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This chapter is from the book

Read-Only Memory (ROM)

Every computer uses both read-write (RW) memory and read-only (R) memory. Optical disks use a similar designation with CD-RW and CD-R designations. Although the acronym RAM stands for random access memory, for the moment you should think of it as read/write memory. ROM is Read-Only Memory. Information can be temporarily stored in RAM, and then a moment later, it can be taken out and new information can be written to the same place (address). ROM doesn't allow changes. When information is placed in memory, we say that we are "writing to" a memory address. When information is retrieved out of memory, we "read from" that memory.

Picture a bulletin board under glass at the back of a classroom. One way to think of ROM is that it's like the hard-copy notes placed under the glass. At the end of the day, they remain unchanged. The next day, the notes are exactly the way they were the day before. We were able to only read them.

If you think of RAM as a blackboard, it starts out blank. During the day, information is written on it, read from it, and maybe even erased. When something is erased, new information is then written to the same place on the board. If a lot of writing and erasing takes place, a chalk buildup forms on the blackboard. This buildup is similar to memory fragments, which can cause computer lockups.

When you go home at the end of the day, you turn off the lights, wash a blackboard clean, and whatever data was on the board goes away forever. This is what happens when you turn off the power to a computer; RAM no longer has the electrical current available to sustain the data in its memory cells.


ROM can have information written into it only one time. From that point on, we can only read the information. No electrical current is required for the information to remain stored—it's nonvolatile. ROM is mostly used for BIOS, although the same concept and acronym apply to commercial compact disks. These are CD-ROM, with the ROM standing for read-only memory.

In some instances, ROM can be changed through the use of certain tools. Flash ROM is nonvolatile memory that occasionally can be changed, such as when a BIOS chip must be updated.

A single letter can really mess you up on the exam if you don't pay close attention. We've seen questions like, "RAM BIOS is used to permanently store instructions for a hardware device: True or False?" (The answer is false.) Keep your eyes peeled, and remember that RAM sounds like RANdom. RAM is never used in BIOS. Since the BIOS instructions are permanent, they almost always use ROM.

RAM is to a computer, like your attention span (i.e., short-term memory) is to your mind. When you cram for this exam, you'll fill your short-term memory with facts and figures just long enough to write them out to a piece of paper in the exam room. Once the data is on the sheet of paper, you can "forget" the information and concentrate on new data, such as the meaning of an exam question. More realistically, your attention span is like a cache (discussed in a moment), and the piece of paper is like RAM. If you were to engrave the information on the surface of the exam-room desk (this is not allowed), it would be more like ROM.

ROM is more like your long-term memory; the things you remember from your past. When you wake up in the morning, you know your name and address. This is like the information stored in BIOS. On the other hand, you may not remember how many glasses of water you had the day before. That information was stored in your short-term memory.

RAM, short-term memory, or attention span, is like a holding tank for data on its way to becoming information. Data becomes information when it takes on context (surrounding circumstances). 76, on its own, means nothing other than the fact that it's a number. Surround that number with context, "Tomorrow, the temperature is expected to reach 76," and it becomes information.

If someone were to ask you to repeat every word on the last page of the previous chapter, you would have to stop what you were doing; turn the pages of this book until you found the requested page, and then read each word. If you were a computer, you would then pause and wait for a new instruction. A human mind continually extracts meaningful information from words, paying little attention to the words themselves. A computer only manipulates bits of data.

Basic Input/Output System (BIOS)

When you turn on a PC, the processor first looks at the basic input/output system (BIOS) to determine the machine's fundamental configuration and environment. This information is stored in a ROM BIOS chip, and largely determines what peripherals the system can support. BIOS instructions are updated regularly by the manufacturer, and if the chip is made to be updated (re-programmed) by the end-user, it is often called Flash BIOS. These programmable chips are often referred to as EEPROM (pronounced ee-prom) chips, which we discuss later in this chapter.


We examine the boot process in depth in Chapter 11, "Booting, Windows 3.x, and Memory."

