With the release of the Atari 1200XL, the Memo Pad present in the Operating System of the Atari 400 & 800 computers was replaced by another program, the "Self-Test".
As its name suggests, "Self-Test" allows the computer to test itself. At start-up, the computer performs a few very quick RAM/ROM integrity checks. If it detects an anomaly, it does not give control to the user but switches directly to "Self-Test", to carry out more complete tests.
How to run it manually?
The "Self-Test" can also be started manually by the user of an XL/XE computer. This can be very useful if you are buying a 2nd hand computer these days: the "Self-Test" allows you to quickly check that the ROM & RAM, keyboard, display, sound of this computer are in working order.
If the Atari BASIC is loaded, the
BYE instruction is used to go to the "Self-Test".
If the computer is turned off with no Atari BASIC cartridge plugged and no floppy disk drive #1 connected, simply press and hold the "OPTION" key for 5 seconds while you turn the computer on.
On the Atari 1200XL, it is also possible to press the "HELP" key when the Atari Rainbow Logo appears.
How to run it automatically?
On an Atari 1200XL, a jumper on the motherboard can force the computer to run the "Self-Test" program directly, ignoring cartridges, cassette tapes and diskettes. If your 1200XL stubbornly refuses to launch anything and systematically shows you the "Self-Test", it is not necessarily because a problem has been detected, it may simply be due to this jumper on the motherboard. Read the "OS Manual XL addendum" page 15 for details.
How much RAM is tested?
With the 600XL, the "Self-Test" will offer to test the whole 16 KiB of RAM.
For the video: Altirra emulator, Atari 600XL, NTSC, 1983 OS rev. 2, running the "Self-Test".
With the 1200XL/800XL/65XE/XEgs, if the Atari BASIC is loaded in memory, the "Self-Test" will offer to test only 40 KiB of RAM.
With the 1200XL/800XL/65XE/XEgs, if the Atari BASIC is not loaded in memory, the "Self-Test" will offer to test 48 KiB of RAM.
In other words, a maximum of 48 KiB of RAM can be tested, even with computers equipped with 64 KiB of RAM. The extra RAM is in the computer, but cannot be tested with the "Self-Test" program. This 16 KiB memory space is used by the OS in ROM.
For the video: Altirra emulator, Atari 1200XL, NTSC, 1982 OS rev. A, running the "Self-Test".
With the 130XE — the first computer equipped with 128 Kib of RAM — the "Self-Test" will also test 4 additional blocks of 16 Kib, i.e. the additional 64 KiB, which none of the other models are equipped with. As for computers equipped with 64 KiB, the "Self-Test" will test the first 48 KiB of RAM, skip the 16 KiB memory space used by the OS in ROM, then test the additional/remaining 64 Kib of RAM.
For the video: Altirra emulator, Atari 130XE, NTSC, 1985 OS rev. 3, running the "Self-Test".
Specificities and legacies of the 1200XL's Self-Test
On the 1200XL, the keyboard test allows you to test the function keys F1 to F4, that were introduced with the 1200XL. These keys will be physically removed from the 600XL and 800XL onwards but the "Self-Test" keyboard test will not be updated immediately, offering to test keys that no longer existed...
On the 1200XL, the RAM test will cause the L1 and L2 LEDs above the keyboard to flash alternately. As the 1200XL is the only model equipped with these L1 and L2 LEDs, this specificity will disappear with the 600XL & 800XL.
Also, on the 1200XL, Scott Stilphen mentioned in the FAQ (section: "What is the Atari 1200XL?") that if you select "All tests", when it gets to the keyboard test it'll type out the programmer's name: Michael Colburn. The updated version removes this name and types in some letters that don't mean anything.
Knowledge base article: kb-software-0003-atari-8bit-xl-xe-self-test