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 in which programs and their data could reside. As hardware and software evolved, techniques such as upper memory blocks, memory managers, and memory technologies like Expanded Memory (EMS) and XMS were developed to access additional memory beyond the 640 KB boundary. These approaches allowed more space for programs and drivers by relocating or bank-switching memory modules away from conventional memory.

The phrase 640 KB as a memory barrier also entered popular culture through a widely circulated but disputed attribution to figures such as Bill Gates. There is no verifiable record of Gates saying that, and Gates has denied the quote. Regardless of its origin, the 640 KB figure is commonly cited as a historical illustration of early computer memory constraints and the shift toward larger address spaces in later systems.

Today, the notion of a 640 KB limit is largely obsolete, reflecting the substantial growth in available memory and changes in how operating systems and applications manage memory beyond the era of DOS.