CONFIG_MACH_DECSTATION is not available for thedefaultarchitecture x86.
Result is shown for architecture mips



System type
In linux kernel since version 2.6.12  
This enables support for DEC's MIPS based workstations. For details
see the Linux/MIPS FAQ on <> and the
DECstation porting pages on <>.

If you have one of the following DECstation Models you definitely
want to choose R4xx0 for the CPU Type:

DECstation 5000/50
DECstation 5000/150
DECstation 5000/260
DECsystem 5900/260

otherwise choose R3000.