 Some processors, such as DEC Alpha processors and the CMOS microprocessors on later IBM System/390 mainframes and z/Architecture mainframes, use machine code, running in a special mode that gives it access to special instructions, special registers, and other hardware resources not available to regular machine code, to implement some instructions and other functions, such as page table walks on Alpha processors.This is called PALcode on Alpha processors and millicode on IBM mainframe processors.Another form of vertical microcode has two fields