Computer Storage Basics

Figure 1Computers are designed to allow data to be input, processed, stored, and output. In this article we'll be taking a casual look at a computer's design for storing data.

There are primarily two types of storage a computer uses when it boots, loads an operating system like Windows and begins to perform useful functions: 1) primary storage, which is more popularly called simply memory; and 2) secondary storage, which is more popularly referred to as simply storage. In this article we'll also briefly discuss tertiary storage and offline storage.

Primary storage is where a computer stores data on a temporary basis so it can process the data. Think of primary storage as "short term memory". Primary storage is a type of memory that is directly accessible to a computer processor and it is volatile because it is temporary in nature and is erased when the power is turned off.

Data the computer is currently processing or data which the computer knows it is about to need for processing is stored in primary storage. Memory in primary storage can be accessed quickly by the CPU. Its storage capacity, however, is much smaller than what can be stored in secondary or tertiary storage. Computers need just enough primary storage to function and temporarily hold anticipated amounts of data for processing.

Secondary storage is where a computer stores data it is not currently processing but which it may need at some later time. Secondary storage can be thought of as "long term memory", or storage, and it is non-volatile in nature because data remains intact even when power to a computer is turned off. Operating systems, documents, music files and so on are typically stored in a secondary storage device such as a hard drive.

Tertiary storage is storage which is not connected to the computer but is physically "near" the computer so that its media can be retrieved by mechanical means and brought to the computer and loaded into it.

Offline storage is storage media which can be inserted into the computer or connected to the computer and used but which can then be removed from the computer and stored elsewhere.

Primary Storage

RAM Memory

Figure 2Data in a computer's primary storage is stored for very fast retrieval. It is called Random Access Memory because any of the data in RAM can be accessed just as fast as any of the other data, hence the designation Random Access.

Primary Storage (RAM) is designed to be large enough and accessible enough to cooperate with and complement the speed of the computer's Central Processing Unit (CPU) and its cache memory (L1 cache, L2 cache, etc.) as the CPU processes commands and data.

Data stored as RAM is stored in two types of RAM, though there are many variations of RAM which are beyond the scope of this article. The two types of RAM we will consider are dynamic RAM and static RAM. Primary memory is stored largely as dynamic RAM, or DRAM. It is cheaper than static RAM, SRAM, and it cannot be accessed quite as quickly.

The computer uses SRAM in its L1, L2, and L3 cache memory. It is much more costly than DRAM but it can be accessed faster by the CPU. The computer needs data stored in SRAM right at the point where it will be loaded in the CPU's registers for processing.

Much is said about a computer's RAM storage capacity. In fact, RAM upgrade is one of the few computer memory upgrades which is considered to be worth it. Most of this RAM memory capacity comes from Dual Inline Memory Modules (DIMMS) whose pins plug into the RAM slots on the computer Motherboard. DIMMS provide the CPU a 64 bit path to the data they store. RAM temporary storage ranges typically from 256 MB to 4 GB.

ROM Memory

A CPU will also make use of some ROM, or Read Only Memory, in primary storage. This memory is used as the computer begins to boot up. Small programs called firmware are often stored in ROM chips on hardware devices (like a BIOS chip), and they contain instructions the computer can use in performing some of the most basic operations required to operate hardware devices. ROM memory cannot be easily or quickly overwritten or modified.

The Computer's Central Processing Unit, or CPU

Figure 3The Central Processing Unit is the computer's brain. It does the computers "thinking". Each CPU has an internal clock which determines how fast the CPU can process data. At each tick of its internal clock, the computer can perform an action of some type. CPU clock speed ranges from about 700 MHz to about 3 GHz. For example, if the CPU is a 2 GHz CPU, it can perform an action 2 billion times a second.

The CPU is a small chip inside a box which plugs into the Motherboard of the computer. You can learn more about the CPU and the Motherboard (which is like the computer's Central Nervous System, a nexus connecting all the computer's capabilities- video, sound, printer, drives etc. with the CPU brain) by reading Introduction to Computers.

The CPU has direct access to data in primary storage (ROM, DRAM, and SRAM). It does not have direct access to data in secondary storage. To access data in secondary storage, the CPU must communicate with the storage device's storage controllers and request specified data.

The CPU has a Control Unit which interprets and controls the execution of computer instructions it has loaded into RAM/ROM from the hard drive. It also has one or more Arithmetic Logic Units (ALUs) which carry out operations like addition/subtraction and an Input/Output Unit.

The CPU's Registers

Part of a computer's primary storage system are the registers inside the Central Processing Unit. There are usually less than 100 registers. Data the CPU is processing is stored in these registers.

They may be 8 bit registers; 16 bit registers; 32 bit registers; or 64 bit registers and the CPU may be classified according to the bit capacity of its registers. For instance, an 8 bit CPU will have 8 bit registers; a 16 bit CPU 16 bit registers; and so forth.

