More About Memory

lled modules that plug into the motherboard. This makes it very convenient to add more memory by plugging in another module. Older computers used a SIMM, which stands for Single Inline MemoryModule, and later ones use a DIMM or Dual Inline Memory Module. The difference is that DIMMs have different signals on each side of the module connector, and the additional signals allow more memory addresses and more data lines going in and out.

The amount of memory is always is some multiple of two bytes. The most common values are 32, 64, 128, 256 or 512 megabytes.

As it is used by the system, RAM is divided into base memory and extended memory. Base memory is the first megabyte, and extended memory is everything over that. The very first PCs had only base memory, and then a scheme was developed called expanded memory that tricked the computer into allowing more than 1 MB of memory. Once extended memory was developed though, expanded memory quickly became obsolete and will only be found in older computers.

Another memory term you will encounter is cachememory. This refers to memory that is not on DIMMs but is either built into the CPU or connected directly to the motherboard near the CPU. It provides even faster access than RAM for the data that the CPU expects to need next. The contents of the cache will change constantly depending on what the CPU is doing.

More About Disk Drives

Floppies – Although floppy drives are being phased out in some new computers, there are still millions of them out there and you should know something about them. The floppy drive has a little slot on the face of the computer cabinet, and into this slot you can slide a floppy diskette like the one shown here. One of the reasons floppy drives are still around is that it is very easy to take a floppy diskette from one system to another.

Inside the floppy diskette is a round flat disk coated with iron oxide on each side so that data can be stored on it magnetically. This disk is called a platter, and it spins underneath an electro-magnet called the write head that puts data onto the platter surface. There is another head called the read head that copies data from the platter.

Once the disk has made one complete revolution, data is written all the way around. That is called a track. The head then moves a bit and writes another circle of data to create a second track. Altogether, there are 80 tracks on each side, for a total of 160. Altogether, the floppy can hold 1.44 MB (megabytes) of data.

If we are looking for just a few bytes out of 1.44 million, it’s not enough to know which track it is in. To help narrow the search, the track is divided into 18 pieces, calledsectors, which look much like a slice of pie. Each sector holds 512 bytes of data, so if we know the track and sector number of the data we want it won’t be hard to find.

Hard Drives – On a hard drive, data is also organized into tracks and sectors. While each sector still holds 512 bytes, there can be many more tracks and sectors on a platter. There are also multiple platters, one on top of the other like a stack of pancakes. Hard drives can hold much more data than floppies, sometimes into the billions of bytes, calledgigabytes (GB).

Multiple platters require multiple read and write heads, all attached to the same arm so they move together. It’s called an actuator arm. When we are reading track number 10 on the top platter, the other heads are also positioned over track 10 of the other platters, and together all of these track 10s make up a cylinder. To specify the location of data on a hard drive it is necessary to say what cylinder, then the track and sector. Moving the heads from one cylinder to another is called a seek, and the amount of time this takes is the average seek time.

Although hard drives can hold much more data than floppies, the platters are sealed into a metal case that is fastened inside the computer cabinet, so it’s not an easy matter to move from one system to another like you can with floppies. A hard drive is sometimes called a fixed disk for this reason.

Operating systems use a couple of different methods to keep track of what data is stored where on a drive. One common method uses a table called a File Allocation Tableor FAT, which is a section of the disk with pointers to data locations. There are two versions, called FAT16 and FAT32. Windows NT, XP and 2000 use a similar method called NTFS.

There are two different interfaces commonly by hard drives to talk to the rest of the system. These are called IDE for Integrated Drive Electronics, and SCSI for SmallComputer System Interconnect. The technical differences are not important at this point, but you should know about the two types because they are not interchangeable.

Figuring out where the heads should go next and then moving them there is the job of some electronic circuitry called the disk controller. Every disk drive has its own controller, which may be on the motherboard or inside the drive itself, depending on the type of drive.

There are a few more things you should know about disk drives before we leave the subject. The first sector of Cylinder 0, Track 0 is called the boot sector, and it contains aMaster Boot Record (MBR) that shows whether the disk contains an operating system and the location of the code. If there is more than one operating system, the drive must be divided into multiple partitions. If not, then the whole drive will be a single partition. All of the disk space assigned to a partition is called a volume.

Another term you will encounter is a disk format. There is a high-level format, which creates a new file allocation table and is done with a FORMAT command. There is also alow-level format that creates a new pattern of sectors. A low-level format must be followed by an FDISK command to create a new Master Boot Record and partitions.

Last, we have the word media. This refers to the actual surface holding the data, which is the platter in the case of a disk drive. Because the floppy platter can be taken out of the drive, it is called removable media, while a hard drive is called fixed media.

Other Drives – Most systems today, especially home systems, have additional storage drives that use CD or DVD discs. The technology for both is similar but DVDs hold much more data. These drives do not store data magnetically but use optical markings that are read with a laser. They are mostly used just to read data and not to write it. The full name for CD in fact is CD-ROM, which stands for Compact Disc - Read Only Memory. However, there are versions that can be used to write also, and these are called CD-RW and DVD-RW. Even so they are mostly used to write just once for permanent storage, and are not practical for constantly changing data.

