is the read-only bit, thus odd tags indicated control words that cannot be written by a user-level program. Code words are given tag 3. Here is a list of the tags and their function: The current incarnation of these machines, the Unisys ClearPath has extended tags further into a four bit tag. 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. Display registers A stack hardware optimization is the provision of D (or "display") registers. These are registers that point to the start of each called stack frame. These registers are updated automatically as procedures are entered and exited and are not accessible by any software. There are 32 D registers, which is what limits to 32 levels of lexical nesting. Consider how we would access a lexical level 2 (D[2]) global variable from