Early Tandy 1000 models used Phoenix BIOS. Common models of the machine included the Tandy 1000, 1000 EX, 1000 HX, 1000 SX, 1000 TX, 1000 SL, 1000 RL, and 1000 TL. With the exception of the RLX and RSX, the Tandy 1000 machines are XT-class machines, which cannot support extended memory despite some models using 80286 processors. The RLX is an oddity, as it is an XT-class machine that supports 384 KB of extended memory, and the RSX is a fully AT-class machine which can support up to 9 MB of extended memory.