Like hard drives, CD-ROM drives can use either an IDE or SCSI interface. The version of IDE for CD-ROM drives is called ATAPI, and for SCSI the CD-ROM version is ASPI.

Because the discs can be removed, CD-ROM and DVD are considered removable media. There are other types of removable media also that are not as common, such as tape drives and Zip disks, which are similar to floppies but with a storage capacity of 100 or 250 MB. Zip disks and tape drives also use the ATAPI interface.

More About Video

The monitor is a passive device that just displays the video output from the system. However, so much data is needed for the constantly changing screen display that special provisions are made for it.

The video card (or video circuitry on the motherboard) has its own RAM memory just to hold the display information, and its own ROM BIOS to control the output. Some motherboards even have a special high-speed connection between the CPU and the video. It’s called the AGP, or Accelerated Graphics Port.

The important numbers in evaluating a video display are how many distinct colors can be displayed and also the resolution, which is how many pixels the image contains across and from top to bottom. Each dot of color making up the image is one pixel. As video technology evolved there have been a number of standards, and each one has its own set of initials like EGA, CGA or VGA. A common one is SVGA, which stands for SuperVideo Graphics Array and has a resolution of 800x600 (that’s 800 pixels across and 600 down). Some high-performance monitors use SXGA (1280x1024) or even UXGA with a resolution of 1600x1200.

CMOS and RTC

There is other start-up information that normally stays the same but that we might want to change once in a while. This includes info about the various pieces of hardware connected to the system, which disk drive to check first for the operating system and that sort of thing. This data can’t be stored on the hard drive because we need it to boot up. It can’t be stored in RAM because it will be lost at power-off, and it can’t be stored in the BIOS because we might need to change it.

The problem is solved by a type of RAM chip that uses very low power, and it is connected to a battery. This type of low-power memory chip is called CMOS. It stands for the type of technology used in the chip, which is Complementary Metal OxideSubstrate. This is probably more than you need to know, but I’m a fanatic about defining things. By the way, since batteries don’t last forever, if you leave your computer unplugged for about 5 years you’ll find it needs a bit of trickery to get it to boot again, because the CMOS information will be gone.

There is another feature in the computer that has the same requirements as CMOS, and that is the date and time function. This obviously needs to change very minute, but we don’t want to lose track when the computer is turned off. The circuitry for this is called the RTC or Real Time Clock, and for convenience it is usually included in the same chip with the CMOS. A little trickle of juice from the CMOS battery keeps the clock running, and when you turn the computer on again it knows exactly what time and day it is. Convenient, isn’t it?

The BIOS

As we mentioned earlier, the computer knows what to do by taking instructions from programs stored in RAM. The main instructions come from a program called the operating system, and those instructions direct traffic for other programs called applications.

When the computer is turned off, all the instructions copied into the RAM are gone. When the system is turned on again, it needs to go out to the disk, get the operating system and load it into RAM, but there are no instructions in the RAM to tell it how to do this. The solution to this problem is a set of instructions that stay in memory and don’t get lost when the computer is turned off.

This set of instructions is called the BIOS, for Basic Input Output System. Since the instructions don’t need to change, they can be stored in a different kind of chip than we use for RAM. It’s called ROM, for Read Only Memory. We say that the instructions in the BIOS are hard-wired, and instead of software they are called firmware.

The computer goes through a process called booting up when it is first turned on. This involves executing the BIOS instructions, loading the operating system from disk into RAM, and then turning control of the computer over to the operating system after everything checks out OK. The term refers to somebody pulling themselves up by their own bootstraps (without outside help, in other words). Any computer term that includes ‘boot’ will have something to do with this start-up process.

Ports

In addition to the I/O ports mentioned in the last section which are really addresses, there are physical connections on the back of the computer that are also called ports, and various devices connect to the computer through cables attached to these ports. One of these connections is called the Serial Portbecause data goes over a single signal line as a series of bits, one right after the other. Serial port connectors have either 9 or 25 pins, with the male connector on the back of the computer and a female connector on the cable. The 9-pin version is more common, and is often used to connect a mouse if there is not a separate mouse connection.

Another connection is called the Parallel Portbecause its cable has 8 parallel signal lines to transmit 8 bits at a time. The computer has a 25-pin female connector on the back, so it won’t be confused with the 25-pin male serial connector. The parallel port is most often used to connect a printer. Computers sold in the last few years will probably also have a connection called USB, for Universal Serial Bus. It is a bus because several devices can be connected on the same cable, but it’s also a port because data goes into and out of the computer cabinet through its connector. There is USB1 and USB2. USB2 is newer and much faster.

The Motherboard

The biggest and most important circuit board in a computer is the motherboard. It's called the motherboard because every other part of the computer is connected to it. The CPU plugs into the motherboard, and so does the memory. And all the components that plug into the back of the case, like the keyboard and monitor, are connected from there to the motherboard. It is also called the system board.

There is a set of slots near the back edge of the board for plugging in other circuit boards, calledadapter cards. These plug-in slots are for things like modems, sound cards, network cards and just about any other add-on feature you can think of. There is a set of traces connecting these slots to each other. A trace goes from Pin 1 of the first slot to Pin 1 of each of the other slots, and other traces connect each of the Pin 2s, each of the Pin 3s, etc. These traces then go on to connect to the memory, CPU, disk drives and other parts of the computer.