Running just one program at a time while sharing memory with the operating system is the most basic memory management strategy. Figure illustrates three versions on this theme.
The operating system could be in RAM (Random Access Memory) at the bottom of memory, as shown in Fig. (a), or it could be in ROM (Read-Only Memory) at the top of memory, as shown in Fig. (b), or the device drivers could be in ROM at the top of memory and the rest of the system in RAM down below, as shown in Fig. (c).
Formerly utilized on mainframes and minicomputers, the first model is now rarely employed.
Some embedded devices and palmtop computers employ the second model.
The third model, known as the BIOS (Basic Input Output System), was utilized by early personal computers (running MS-DOS, for example).
Only one process can be active at a time when the system is set up in this manner. The operating system copies the desired software from disk to memory and starts it running as soon as the user types a command. The operating system displays a prompt character and awaits a new command after the process is complete. It loads a new program into memory and overwrites the initial one when it receives the command.
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