640K

640K refers to a specific amount of computer memory, specifically 640 kilobytes. In the early days of personal computing, particularly with the IBM PC and its successors, this 640KB limit was a significant architectural constraint. This limit arose from the original design of the Intel 8088 processor and the way memory was addressed. The Intel 8088 had a 20-bit address bus, which allowed it to address a maximum of 1 megabyte (1MB) of memory. The initial design of the IBM PC BIOS (Basic Input/Output System) and hardware reserved the upper portion of this 1MB address space for system use, such as ROM and memory-mapped I/O devices. This left the lower 640KB available for the operating system and user programs.

As software became more complex and demanded more memory, the 640KB barrier became a major bottleneck. Programmers developed various techniques to overcome this limitation, including memory management schemes and overlays. The "640K ought to be enough for anybody" quote, often attributed to Bill Gates, reflects the prevailing sentiment at the time, though it later became a symbol of early computing limitations. Modern computers have vastly more memory, measured in gigabytes or even terabytes, making the 640KB limit a historical footnote. However, understanding this limitation is crucial to appreciating the evolution of computer architecture and software development.