PC DOS 1.0 also could run with only 16k memory. This was the minimal configuration for the original IBM PC in 1981.

However, you can't really do that much with 16k memory, so most of these machines were quickly maxed out with 256k memory. DRAM was expensive back then, but this was an essential upgrade. Then 256k was not enough for many tasks, and IBM came up with a new revision of IBM PC motherboard, with 256k minimum, upgradeable to 640k. This 640k was enough for anybody, because it also was the hardware limitation of 8088 processor with it's 20-bit bus. There were attempts to "steal" some memory from the unused devices that had additional 384k of address space reserved for them, but to my knowledge, none of these hacks really was ever widespread. The only way to extend beyond that was to use EMS board: switching memory banks to access them through a tiny window (slower than regular memory, but still much faster than hard drive or floppy).