The original PC/XT-type system was designed to use 1M of memory workspace, sometimes called RAM (random access memory). This 1M of RAM is divided into several sections, some of which have special uses. DOS can read and write to the entire megabyte, but can manage the loading of programs only in the portion of RAM space called conventional memory, which at the time the first PC was introduced was 512K. The other 512K was reserved for use by the system itself, including the motherboard and adapter boards plugged into the system slots.
The term Upper Memory Area (UMA) describes the reserved 384K at the top of the first megabyte of system memory on a PC/XT and the first megabyte on an AT-type system. The way the 384K of upper memory is used breaks down as follows:
 | The first 128K after conventional memory is called Video RAM. It is
reserved for use by video adapters. When text and graphics are displayed
on-screen, the electronic impulses that contain their images reside in this
space. Video RAM is allotted the address range from A0000-BFFFF.
| The next 128K is reserved for the adapter BIOS that resides in read-only
memory chips on some adapter boards plugged into the bus slots. Most
VGA-compatible video adapters use the first 32K of this area for their
on-board BIOS. The rest can be used by any other adapters installed. Many
network adapters also use this area for special purpose RAM called Shared
Memory. Adapter ROM and special purpose RAM is allotted the address range
from C0000-DFFFF.
| The last 128K of memory is reserved for motherboard BIOS, (the basic
input/output system, which is stored in read-only RAM chips or ROM). The
POST (Power-On Self Test) and bootstrap loader, which handles your system at
bootup until the operating system takes over, also reside in this space.
Most systems only use the last 64K (or less) of this space, leaving the
first 64K or more free for remapping with memory managers. Some systems also
include the CMOS Setup program in this area. The motherboard BIOS is
allotted the address range from E0000-FFFFF. | |
A video adapter installed in your system uses some of your system's memory to hold graphics or character information for display. Some adapters, like the VGA, also have on-board BIOS mapped into the system's space reserved for such types of adapters. Generally, the higher the resolution and color capabilities of the video adapter, the more system memory the video adapter uses. It is important to note that most VGA or Super VGA adapters have additional on-board memory used to handle the information currently displayed on-screen and to speed screen refresh.
The memory map on a system based on the 286 or higher processor can extend beyond the 1M boundary that exists when the processor is in real mode. On a 286 or 386SX system, the extended memory limit is 16M; on a 386DX, 486, Pentium, Pentium MMX, or Pentium Pro system, the extended memory limit is 4G (4,096M). Systems based on the new Pentium II processor have a limit of 64G (65,536M).
Some older programs can use a type of memory called Expanded Memory Specification or EMS memory. Unlike conventional (the first megabyte) or extended (the second through 16th or 4,096th megabytes) memory, expanded memory is not directly addressable by the processor. Instead, it can only be accessed through a 64K window and small 16K pages established in the UMA. Expanded memory is a segment or bank-switching scheme in which a custom memory adapter has a large number of 64K segments on-board, combined with special switching and mapping hardware. The system uses a free segment in the UMA as the home address for the EMS board. After this 64K is filled with data, the board rotates the filled segment out and a new, empty segment appears to take its place. In this fashion, you have a board that can keep on rotating in new segments to be filled with data. Because only one segment can be seen or operated on at one time, EMS is very inefficient for program code and is normally only used for data.
The CPU and motherboard architecture dictates a computer's physical memory capacity. The 8088 and 8086, with 20 address lines, can use as much as 1M (1024K) of RAM. The 286 and 386SX CPUs have 24 address lines; they can keep track of as much as 16M of memory. The 386DX, 486, Pentium, Pentium-MMX, and Pentium Pro CPUs have a full set of 32 address lines; they can keep track of 4G of memory, while the Pentium II with 36 address lines can manage an impressive 64G! Whoa!
A RAM chip temporarily stores programs when they are running and the data being used by those programs. RAM chips are sometimes termed volatile storage because when you turn off your computer or an electrical outage occurs, whatever is stored in RAM is lost unless you saved it to your hard drive. Because of the volatile nature of RAM, many computer users make it a habit to save their work frequently. (Some software applications can do timed backups automatically.)
