Back then 640KB was supposed to be enough for anyone.

It wasn't.

HIMEM. SYS, if I recall correctly, allowed you to tell DOS to load as much as possible of itself (and maybe even some drivers?) into “high memory” (within the first, and probably last, megabyte, I believe) if it existed and the processor was at least a 286, freeing more of those precious 640KB for programs to run in (DOS by default didn't give them any means of addressing any more memory, even if it existed).

There was also expanded memory (EMS) and, from the 286 on, extended memory (XMS), different, incompatible, methods of addressing memory above that first MB (up to a whopping 8MB with EMS and an absurd 4GB with XMS), and depending on what the program you wanted to run required you had to choose one or the other (which became much easier once the memmaker utility came along).

Then true 32-bit software able to access the whole 4GB address space in 386s and later came along, and all that became ancient history, until we started needing more than 4GB and had to move to 64 bits.