An 8 bit register can transfer 8 bits of data at one clock stroke of the CPU; 16 bit registers, 16 bits; 32 bit registers, 32 bits; and 64 bit registers, 64 bits. So, the larger the bit capacity of a CPU's registers, the more powerful the CPU will be all other things being equal- the more data that can be sent and received by the CPU. At this point, as we talk about data transfer, we need to divert briefly to talk about something which affects it.

Computers Receive and Send Data by Bus

A computer has a number of buses. A computer bus is a computer's data transmission roadway. Like an interstate highway system which allows cars and trucks to move back and forth across a country, a computer bus system allows data in the form of info, commands, instructions, addresses, data, etc. to move back and forth across its own inner road system.

So, a computer needs a capable bus system so data traffic can move effectively back and forth throughout the system without sluggishness and delay. What are some of a computer's bus systems?

The CPU has a CPU Bus which consists of lanes like lanes on an interstate highway. This allows for data transmission between its Control Unit; its Arithmetic Logic Units; and its registers. It has a CPU Memory Bus which consists of lanes that allow data transmission between it and its Primary Storage locations. It has a System Bus which allows for data transmission between the CPU and all the devices installed on the computer system.

As you might suspect, the wider these lanes are, the more adequately they can handle the flow of data traffic. In our prior discussion of registers, if the CPU has 16 bit registers but the bus only has 8 bit lanes, the CPU can only send half the data (8 bits) in one operation. So, it will have to transmit the 16 bits of data in its register in two operations. This will slow down its performance.

The CPU's Cache Memory

Instructions that are repeatedly required to run programs are stored in cache memory. The L1 cache memory is usually built into the CPU, installed on the die. L1 cache typically has storage capability from about 312 KB to 1024 KB. L2 cache which may be installed inside the CPU housing typically has storage capability of 512 KB to 2 MB. Or it may be installed outside the CPU housing. If the computer has L3 cache, it will be located on a chip outside the CPU or on the Motherboard. Either way it will be physically close to the CPU. Cache is designated L1, L2, L3 based on its nearness to the CPU; L1 being closest.

Secondary Storage

The shelves of books in a library are similar to a computer's secondary storage. They are filled with data which is available to be retrieved. When someone removes a book from the shelves and sits down at a table in the library to read it, we can say the data has been removed from secondary storage and moved to primary storage. The table is the primary storage location.

Figure 4

There are a few things we can take notice of here which are the similar to computer storage. First, primary storage is closer to where the data is being processed than secondary storage. Second, primary storage can store far less data than secondary storage. Third, data in primary storage can be accessed far easier and far faster than data in secondary storage. Fourth, data in primary storage can be accessed as quickly as any of the other data in primary storage. In other words, one book in primary storage can be accessed just as easily as another book. Books on the shelves can only be accessed with far greater difficulty.

Hard Disk Drive (HDD)

Figure 5The computer's largest secondary storage location is its hard disk drive, or just hard drive. Hard drives are platters like dishes which are stacked top, middle, and bottom to make one unit. Hard drives are mechanical devices which store data magnetically. They are considered permanent storage. The capacity of hard drives typically ranges from about 40 GB to 400 GB, or higher up to 2 TB.

Hard disk drives are read/write. They can be read over and over and they can be modified, or written to, over and over.

Tertiary Storage

Tertiary storage is very large storage which is separate from the computer. The most obvious example of tertiary storage is a automated storage facility where mechanical arms retrieve media and load it into large computers. Other tertiary storage may simply be off-grounds locations which allow vital data in various mediums to be safe-guarded for security purposes- fire, theft, etc.

Offline Storage

Offline storage is storage media which can be inserted into the computer and used but which can then be removed from the computer and stored elsewhere. It can also be external sources (ex. a remote hard drive) which are connected to the computer and then disconnected when the user is finished with them.

Floppy drives, CD drives, and DVD drives might also alternately be considered secondary storage because their drives are usually installed in the computer but the key here is the media the data is stored on. The media- the floppy, the CD, the DVD is not a part of the computer and is usually removed from the computer after use. It is, therefore, "offline" storage.

Floppy Drive

Figure 6Most modern personal computers do not have floppy drives anymore. The small, 3 1/2" disks they read only hold 1.44 MB of data so they have become somewhat obsolete when compared to the data storing capabilities of USB flash drives (up to 256 GB) and CD drives (700 MB).

Floppies are non-volatile storage. The data remains when the computer power is turned off. They can be read or written to over and over again.

USB Flash Drive

Figure 7The flash drive is a small removable stick that plugs into a USB port on the computer. Flash drives are non-volatile. Data can be read from and written to them over and over again. They are a convenient, temporary storage medium.

They are, however, considered a poor storage medium for long term storage because they are easily misplaced; somewhat fragile; and they can be corrupted through repeated use, unintended damage and ensuing malfunction. They are, however, good for storing data and carrying it with you from one point (home) to another point (the office) where the data will be used.

Flash drives today typically store somewhere from about 128 MBs to 16 GBs, though a 128 GB drive is advertised for $546 on the net. The largest flash drives hold 256 GBs.

