What are the four voltages produced by an AT power supply?

There are actually five common voltages produced by an AT power supply, not four:

1. +12V: Used for peripherals like floppy drives, hard drives, and fans.

2. +5V: The main power supply for the motherboard and most components.

3. -12V: Used by some older components, though less common in modern systems.

4. -5V: Used by some older components, though less common in modern systems.

5. +3.3V: Used by some components, particularly modern CPUs and other high-speed components.

While +3.3V is less common in the AT standard, it is crucial to remember that it's still produced by most modern power supplies.