To be honest, I remember something different. Dos was originally a single process OS. So memory was allocated like a stack, at least for a .COM. Some detail:

To be honest, I remember something different. Dos was originally a single process OS. So memory was allocated like a stack, at least for a .COM. Some detail:All memory above your starting address is yours. You call int 12 to find out how much base memory is installed, which in later computers was either 640K or 639K if the bios wanted a bit or storage. If you want to start another program, you first resize your part of the memory with INT 21 (ah=4D? Its too long ago, don't quote me on this) Later, there came TSRs, EMS/UMBs/high memory/... to muck this scheme up. A TSR was not allowed to allocate memory after it terminated, exactly because it runs code in another process while it should have been terminated. It had to always stay under all other programs, memory address wise. It was not allowed to do much, in fact, because DOS was not reentrant, and running int21h while running int21h had a good chance of making DOS overwrite its own variables.