There are two basic standards of USB connectivity prevalent today and one is on the way in consumer products in 2010. They are USB 1.0 and 1.1 (an improved version of 1.0); 2.0; and 3.0 coming next year.

USB 1.0 and 1.1 supports theoretical data transfer speeds of 1.5 Mbits/sec (low speed) to 12 Mbits/sec (full speed). It is used in low bandwidth devices such as keyboards, mice, and joy sticks.

USB 2.0 supports theoretical data transfer speeds of 480 Mbits/sec (high speed), a data transfer rate an impressive 40 times greater than USB 1's full speed. It is back ward compatible with USB 1 but at USB 1's slower speed.

USB 3.0 supports theoretical data transfer speeds of 5 Gbits/sec, or 5000Mbits/sec (super speed). Super speed is 10 times faster than USB 2.0 and 417 times faster than USB 1. It is considered to be largely backward compatible at the slower speed but compatibility is not guaranteed. Some 3.0 is incompatible with some older connectors.

CD Drive

Figure 8CD drives store data on shiny 5" discs, 1.2 mm thick. There are variety of disks the CD drive can "read". As already stated, CDs have a storage capacity of 700 MBs. This storage capability is equivalent to about 80 minutes of sound recording if that is what the user records to the CD.

Can be written to once and this occurs in the manufacturing process. The surface of the CD-ROM is actually etched or embedded with the data. The result is that the surface of the CD now has "lands", or raised areas, and "pits" or recessed areas, each representing either a 1 or a 0, which is read by the laser beam passing over it. Whereas most of the other drives mentioned store data magnetically, the CD drive is an optical device.

Most software the user can purchase comes on a CD-ROM which is used to load, or install, the software onto the user's hard drive. Books on CD and musical albums on CD also come on CD-ROM.

R for "Recordable" can be written to once by the user but only once. After that, they can be read over and over again. They are, therefore, a good medium for saving or backing up personal data the user doesn't want to lose. They can be kept and used indefinitely with good care.

RW for "Rewritable" can be modified, written to or over-written, time after time. And, of course, they can be read over and over again. The CD-RW might be for the person who hates to waste all the room he or she did not use on a CD-R but at today's prices, the CD-Rs only cost 10-20 cents each.

DVD Drive

Figure 9As mentioned above, a recordable CD has the capacity to store about 80 minutes of sound. But what if someone wanted to record a full-length movie on CD. That could take up to 7 CDs. That's where DVD disks come in.

Single layer digital video disks, or DVDs have a storage capacity of 4.7 GB; double layer DVDs a storage capacity of 8.5 GBs.

How do DVDs get so much storage space into a 5" shiny disc that looks just like a CD? They use of both the top surface of the disc and the bottom surface of the disc for storage. The data on the disc is more densely packed on the disc and there is a second layer added to DVD discs.

Read only. Can also read CD-ROM.

Write once. Read over and over again.
Same single and double layer. Can also read DVD-ROM drives.

Competing format with DVD-R. Developed 2002.
Can read DVD-ROM; CD-ROM; but not compatible with DVD-RAM

Read and Write. Can be read by most DVD-ROM drives.

Competing with DVD-RW.
Can read DVD-ROM; CD-ROM; but not compatible with DVD-RAM.

Recordable and writable.
Can read DVD-RAM; DVD-R; DVD-ROM; and CD-R discs.

Blu-Ray Drive

Blu-Ray has emerged as the next jump in technology over the traditional DVD formats mentioned above. Something was needed to play and record high definition titles and TV programs. Blu-Ray competed, for a brief time, with HD-DVD before prevailing. Now, according to Wikipedia, there are over 2500 titles available on Blu-Ray for viewers enjoyment. Blu-Ray is also used in the Play Station 3, PS3, which can play Blu-Ray titles.

The popular DVD formats use a red laser to access data on the DVD. Blu-Ray uses a blue-violet laser. The Blu-Ray laser has a shorter wavelength, 405 nanometers, versus the red laser, 650 nanometers. The shorter wavelength means the blue laser can focus on a smaller "data spot" on the disk. This allows the disk to be more densely packed with data. This translates into the Blu-Ray disk holding about 6x more data.

Though the Blu-Ray disks are the same size as standard DVD disks (12 cm), a single layer Blu-ray disk holds 25 GBs of data; a double layer disk 50 GBs. The current maximum recoding speed is 12x, which is about 54 MBs per second. Also available, Mini Blu-Ray disks (8 cm) are single layer 7.8 GBs; double layer 15.7 GBs.

Among the Blu-Ray Disk offerings are:

Used for commercial disks. A special piece of hardware stores a BD-ROM mark on the disk. Copies without this mark cannot be decoded. The mark is added to prevent illegal mass production of the disks.

Can be written once and viewed many times over.

Can be read and written to many times.

BD-R LTH Low to High
Can be written once and read many times over. Record speed 6x.