- Conceptual Overview
- Read-Only Memory (ROM)
- Random Access Memory (RAM)
- Cycles and Frequencies
- Summary—Basic Memory
- Cache Memory
- Memory Pages
- Rambus Memory (RDRAM)
- Double Data Rate SDRAM (DDR SDRAM)
- Video RAM (VRAM)
- Supplemental Information
- Packaging Modules
- Memory Diagnostics—Parity
- Exam Prep Questions
- Need to Know More?
Read-Only Memory (ROM)
Every computer uses both read-write (RW) memory and read-only (R) memory. We see these same designations applied to CD-RW and CD-R, or DVD-RW and DVD-R optical disks. "Writing to" memory is the same as placing information in a memory address. "Reading from" memory is the same as retrieving information from an address. Information can be temporarily stored (written) in RAM, then a moment later, taken out (read). New information can then be written to the same place. Although the acronym RAM stands for random access memory, think of it, for the moment, as read/write memory: It can be both written to and read from.
ROM is read-only memory, and typically, doesn't allow changes. ROM can have information written into it only one time. From that point on, all we can do is read whatever was put there. Imagine a bulletin board under glass at the back of a classroom. ROM information is like hard-copy notes placed under the glass. At the end of the day, we turn out the lights and everyone goes home. The words on the paper remain unchanged. The next day, the notes are exactly the way they were the day before.
ROM is nonvolatile because no electrical current is required for the information to remain stored. ROM chips are mostly used for BIOS, although the same concept and acronym applies to commercial pre-recorded compact disks. The ROM in CDROM stands for read-only memory. Although CD-RW and DVD-RW can be changed, they're referred to as permanent storage media.
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. Because the BIOS instructions are permanent, they almost always use ROM.
RAM, on the other hand, is like a blackboard. It starts out empty, then 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 blackboard. At the end of the day, we turn off the lights and wash off the blackboard. Whatever data was on the board goes away forever. When you turn off the power to a computer, RAM no longer has the necessary electrical current to sustain the data in its memory cells. Once again, RAM is volatile because it can't store information without using electricity.
Windows sets aside (allocates) some amount of memory as resource memory. When we write and erase many times on a blackboard, we get a chalk build-up. Similarly, resource memory can sometimes become disorganized and confusing for Windows to read. You can repair this memory fragmentation either by re-starting the machine or by using specialized third-party software utilities.
RAM Versus ROM
RAM is to a computer as your attention span is to your mind. When you cram for this exam, you'll focus your attention on facts and figures, placing them into short-term memory just long enough to write them out to a piece of paper in the exam room. After the data is stored to the sheet of paper, you can "erase" the information in your attention area and bring in new data. New data might be an exam question, on which you can then perform calculations such as determining a correct answer. When you require the information you wrote to the paper, you can return it to your attention by reading the page.
The tear-away Cram Sheet on the front cover of this book is designed to give you the minimal basics of those difficult-to-remember facts you'll likely want to have handy during the exam. Although you can't bring the sheet into the exam room, you can try to remember them long enough to write them on the blank piece of paper you'll be given when you've entered the exam room.
The piece of paper in the example is similar to a floppy disk. There isn't a lot of room on the paper, but you can carry it easily in your shirt pocket. A loose-leaf binder or notebook would be more like a hard disk. Depending upon the size of the binder, you can store a lot more information than on a single piece of paper. If you were to engrave the information on the desk in the exam room (not allowed), it would be analogous to authoring a CDROM.
ROM is like your long-term memory, holding the things you remember from your past. This is also like the information stored in BIOS and CMOS. When the computer "wakes up," ROM settings provide an awareness of the size of the hard disk, the presence of a sound card, whether or not any memory exists (and how to use it), and simple access routines to permanent hardware.
Basic Input/Output System (BIOS)
When you turn on a PC, the processor first looks at the basic input/output system to determine the machine's fundamental configuration and environment. This information is stored in a ROM chip and largely determines what peripherals the system can support. BIOS instructions are updated regularly by the manufacturer, not by the end user. If the chip is made to be updated (re-programmed) by the end user, it is often called Flash BIOS, or sometimes, Flash ROM. These programmable chips are often referred to as EEPROM (pronounced ee-prom) chips, discussed in a moment.
In a human being, BIOS would be like waking up and learning that you have a head, two arms, and two legs. The POST would be like a quick self- assessment as to whether or not you can move your arms and legs, and how bad a headache you have. CMOS would be like knowing your name, your address, and that you were last configured as a drinking machine.
