The Alpha :

The development of the Alpha chip start in the year 1988 The new chip used 64 bit technology, let users to pack  more  complexity  into  their  programs  than  existing  32  Bit  technology  chips.  In addition, although designed to replace the VAX chip, the Alpha chip would have the power to support a variety of different operating systems, such as Microsoft and UNIX.

But the design forced all current VAX software to be rewritten to run on the new chip. If DEC wanted other operating systems, such as Windows NT or UNIX, to run on the chip, new versions of the operating systems would need to be developed particularly for the Alpha chip. The software would have to be tuned for the chips 64 Bit speed, which was a disadvantage.