In: Categories » Computers and technology » Memory Processor Motherboards and buses » What is the Extended Memory
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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.
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