Memory chips (DIPs, SIMMs, SIPPs, and DIMMs) are organized in banks on motherboards and memory cards. You should know the memory bank layout and position on the motherboard and memory cards.
One standard IBM set for the industry is that the memory chips in a bank of nine each handle one bit of data: eight bits per character plus one extra bit called the parity bit. The parity bit enables memory-control circuitry to keep tabs on the other eight bits--a built-in cross-check for the integrity of each byte in the system. If the circuitry detects an error, the computer stops and displays a message informing you of the malfunction. If you are running a newer operating system such as Windows or OS/2, a parity error will generally manifest itself as a locked system. When you reboot, the BIOS should detect the error and display the appropriate error message.
For memory storage, most modern systems have adopted the single in-line memory module (SIMM) or dual in-line memory module (DIMM) as an alternative to individual memory chips. These small boards plug into special connectors on a motherboard or memory card. The individual memory chips are soldered to the SIMM/DIMM, so removing and replacing individual memory chips is impossible. Instead, you must replace the entire module if any part of it fails. The SIMM/DIMM is treated as though it were one large memory chip.
SDRAM is similar to EDO RAM in that it has a dual-stage pipeline structure. SDRAM delivers information in very high speed bursts using a high-speed, clocked interface. Like EDO RAM, your chipset must support this type of memory. Intel's 430TX and 430VX Triton II chipsets fully support SDRAM. Performance of SDRAM is similar to EDO RAM, with the exception that SDRAM supports bus speeds of up to 100MHz. It is anticipated that in the near future this figure will be pushed to 200MHz in order to keep up with faster systems of the future. SDRAM is limited primarily to DIMMs at this point, but prices are not appreciably higher than for other types of memory.
There is a newer type of memory being offered for Pentium systems called EDO (Extended Data Out) RAM. These are 72-pin SIMMs and 168-pin DIMMs with specially manufactured chips that allow for a timing overlap between successive accesses. EDO RAM has a dual-pipeline architecture that allows the unit to simultaneously read new data while discharging the old. This allows for a tighter coupled access cycle and a performance improvement of 20 percent or so over regular non-EDO SIMMs. EDO RAM is ideal for systems with bus speeds of up to 66MHz, which fits perfectly with the current and future Pentium and higher processor architectures. Unfortunately, all EDO RAM offered today is non-parity, and it is incapable of supporting systems with a bus speed higher than 66MHz.
DRAM stands for dynamic random access memory, a type of memory used in most personal computer.
Short for Double Data Rate-Synchronous DRAM, a type of SDRAM that supports data transfers on both edges of each clock cycle, effectively doubling the memory chip's data throughput. DDR-SDRAM is also called SDRAM II.
Short for Rambus DRAM, a type of memory (DRAM) developed by Rambus, Inc. Whereas the fastest current memory technologies used by PCs (SDRAM) can deliver data at a maximum speed of about 100 MHz, RDRAM transfers data at up to 600 MHz.
In 1997, Intel announced that it would license the Rambus technology for use on its future system boards, thus making it the likely de facto standard for memory architectures. However, a consortium of computer vendors is working on an alternative memory architecture called SyncLink DRAM (SLDRAM).
RDRAM is already being used in place of VRAM in some graphic accelerator boards. As of late 1999, Intel has been using RDRAM in its Pentium III Xeon processors and more recently in its Pentium 4 processors. Intel and Rambus are also working a new version of RDRAM, called nDRAM, that will support data transfer speeds at up to 1,600 MHz.
Short for static random access memory, and pronounced ess-ram. SRAM is a type of memory that is faster and more reliable than the more common DRAM. The term static is derived from the fact that it doesn't need to be refreshed like dynamic RAM.
While DRAM supports access times of about 60 nanoseconds, SRAM can give access times as low as 10 nanoseconds. In addition, its cycle time is much shorter than that of DRAM because it does not need to pause between accesses. Unfortunately, it is also much more expensive to produce than DRAM. Due to its high cost, SRAM is often used only as a memory cache.
02/21/2001