What Is Extended Memory?

The relentless drive for new technologies and terminology has dominated the computing industry since its inception. The term extended memory is precisely one such case, being relevant for less than a decade.

The IBM PC was trendy when it was released in 1981. It utterly dominated the home computer market. Its dominance inevitably resulted in an army of clones. It also resulted in many software being designed to be compatible with it. This locked in some design issues for the best of a decade.

Limits of Memory

The original PC used Intel’s 8088 CPU. This CPU had a hard limit on the amount of memory that it could address, 1MiB. While that may sound like a huge issue nowadays, it wasn’t when the PC was released. Memory was expensive, and small amounts were standard. This memory wasn’t just used for RAM, either. Some of it was allocated for use by graphics adaptors, the BIOS, the operating system, and other expansion utilities.

Of the 1MiB of memory, 680KiB was allocated to RAM. This was known as the conventional memory area. The remaining space was referred to as the upper memory area. In time, expanded memory offered ways to swap extra memory in via a memory window. This was, however, a bit of a kludgy solution. The real solution was to allow more memory.

Later successors to the PC utilized newer CPUs. Technical advances allowed them to address more memory. To differentiate it from the conventional and upper memory area, all of the above 1MiB was referred to as extended memory.

Rough Starts

Technically the first 65520 bytes of the extended memory area and the last 16 bytes of the upper memory area have another name. This is called the High Memory Area or HMA. It’s another legacy of the 8088. Due to the 20-bit addressing scheme and the offset used, the PC could address more than the 1MiB of memory installed. To allow this, the last 64 KiB was wrapped around the first 64.

Unfortunately, some software developers decided that this was a feature they should use. This meant that when newer models came around with more memory, they weren’t compatible, as that address no longer wrapped around. To prevent the surprisingly large array of software that did this from not working on newer models, IBM decided to artificially replicate the same issue, wrapping around that 64KiB of memory space.

Many software isn’t designed to support larger memory pools that PC successors offer with extended memory. For that reason, expanded memory also became popular.

Demise?

Just like expanded memory, extended memory was a term associated with IBM PCs and the DOS operating system. The advent of graphical operating systems like Windows enabled the use of protected mode and virtual memory addresses at all times. This removed all need to define different memory sections for different purposes. The increase to 4GiB of RAM with the 32-bit CPUs and operating systems also meant that there was 4096 times more memory space than there was in the original PC.

Technically this means that the term extended memory is obsolete. It is possible to interpret the definition as referring to all memory above 1MiB. However, even if those sections below it are no longer separately labeled. It’s probably better to treat the term as obsolete, though. The memory space is no longer treated differently, so the need for differentiation has gone.

Conclusion

Extended memory refers to memory after the first 1MiB of memory on DOS computers. It could be interpreted as referring to any memory above 1MiB, no matter the operating system. The term was, however, intended to differentiate from the conventional memory area, upper memory area, and expanded memory of the PC-compatible DOS systems.

As Windows and other graphical operating systems have superseded DOS, that entire vocabulary has been obsoleted. It was all based on restrictions required by the Intel 8088 CPU used in the IBM PC and memory layout choices within that restriction. Moving on from DOS eliminated the need for compatibility with those issues, as virtual memory addresses allowed the underlying memory to be treated as uniform.