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The "extra memory" can be used in the page cache to cache frequently used files and metadata, such as directory information, from secondary storage. If the processor and operating system support multiple virtual address spaces, the "extra memory" can be used to run more processes.
Most, but not all, instructions that use DS by default will accept an ES override prefix. Processor stack references, either implicitly (e.g. push and pop instructions) or explicitly (memory accesses using the (E)SP or (E)BP registers) use the stack segment specified by the segment selector held in the SS register.
The memory subsystem manages the physical memory and the virtual memory of the system (both part of the hardware resource). The virtual memory extends physical memory by using extra space on a peripheral device, usually disk. The memory subsystem is responsible for moving code and data between main and virtual memory in a process known as ...
Rarely do processes require the use of an exact number of pages. As a result, the last page will likely only be partially full, wasting some amount of memory. Larger page sizes lead to a large amount of wasted memory, as more potentially unused portions of memory are loaded into the main memory.
However, on the 80386, with its paged memory management unit it is possible to protect individual memory pages against writing. [4] [5] Memory models are not limited to 16-bit programs. It is possible to use segmentation in 32-bit protected mode as well (resulting in 48-bit pointers) and there exist C language compilers which support that. [6]
Registered memory (also called buffered memory) is computer memory that has a register between the DRAM modules and the system's memory controller. A registered memory module places less electrical load on a memory controller than an unregistered one.
The Expanded Memory Specification (EMS) is the specification describing the use of expanded memory. EMS functions are accessible through software interrupt 67h. Programs using EMS must first establish the presence of an installed expanded memory manager (EMM) by checking for a device driver with the device name EMMXXXX0.
On IBM XT computers, it was possible to add more memory to the motherboard and use a custom address decoder PROM to make it appear in the upper memory area. [4] As with the 386-based upper memory described above, the extra RAM could be used to load TSR files, or as a RAM disk .