Extended memory refers to any amount of memory that is available for use over the main memory that is inherent in most DOS systems. In most cases, this means that any memory over the 1 megabyte (MB) that is supported by DOS would be considered extended memory. Memory of this type is supported by the use of certain types of microprocessors that allow loading and access to the extended memory. Along with the extended form of memory, many systems are also able to support what is known as expanded memory.
One of the chief differences between extended and expanded memory is that expanded memory is configured to meet a specific published standard that is known as EMS. This standard works with DOS to allow access to the extra memory and make it possible to use that memory for certain tasks. In contrast, extended memory does not have to make use of EMS or a particular published standard. Instead, the ability to make use of that extra memory via the microprocessor is managed with the use of an application programming interface that allows communication between the standard and the extended memory.
The function of extended memory makes it possible for a system to allocate memory resources with greater efficiency. As part of the allocation, the processor is able to determine when the standard memory is insufficient to manage certain tasks and make use of a portion of the added memory resources to complete the necessary steps in the sequence of the task. To end users, the perception is that the total memory is insufficient to drive the functions required, without any real need to distinguish if the resources involved are from the standard or the extended memory. The end result, a completed task, is still the same.
Over time, extended memory has become more common that the approach of using expanded memory. This is because newer systems make use of more powerful processors that render the need to utilize expanded memory obsolete. As a result, the more powerful processors can support the installation of significantly larger amounts of memory, which in turn allows the system to manage a greater number of tasks simultaneously, without negatively impacting the overall efficiency of the system. The ability to have access to increasingly larger amounts of memory has led to end users being able to engage in tasks with desktop and laptop systems that would have been impossible as recently as a decade ago. As the use of computers in everything from business tasks to accessing entertainment options continues to grow, processors capable of managing larger amounts of memory will be developed.