In 1991 MIPS released the first 64-bit CPU design, the R4000.
The design was so important to SGI, at the time one of their
only major customers, that SGI bought the company outright in
1992 in order to guarantee the design would not be lost given
the financial difficulties MIPS had while building the design.
Becoming an internal group at SGI, the company was now known as
MIPS Technologies. Outperforming the R6000 in a compact single
chip solution it set an end to its short and unsuccessful life.
The R4000's successor is the R4400 which is virtually identical.
Aside of bug fixes and higher clock rate the most significant
difference between the R4000 and the R4400 is the cache size.
R4000 processors come with each 8kB instruction and data cache
while R4400 processors double that to 16kB each cache.