A microprocessor including memory storage into which ISA customer code routines can be stored after having been decoded into their machine-native microinstructions. The customer code store is not subject to eviction and the like, as a cache memory would be. ISA level code can specify a routine for storage into the customer code store, at a time prior to its execution. The customer code store thus serves as a write-once execute-many library of pre-decoded routines which ISA level applications can subsequently use, permitting a system manufacturer to create a highly customized and optimized system.