Instruction manual

Table 2 lists the conventions for naming bits:
Table 2 Bit Name Conventions Used in this Guide
Convention Description
0 Describes a bit that is ignored on write operations and is
read as 0.
1 Describes a bit that is ignored on write operations and is
read as 1.
R/W Read/write. A bit or field that may be read or written by
software.
RO A read-only bit that can be read by software. It is written
by hardware. Software writes are ignored.
WO Write-only. A write-only bit that can be written by
software. It is used by hardware. Reads by software
return unpredictable results.
W A write bit that can be written by software. It is used by
hardware. Reads by software return a 0.
WC Write-one-to-clear. Software writes of a 1 cause this bit
to be cleared by hardware. Software writes of a 0 do not
modify the state of the bit.
W0C Write-zero-to-clear. Software writes of a 0 cause this bit
to be cleared by hardware. Software writes of a 1 do not
modify the state of the bit.
WA Write-anything. Software writes of any value to the
register cause the bit to be cleared by hardware.
RC Read-to-clear. The value is written by hardware and
remains unchanged until read by software, at which point
hardware may write a new value into the field.
IGN These bits or fields are ignored when written.
RAZ Read-as-zero. These bits or field return a zero when read.
MBZ Must-be-zero. These bits or fields must never be written
by software with a non-zero value. A reserved operand
exception occurs if a non-zero value in a MBZ fields is
encountered by the processor.
SBZ Should-be-zero. These bits or fields should be filled by
software with a zero vlue. These fields may be used at
a future time. Nonzero values in SBZ fields produce
unpredictable results.
RES/Reserved Reserved. These bits or fields are reserved for future
expansion.
X A don’t-care bit. The value of a don’t-care bit is ignored.
UNDEFINED Triggered by privileged software, this type of value may
halt the processor or cause it to lose information.
UNPREDICTABLE Triggered by either privileged or unprivileged software,
this type of result or occurrence does not disrupt the basic
operation of the processor. Unpredictable results may
acquire arbitrary values.
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