Owner`s manual

Table 4–1: Types of Registers, Bits, and Fields

Type Description

MBZ Must be zero. Bits and ﬁelds speciﬁed as MBZ must never be

ﬁlled by software with a nonzero value. If the CPU encoun-

ters a nonzero value in a bit or ﬁeld speciﬁed as MBZ, a Re-

served Operand Exception occurs.

SBZ Should be zero. Bits and ﬁelds speciﬁed as SBZ should be ﬁlled by soft-

ware with a zero value. If CPU encounters a nonzero value

in a bit or ﬁeld speciﬁed as SBZ, UNPREDICTABLE results oc-

cur.

0 Initialized to logic level zero

1 Initialized to logic level one

X Initialized to either logic level

RO Read only

R/W Read/write

R/Cleared on W Read/cleared on write

R/W1C Read/cleared by writing a one

WO Write only

4–2 VAX 6000 Model 500 Mini-Reference