640kilobyte
640 kilobyte refers to 640 × 1024 bytes, equal to 655,360 bytes. In the context of early personal computers, it is most often discussed as the conventional memory limit of IBM PC–compatible systems. These machines used a 20‑bit address bus, which could address up to 1,048,576 bytes of memory, i.e., 1 MB. Of that address space, the first 640 KB (from 0x00000 to 0x9FFFF) was allocated as conventional RAM for running software, while the remaining 384 KB (from 0xA0000 to 0xFFFFF) was reserved for system firmware, video memory, and memory-mapped I/O.
The practical implication was that, in real mode, DOS software had a hard limit on available memory
The phrase 640 KB as a memory barrier also entered popular culture through a widely circulated but
Today, the notion of a 640 KB limit is largely obsolete, reflecting the substantial growth in available