2 GBytes for a typical microcomputer (personal computer), the primary memory of a mainframe or supercomputer may reach Gigabytes in size.

The cache memory is typically of the order of a few MBytes these days.

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Virtual Memory

Often, applications are too large to be stored in RAM in their entirety or more than one application is running at the same time. There are several ways to handle this but all involve keeping sections of primary memory on high speed secondary storage. Thus, if you are using a Word processor and not using the spellcheck, that part may be held on disc and only loaded in when needed.

The operating system will use hardware-based memory management schemes to swap memory in and out as neccessary.

In the popular Windows OS this area of disc is known as the swap file.

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Secondary Memory

Backing storage (secondary memory) is required to keep information which is not needed in primary memory all of the time and which may be too large to fit into the memory of the computer.

Both programs and data are held on backing store. The two main ways of storing data on backing store are serial access and direct access.

For example, a payroll program has to access the data file containing all the data on all of a company's employees, it accesses this data one record at a time, one after the other, this is serial access.

Direct access (sometimes called random access) would be used, for example, in a supermarket where details of all of the items for sale are held in a file, the computer needs to locate an item quickly by moving directly to its record.

Direct access requires some kind of index to be held on the medium (the material storing the data).

Flash Memory

This is a solid state memory (no moving parts) which is relatively new and increasingly available for personal computers. It is random access and writeable yet non-volatile so combines the best features of the more traditional RAM and ROM.

It requires very little power to operate and is very compact so is also ideal for use in handheld devices like mobile phones.

Commonly known as flash drives, pen drives or thumb drives.

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Discs

There are two main types of magnetic disc in general use, the removable floppy disc and the fixed (or hard) disc.

A floppy disc stores up to 1.5 Mbytes or so, a typical pc hard disc stores up to 160 Gbytes or so (these things change quickly, of course), a mainframe disc pack may well store several hundred Gigabytes.

These days larger sizes are often achieved by using a set of hard disc drives in conjunction (an array of discs or a disc farm).

A disc pack (and the pc hard disc) consists of several plates of magnetic material arranged on a single spindle:

The disc surfaces themselves are divided up into discrete blocks each with a unique address. Therefore it is possible to store the addresses of files, and their sizes, in an index on the disc itself and data (or programs) can be retrieved by looking up the address in the index .

Optical devices

Essentially CD ROM's and related technology such as DVD's . The CD ROM in your pc is the same as those used for playing music but the data is stored in a different format. The surface of a CD/DVD has lands and pits in the surface and the transition from one to the other can represent binary states.

These very small irregularities are read by a reflected laser beam. In practical terms optical disks are useful for distributing information because they are portable and rugged (unlike, say floppy discs) and they store much more data (from around 600 Mbyte for conventional CD up to 2.7 Gbytes for DVD formats).

There are also CD-R's and DVD-R's (recordable CD's and DVD's) on the market which you can write to once but read many times (Write Once Read Many = WORM , although this term seems not to be used much any more).

Also now common are the re-recordable optical disks which can be used like a (very) large floppy disc. These have pretty much driven magnetic media like ZIP discs which typically hold a few hundred Mbytes of data off the market in recent years due to their convenience (every computer has a CD/DVD drive these days) and price ( CD's are a few cents each whereas ZIP discs are several dollars or more each).

Tape

Although the days of big computers spinning reels of tape are long gone (ever seen the movie "Billion Dollar Brain"? - I thought not) tape still has its place in some systems.

Once upon a time, magnetic discs were very, very expensive devices and tape was cheap and easy to manufacture. Also tapes have a relatively high storage density.

The main use in a modern system would be for backing up network servers. Since tape is not a direct access medium these backed up files are not that easily retrieved but hopefully they will never need to be.

Tape can be re-used quite easily and is a compact storage medium . Systems have also been built for archiving high volumes of data which involve robot -controlled retrieval of tape cartridges (a large warehouse full of tape cartridges could store Petabytes (thousands of Terabytes ) of archived data.

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Example applications of secondary memory

Serial access using tapes has it's disadvantages, particularly if you are in a hurry. If you are running some kind of information system in which speed is important you want to be able to locate a given record very much more quickly than be using serial access on magnetic tapes.

You can compare serial access and direct access by using a musical analogy, to find a given song on a tape cassette requires you to wind on through all those intervening tracks. On a CD player however, you can move directly to the piece you want to hear.

Next, we look at the versatile microprocessor .

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related: [ Common core home | previous: architecture | next: microprocessor ]

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