ATX is a "form factor", or standard size, of motherboard used in mid-tower and full-tower PCs. ATX measures approximately 305 mm × 244 mm (12" x 9.6").
Advance Technology eXtended. It is the...
AT- hardware power control. ATX- software power control, different power connectors, different keyboard connection, different size. AT is the older size/format, mainly use up till the late 90s. ATX...
BIOS is not dependent on the form factor (ATX) of the motherboard. BIOS are chips with information on how to run certain hardware, these are usually American Megatrends, Award, or Phoenix.