Classic Computer Magazine Archive COMPUTE! ISSUE 65 / OCTOBER 1985 / PAGE 10

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Atari Disk RAM?
I have an Atari 600XL and 1050 disk drive. My 600XL has 16K RAM. Does my disk drive add any RAM to the computer? If so, how much does it add?
Doug Howard

Strictly speaking, you lose some usable computer memory when operating an Atari (or most other computers) with a disk drive. To use the drive, you must load DOS (the Disk Operating System) into memory. DOS is a machine language program that on the Atari is roughly 6K long. Therefore, when DOS is present, you lose memory that's otherwise available.
    However, in a broader sense the disk drive expands system memory tremendously. A disk drive lets you run much larger programs (and process much more data) than the computer could otherwise handle. For example, a program that's too big to fit into memory can be broken into two separate parts or modules. When the first program module is finished, it loads and runs the second module. The second module could in turn load a third, and so on. Since the program modules link themselves together as they go, this technique is often called chaining.
    Though many BASIC programs store data within the program itself (usually in DATA statements), you can also store data outside the computer in a disk file. An Atari 1050 drive with DOS 2.5 or 3.0 stores about 126K of data on each disk. An 810 drive (or 1050 drive with DOS 2.0 or 2.5 formatted for single density) stores about 88K on a disk. Of course, the computer's memory can't hold that much data all at once. But it can access parts of it whenever it wants. When one disk fills up with data, you start filling another, and so on. In this sense, a disk drive extends system memory to infinity.