CMOS Settings

Although a basic motherboard can be pretty standard, the system can vary in components like hard drives; floppy, CD, or DVD drives; memory; and so forth. The complimentary metal oxide semiconductor (CMOS) is a small memory chip that stores the optional system settings (e.g., hard drive specifications, amount of memory, and so forth). Because these settings are held in CMOS memory with a small electrical charge, CMOS is volatile. However, this trickle charge comes from a battery installed on the motherboard, so even when the main power is turned off, the charge continues. If the battery power fails, all CMOS information vanishes.


Technically, CMOS is different from ROM BIOS in that the CMOS settings require some source of electrical power. Nonvolatile memory doesn't require electricity at all.

Older computers, such as some IBM PS/2 models and the original IBM AT, required a setup program stored on a special floppy disk. When you ran the program, a setup screen allowed you to configure the machine. These configurations were stored in special files on the hard drive. Compaq continued the idea of putting a setup program on a disk. Typically, Compaq's CMOS settings were held in a dedicated 3–4MB, non-DOS partition on the hard drive.


Today, most computers use a keystroke combination, such as ESC, Del, F1, or F2 to access the CMOS. The keys are pressed at startup, before the BIOS transfers control to the operating system. (This is not the same as accessing Windows Safe Mode.)

The CMOS settings are essential to the hardware configuration of any personal computer. A typical symptom of a fading CMOS battery is that the system date begins to fluctuate, sometimes by months at a time. Backing up files and software are a standard part of keeping a current backup, but you should also have a report of the current CMOS settings. On many modern PCs, this can sometimes be done by turning on a local printer, restarting the machine (as opposed to a first-time boot) and going into the CMOS settings. At each screen, press the Print Screen key.

When you exit out of the CMOS setup, the machine will most likely restart. If you boot to DOS, you can send an end-of-form page request to the printer to print the last page being held in printer memory. This can be done by typing "echo ^L > prn" (without the quotes). The "^L" is actually created by pressing the Ctrl+L key. From within Windows, open a text editor (for example, Notepad) and print a blank page. The stored page in the printer will come out as part of the print job.


If the Print Screen function doesn't load on a particular machine, the only other way to store the CMOS settings is to manually write them down on a piece of paper. We discuss problems with CMOS in Chapter 14 "Troubleshooting."

Flash BIOS

With advances in technology, most BIOS chips became Flash EEPROM (electrically erasable programmable ROM). These new EEPROM chips made it easier to change the BIOS. Rather than pulling out the ROM chip and replacing it with an updated one, upgrades could be downloaded through the Internet or a bulletin board service (BBS). A small installation program changed the BIOS programming, eliminating the need for pulling apart hardware.


BIOS determines compatibility. Modern BIOS is often stored in the CMOS, whereas older BIOS was stored in nonvolatile ROM, often soldered into the motherboard. Remember that the CMOS is almost always where the computer's configuration is stored. BIOS is where the software instructions for the basic input/output (I/O) operations are stored (for example, COM and LPT ports, expansion bus, and so forth).

Programmable ROM

Compact disks (CD-ROMs) offer another common use for one-time, read-only memory. In the same way that rewriteable CDs (CD-RW) changed the way that we use the disks, programmable ROM chips changed the way BIOS was stored. The formal name for a chip that cannot be modified is mask ROM (from the manufacturing mask). ROM chips have a varying capacity for change, named in the following manner:

  • Programmable ROM (PROM)—Requires a special type of machine called a PROM programmer or PROM burner (like a CD burner) and can only be changed one time. The original chip is blank, and the programmer burns in specific instructions. From that point, it can't be changed.

  • Erasable programmable ROM (EPROM)—Uses the PROM burner, but can be erased by shining Ultraviolet (UV) light through a window in the top of the chip. Normal room light contains very little UV light.

  • Electrically erasable programmable ROM (EEPROM)—Can be erased by an electrical charge and then written to by using slightly higher-than-normal voltage. EEPROM can be erased one byte at a time, rather than erasing the entire chip with UV light. Because these chips can be changed without opening a casing, they are often used to store programmable instructions in devices, such as printers and other peripherals.

Flash ROM

This type of chip is sometimes called Flash RAM or Flash memory and stores data much like the EEPROM. It uses a super-voltage charge to erase a block of data (rather than a byte). Flash ROM and EEPROM can perform read/write operations, but can only be erased a certain number of times. Flash BIOS relies on this ability to change the program instructions in a ROM chip.

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