On this theme, there are three variations as shown in the figure given below:
As shown in the first figure, the OS may be at the bottom of the memory in RAM. Here RAM stands for Random Access Memory.
And as shown in the second figure given above, the OS may be at the top of the memory in ROM. Here ROM stands for Read Only Memory.
And as shown in the last or third figure given above, the device drivers may be at the top of the memory in a Read Only Memory (ROM), and rest of the system in Random Access Memory (RAM) lies below the device drivers.
The first model is rarely used now-a-day, but was formally used on mainframes and minicomputers.
The second model is used on few palmtop computers and embedded systems.
And the third model was used by early PC, for example MS-DOS. Here PC stands for Personal Computer and MS-DOS stands for Microsoft-Disk Operating System. In that model, BIOS was the portion of the system in Read Only Memory (ROM). Here BIOS stands for Basic Input/Output System.
Only one process can be running at a time when the system is organized in this way. As soon as the user types a command, the OS copies the requested program from disk to memory and then executes it.
The OS displays a prompt character and waits for a new command when the process finishes. When it receives a command, then it loads a new program into memory, overwriting the first one.