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Introduction 1–9
1.6.6 Must Be Zero (MBZ)
Fields specified as Must be Zero (MBZ) must never be filled by software with a non-zero
value. These fields may be used at some future time. If the processor encounters a non-zero
value in a field specified as MBZ, an Illegal Operand exception occurs.
1.6.7 Read As Zero (RAZ)
Fields specified as Read as Zero (RAZ) return a zero when read.
1.6.8 Should Be Zero (SBZ)
Fields specified as Should be Zero (SBZ) should be filled by software with a zero value. Non-
zero values in SBZ fields produce UNPREDICTABLE results and may produce extraneous
instruction-issue delays.
1.6.9 Ignore (IGN)
Fields specified as Ignore (IGN) are ignored when written.
1.6.10 Implementation Dependent (IMP)
Fields specified as Implementation Dependent (IMP) may be used for implementation-specific
purposes. Each implementation must document fully the behavior of all fields marked as IMP
by the Alpha specification.
1.6.11 Illustration Conventions
Illustrations that depict registers or memory follow the convention that increasing addresses
run right to left and top to bottom.
1.6.12 Macro Code Example Conventions
All instructions in macro code examples are either listed in Chapter 4 or are stylized code
forms found in Section A.4.6.