What is the Extended Memory

by Kyle Duke.
Share
|
Homepage | Submit your article | Contact | TOS
More articles on memory processor motherboards and buses  

You are here: Categories » Computers and technology » Memory Processor Motherboards and buses

The memory map on a system based on the 286 or higher processor can extend beyond the 1MB boundary that exists when the processor is in real mode. On a 286 or 386SX system, the extended memory limit is 16MB (24-bit addressing); on a 386DX, 486, Pentium, or Pentium MMX system, the extended memory limit is 4GB (4,096MB using 32-bit addressing). Systems based on the Pentium Pro, Pentium II, and newer processors have a limit of 64GB (65,536MB using 36-bit addressing).

Note

36-bit memory addressing is enabled through a method called physical address extension (PAE). PAE is an Intel-provided memory address extension that enables support of up to 64GB of physical memory for applications running on most 32-bit Intel Pentium Pro and later processors. PAE enables the processor to expand the number of bits that can be used to address physical memory from 32 bits to 36 bits, but it requires operating system support as well. PAE support for more than 4GB is provided only in server-oriented operating systems such as Windows 2000 and later versions of the Advanced Server and Datacenter Server operating systems. Without PAE support, memory addressing is limited to 32 bits or 4GB of physical RAM.

For a system to address memory beyond the first megabyte, the processor must be in protected modethe native mode of 286 and higher processors. On a 286, only programs designed to run in protected mode can take advantage of extended memory; 386 and higher processors offer another mode, called virtual real mode, which enables extended memory to be, in effect, chopped into 1MB pieces (each its own real-mode session). Virtual real mode also enables several of these sessions to be running simultaneously in protected areas of memory. They can be seen as DOS prompt sessions or windows within Windows 9x/Me, NT, 2000, XP, or OS/2. Although several DOS programs can be running at once, each is still limited to a maximum of 640KB of memory because each session simulates a real-mode environment, right down to the BIOS and Upper Memory Area. Running several programs at once in virtual real mode, called multitasking, requires software that can manage each program and keep them from crashing into one another. OS/2; Windows 9x/Me; and Windows NT, 2000, and XP all do this.

The 286 and higher CPU chips also run in what is termed real mode, which enables full compatibility with the 8088 CPU chip installed on the PC/XT-type computer. Real mode enables you to run DOS programs one at a time on an AT-type system just as you would on a PC/XT. However, an AT-type system running in real mode, particularly a system based on the 386 through Pentium 4 or Athlon, is really functioning as little more than a turbo PC. In real mode, these processors can emulate the 8086 or 8088, but they can't operate in protected mode at the same time. For that reason, the 386 and above also provide a virtual real mode that operates under protected mode. This enables real-mode programs to execute under the control of a protected-mode operating system, such as Win9x/Me or NT/2000/XP.

Note

Extended memory is basically all memory past the first megabyte, which can be accessed only while the processor is in protected mode.

Share
Leave a comment or ask a question
Total comments: 0

Memory Processor Motherboards and buses Disclaimer

  • The e-articles directory is not responsible for any and all copyright infringements by writers and authors. If you suspect the information contained by this page for any copyright infringements, please contact us to investigate the issue
EDORAM ~ Extended Data Out RAM - In 1995, a newer type of memory called extended data out (EDO) RAM became available for Pentium systems. EDO, a modified form of FPM memory, is sometimes referred to as Hyper Page mode. EDO was inv (more...)
Troubleshooting Memory - Memory problems can be difficult to troubleshoot. For one thing, computer memory is still mysterious to people because it is a kind of "virtual" thing that can be hard to grasp. The other difficult (more...)
Guidelines when upgrading memory on older systems - When upgrading an older motherboard, it is sometimes impossible to match the installed memory. Some motherboards have 30-pin and 72-pin SIMM sockets, and many have both SIMM and DIMM sockets. I (more...)
Guidelines when upgrading memory in Pentium II/III/IV Celeron and Athlon systems - These systems are excellent candidates for memory upgrades. Early Pentium II systems often have only 16 MB of RAM. In the price-sensitive consumer Celeron market, many early systems shipped wit (more...)
Power6 Processor: World`s Fastest Processor - Increases the processing speed to 4.7 GHz A revolutionary device in the form of tiny chip has been designed by IBM which has put the world in wonders. Power6 is really a mind-blow (more...)
About Clocking - In computers, a serial signal is one in which the bits of data of the digital code are arranged in a series. They travel through their medium or connection one after another as a train of pulses (more...)
Hardware Flow Control in the Serial Interface - Several of the signals in the serial interface are specifically designed to help handle flow control. Rather than a simple on and off operation, however, they work together in an elaborate ritua (more...)
Software Flow Control in the Serial Interface - The alternate means of handshaking, software flow control, requires your serial peripheral and PC to exchange characters or tokens to indicate whether they should transfer data. The serial perip (more...)
IrDA overview - The one thing you don't want with a portable PC is a cable to tether you down, yet most of the time you must plug into one thing or another. Even a simple and routine chore like downloading file (more...)
USB: Universal Serial Bus Overview - Three drawbacks head any list of the most aggravating aspects of serial ports: low speed, complex cabling, and the limited number of ports. The Universal Serial Bus breaks through all three, com (more...)
 
free content
    Copyright © 2006 - 2012 e-articles.info.
The texts, articles and tutorials in the directory are property of their respective owners and authors.