640

There are 640 acres in a square mile (United States and the United Kingdom). The acre began as the area one team of oxen could plough in a day, which is why it is a long thin shape — one furlong by one chain — rather than a square.

VGA, introduced by IBM in 1987, made 640 × 480 the baseline display mode that every PC could be assumed to support, and it stayed the safe-mode fallback in Windows for two decades. Its 4:3 shape and 307,200 pixels set the reference point later modes were described against — 800 × 600 and 1024 × 768 are both steps up the same ladder. The number is unrelated to the 640 KB memory limit despite both belonging to the same machine.

The 8088 could address one megabyte and the architecture reserved everything above 640 KB for hardware, so software received what was left. The limit was genuine and shaped a decade of DOS programming. The famous remark that 640K ought to be enough for anybody is attributed to Bill Gates, who has denied it, and no contemporary source has ever been produced.

640 is 128 × 5, which is why it appears at the boundaries of byte-organised systems without being a power of two itself: it is the largest multiple of a power of two that fits under a round decimal figure in several of the places it turns up. 640 KB is 655,360 bytes, and 640 × 480 is 307,200 pixels — neither is a round number in the units that matter to the hardware.