Complex sentence: In the DOS era, overlays were popular because the operating system and many of the computer systems it ran on lacked virtual memory and had very little RAM by current standards--the original PC had between 16K and 64K depending on configuration.
Split into simpler sentences: In the DOS era, overlays were popular because the operating system and many of the computer systems it ran on lacked virtual memory. Early PCs had very little RAM by current standards--the original PC had between 16K and 64K depending on configuration.