Early disk operating systems (DOS) by Microsoft restricted the main memory of a computer to how many kilobytes?

In early Microsoft disk operating systems (DOS), the limitation of main memory was set to 640 kilobytes. This limitation was established primarily due to the architecture of Intel's x86 processors, which were commonly used during that time. The first 640 kilobytes of memory were designated as conventional memory and were directly accessible for use by DOS and applications.

Beyond this 640 kilobytes, the memory space became complicated because the upper memory regions—ranging from 640 kilobytes to 1 megabyte—were often reserved for system resources and hardware, including things like video memory and system ROM. While there were methods developed (such as using upper memory blocks) to utilize more than 640 kilobytes, the standard accessible memory for programs in DOS without additional configuration originally remained capped at 640 kilobytes.

This limitation significantly influenced software design and performance during the early years of personal computing, as developers had to create applications that functioned effectively within this restricted memory environment.