Expanded memory

Computer Memory

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Expanded memory refers to a computer memory management technique that was developed in 1984 by the Lotus Development Corporation. It allowed IBM-compatible personal computers to access more memory than the 640 KB limit imposed by the original IBM PC architecture. This technique was widely used during the 1980s and early 1990s, before it was eventually replaced by more advanced memory management technologies.

Overview

The original IBM PC architecture, introduced in 1981, limited the amount of memory that could be directly accessed by the operating system and applications to 640 KB. This limitation was due to the use of a 20-bit memory addressing scheme, which could only address up to 1 MB of memory. However, as software applications became more complex and memory-intensive, the need for additional memory became apparent.

Expanded memory provided a solution to this problem by utilizing a technique known as bank switching. It allowed the computer to access memory beyond the 640 KB limit by dividing the additional memory into 16 KB blocks, or "pages". These pages could be switched in and out of the 640 KB address space, effectively expanding the available memory.

Implementation

To implement expanded memory, a computer required both hardware and software support. The hardware component consisted of an expanded memory controller, which was typically a separate expansion card that plugged into one of the computer's expansion slots. The software component included a device driver that communicated with the expanded memory controller and provided the necessary interface for applications to access the expanded memory.

Applications that supported expanded memory could allocate and use the additional memory by making specific function calls to the device driver. The device driver would then handle the necessary bank switching operations to ensure that the requested memory pages were accessible to the application.

Advantages and Limitations

The use of expanded memory offered several advantages. It allowed software developers to create more memory-intensive applications, such as graphics and multimedia software, that would not have been possible within the 640 KB limit. It also improved overall system performance by reducing the need for disk swapping, as more data could be stored in memory.

However, expanded memory also had its limitations. The bank switching technique introduced additional complexity and overhead, which could impact system performance. Furthermore, not all software applications were compatible with expanded memory, requiring developers to specifically add support for it.

Legacy

As computer technology advanced, expanded memory was gradually replaced by more advanced memory management technologies, such as extended memory (XMS) and expanded memory specification (EMS). These technologies provided more efficient and standardized methods for accessing memory beyond the 640 KB limit.

Today, expanded memory is largely obsolete, as modern operating systems and applications have access to much larger memory capacities. However, it played a significant role in the evolution of computer memory management and paved the way for the development of more advanced memory technologies.

See Also

• Computer memory
• Memory management
• Extended memory
• Expanded memory specification

References