IBM’s PC AT was basically the “first upgrade from the basic PC architecture introduced in 1981 (the XT expanded storage options, but not the processor, bus, chipset, etc.). Video after the jump.
The first ATs were released with a conservative 6 MHz processor which was quickly upgraded to 8 MHz. The BIOS allowed for the definition of aftermarket hard drives. The AT also introduced the 1.2 MB high-density 5.25″ floppy drive which became an industry standard until the advent of 3.5″ disk drives