The microcode level that specified four bit tags was referred to as level Gamma. Even-tagged words are user data which can be modified by a user program as user state. Odd-tagged words are created and used directly by the hardware and represent a program's execution state. Since these words are created and consumed by specific instructions or the hardware, the exact format of these words can change between hardware implementation and user programs do not need to be recompiled, since the same code stream will produce the same results, even though system word format may have changed. Tag 1 words represent on-stack data addresses. The normal IRW simply stores an address couple to data on the current stack. The SIRW references data on any stack by including a stack number in the address. Tag 5 words are descriptors, which are more fully described in the next section. Tag 5 words represent off-stack data addresses. Tag 7 is the program control word which describes a procedure entry point. When operators hit a PCW, the procedure is entered. The ENTR operator explicitly enters a procedure (non-value-returning routine). Functions (value-returning routines) are implicitly entered by operators such as value call (VALC). Global routines are stored in the D[2] environment as SIRWs that point to a PCW stored in the code segment dictionary in the D[1] environment. The D[1] environment is not stored on the current stack because it can be referenced by all processes sharing this code. Thus code is reentrant and shared. Tag 3 represents code words themselves, which won't occur on the stack. Tag 3 is also used for the stack control words MSCW, RCW, TOSCW. Descriptor-based architecture The figure to the left shows how the Burroughs Large System architecture was fundamentally a hardware architecture for object-oriented programming, something that still doesn't exist in conventional architectures. Instruction sets There are three distinct instruction sets for the Burroughs large systems. All three are based on short syllables that fit evenly into words. B5000, B5500 and B5700 Programs on a B5000, B5500 and B5700 are made up of 12-bit syllables, four to a word. The architecture has two modes, Word Mode and Character Mode, and each has a separate repertoire of syllables. A processor may be either Control State or Normal State, and certain syllables are only permissible in Control State. The architecture does not provide for addressing registers or storage directly; all