In Chapter 2, "Motherboards," we pointed out that memory speed has usually been measured in nanoseconds (billionths of a second). We measure processor speeds in megahertz (millions of cycles per second) or gigahertz (billions of cycles per second). Although gigahertz CPUs operate in billionths of a second, instructions executing out of other processors, such as BIOS chips, execute quite a bit slower. The CPU and other devices may have to repeatedly query the BIOS chip for simple but permanent instructions, thereby reducing system performance. Shadow RAM is a method of storing a copy of certain BIOS instructions in main memory, rather than leaving them in a chip. The process improves execution speed and avoids constant calls to the slower chip. Many computers provide an option to shadow both the BIOS and certain video functions.
DRAM, fast page mode (FPM), and extended data output (EDO) mode all measured memory access times in nanoseconds. A 70ns unit would be labeled a "7." A 60ns unit would be labeled a "6," and so on. The lower the number, the faster the memory (shorter access time). With the introduction of SDRAM, these time measurements became less accurate. At such short intervals, fractions began to lose any real meaning. Instead, it began to make more sense to use speed measurements in the same way as CPUs. For this reason, SRAM and SDRAM modules use ratings such as 66MHz, 100MHz, 133MHz, or 800MHz.
Here's an example of one-time, read-only memory: storing a book on CDROM. Technically, write-once, read-many (times) is written as WORM. A magnetic disk is write-many, read-many, but you won't see a WMRM acronym. Instead, we speak of re-writeable optical disks. CD-RW changed the way that we use CDs and DVDs, just as programmable ROM chips changed the BIOS.
A manufacturing mask is the photographic blueprint for the given chip. It's used to etch the complex circuitry into a piece (chip) of silicon. The overall combination of silicon wafers, circuits, and microscopic components making up a CPU is called the die (like one of a pair of dice). The formal name for a chip that cannot be modified is mask ROM (from the manufacturing mask). The following types of chips offer varying degrees of programmability:
Programmable ROM (PROM)Requires a special type of machine called a PROM programmer or PROM burner (like a CD burner) and can be changed only one time. The original chip is blank, and the programmer burns in specific instructions. From that point, it cannot 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, then written to by using 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're often used to store programmable instructions in devices such as printers and other peripherals.
With advances in technology, most BIOS chips became Flash EEPROM. These chips make it easier to change the BIOS. Rather than pulling out an actual chip and replacing it with a newer one, upgraded programming can be downloaded through the Internet or a bulletin board service (BBS). A small installation program changes the actual program instructions, eliminating the need for opening the computer case.
These types of chips are sometimes called Flash ROM or Flash memory, and store data much as EEPROM does. They use a super-voltage charge to erase a block of data. However, as we said earlier, EEPROM can be erased only one byte at a time. Although both Flash ROM and EEPROM can perform unlimited read/write operations, they can be erased only a certain number of times. (Be aware that Flash memory is not the same thing as nonvolatile memory cards used in such devices as digital cameras.)
As you know, basic motherboards vary in components such as CD or DVD drives, hard drives, memory, and so forth. The CMOS chip is a particular type of memory (static RAM) used to store optional system settings for those components. For example, the board might have a floppy drive and some memory chips. The BIOS stores instructions as to how to reach those components, and the fact that they exist. The CMOS stores variable settings, such as the disk size, the number of platters, and how much memory happens to be installed.
CMOS tends to store information about "unexpected" devices, and settings are held in memory through the use of a small electrical charge. Although CMOS is technically volatile memory, a trickle charge comes from a battery installed on the motherboard. Even when the main power is turned off, the charge continues to maintain the settings. However, if the battery power fails, all CMOS information vanishes.
BIOS determines compatibility. Some modern BIOS settings are often stored in the CMOS chip. Older BIOS was completely stored in nonvolatile ROM chips, often soldered right onto the motherboard. Remember that the CMOS is almost always where the computer's configuration settings are stored. BIOS is where basic input/output routines for the computer are stored.
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. CMOS settings are essential to the configuration of a specific computer. BIOS instructions typically work with a generic type of motherboard and its chipset.
A 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 PCs, turning on a local printer, re-starting the machine (as opposed to a first-time boot), and going into the CMOS settings can generate this type of report. Press the Print Screen key at each screen.
When you exit out of the CMOS setup, the machine will most likely restart. From within Windows, open a text editor (for example, Notepad) and print a blank page. The stored page in the printer comes out as part of the print job. From a DOS command line, you can send an end-of-form page request to the printer to print the last page in the printer's memory. The following ^L is actually created by pressing the Ctrl+L key. Type echo ^L > prn.
Most computers cannot access the PRN device before a successful boot process. Therefore, the Print Screen function may not work. However, a warm reset, as opposed to a power-down and cold reboot, often allows the Print Screen function to remain in low memory. If the Print Screen function doesn't remain loaded 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 other problems with CMOS in Chapter 16, "Troubleshooting."