Types of Memory

Read-only memory

Read-only memory (ROM) is a type of memory that cannot be written to. Information is written to ROM chips by the manufacturer, and this information cannot be changed. In the past, if ROM information needed to be updated, you had to remove the original chip and replace it with an updated ROM chip. Software written to ROM chips is called firmware. One of the major uses for ROM is to store the system BIOS, which contains Power- On Self-Test (POST) routines and other routines that initiate the loading of the operating system. The BIOS also contains the low-level code that makes communication possible between the central processing unit (CPU) and the hardware devices. Because the system BIOS is stored in ROM, you must remove and replace the ROM chip to update the BIOS software.

EPROM

Erasable Programmable Read-Only Memory (EPROM) is a type of memory that normally cannot be written to because it is a variation of ROM. An EPROM chip is a special ROM chip on which the manufacturer can rewrite information with a special programming device that uses ultraviolet light to erase all the data.

EEPROM

A new implementation of ROM is called Electrically Erasable Programmable ROM (EEPROM), or flash ROM. The manufacturer writes the software instructions into the ROM chip, but you can update these instructions by running special software provided by the manufacturer. The special software is usually provided on a disk or can be downloaded from the manufacturer’s Web site.

EEPROM has become the typical way to update your system BIOS. When my 486SX25 was designed, for example, it was not “aware” of my brand-new 6GB hard disk that I had purchased and planned to install in that computer. The manufacturer of the BIOS chip that’s installed in my 486SX is well aware of current devices. Therefore, the manufacturer places BIOS updates on its Web site for computer users running those BIOSs. I just have to download the update to a floppy disk and then run the BIOS update. The update rewrites the BIOS instructions, making the computer “more aware” of today’s hardware.

Random access memory

ROM is permanent memory, or permanent storage of information. RAM, or random access memory, is information stored temporarily. RAM is volatile, meaning that it needs constant current to maintain the information that resides in its chips. If current is lost, the contents of RAM will be erased. This section will discuss the different flavors of RAM. On the exam, you can expect a few questions on the different types of memory.

DRAM

Dynamic RAM (DRAM) is probably the most popular type of memory today and the one that you are most often going to upgrade. When someone says to you, “I have 128MB of RAM!” they are talking about DRAM. Dynamic RAM gets its name from the fact that the information stored in DRAM needs to be constantly refreshed. Refreshing involves reading the bits of data stored in DRAM and then rewriting the same information back. Because the CPU accesses data in memory, we measure the memory’s performance based on the time it takes the CPU to access that data. The measurement used to determine the speed of memory is nanoseconds (ns). One nanosecond equals a billionth of a second. If I have memory that is 50ns and my best friend has memory that is 70ns, we can assume that my memory is faster. My CPU receives the information from memory after waiting only 50 billionths of a second, whereas my best friend’s CPU waits 70 billionths of a second.

The speed of DRAM ranges from 60ns to 80ns. The lower the number of nanoseconds, the better the performance.

CMOS RAM

The Complementary Metal-Oxide Semiconductor (CMOS) is the area where the computer stores its configuration information, such as whether or not it has a floppy drive, the date and time for the system, and the number and size of the hard drives that are installed. Think of the CMOS information as an inventory list for the majority of components that are installed on the computer.

Is the CMOS information stored in the BIOS chip, or perhaps another ROM chip? The answer is no. In fact, if the information were stored in a ROM chip, you wouldn’t be able to go into the CMOS Setup program and change the configuration. The CMOS configuration information is stored in a type of RAM called CMOS RAM. CMOS RAM is a special volatile RAM chip that stores the CMOS information. Volatile means that if power is lost, the information is wiped out. This could present a problem with regard to CMOS configuration, since if the CMOS RAM is wiped out, the computer forgets its inventory information and has to relearn it. Thus, the computer has a small battery on the motherboard that maintains enough of a charge to avoid CMOS RAM erasure.

Shadow RAM

Part of the boot process involves copying some of the BIOS instructions from ROM up to RAM and then executing those instructions from RAM rather than from the ROM chip. Why? Because ROM is much slower than RAM, you will get a performance increase because you are executing the instructions from (faster) RAM, as opposed to (slower) ROM. This entire process is called shadowing, in which a copy of the BIOS instructions are shadowed, or copied, to an area of memory called shadow RAM.

SRAM

With speeds running at 10ns to 20ns, SRAM is much faster than DRAM. This speed increase comes with a price, however, which is why people add DRAM to their systems more often than they add SRAM. SRAM is typically used for cache memory. Cache memory is used to store frequently used data and program code from slower DRAM. Think of cache memory as a bucket that sits beside the CPU and stores frequently used information. After the system has searched through DRAM once for specific information, it can store that information in the bucket for easy access later. Because cache memory is much faster than DRAM, the CPU retrieves information from cache rather than from DRAM; thus, system performance is increased due to the reduced wait time.

VRAM

Video RAM (VRAM) is dual-ported memory, meaning it can be read from and written to at the same time. DRAM is single-ported (which means that the memory can be written to and read from, but not simultaneously; it has to go one way at a time). VRAM is most commonly used on video accelerator cards and is used to store the values of the pixels on the screen for refresh purposes. VRAM is the favored memory for video purposes because it outperforms the other memory types by being dual ported.

WRAM

Windows RAM (WRAM), also known as Windows Accelerator Card RAM, is a modification of VRAM and is also used for video display purposes. Like VRAM, WRAM is dual-ported memory, but runs about 25 percent faster. In general, WRAM offers better performance than VRAM (and at a cheaper price).

RIMM

There is a new high-speed flavor of DRAM on the market called Rambus DRAM (RDRAM), which runs at speeds around 800 MHz! The RDRAM chips have a 16-bit internal bus width and are packaged together in a 184-pin (gold plated) memory module called a Rambus Inline Memory Module (RIMM). In order to take advantage of this type of memory, you will need a motherboard and chipset that support RDRAM.