Banked register

Is a register that has multiple instances, with the instance that is in use depending on the processor mode, security state, or other processor state.

IMP

Is an abbreviation used in diagrams to indicate that the bit or bits concerned have implementation defined behavior.

IMPLEMENTATION DEFINED

Means that the behavior is not architecturally defined, but should be defined and documented by individual implementations.

Observer

A processor or mechanism within the system, such as peripheral device, that is capable of generating reads from or writes to memory.

Read-As-One (RAO)

In any implementation, the bit must read as 1, or all 1s for a bit field.

Read-As-Zero (RAZ)

In any implementation, the bit must read as 0, or all 0s for a bit field.

RAO

See Read-As-One.

RAO/WI

Read-As-One, Writes Ignored.In any implementation, the bit must read as 1, or all 1s for a bit field, and writes to the field must be ignored.

Software can rely on the bit reading as 1, or all 1s for a bit field, and on writes being ignored.

RAZ

See Read-As-Zero.

RAZ/WI

Read-As-Zero, Writes Ignored.In any implementation, the bit must read as 0, or all 0s for a bit field, and writes to the field must be ignored.

Software can rely on the bit reading as 0, or all 0s for a bit field, and on writes being ignored.

Reserved

Registers and instructions that are reserved are unpredictable unless otherwise stated. Bit positions described as Reserved are UNK/SBZP.

SBZ

See Should-Be-Zero.

SBZP

See Should-Be-Zero-or-Preserved.

Security hole

Is a mechanism that bypasses system protection.

Should-Be-Zero (SBZ)

Should be written as 0 (or all 0s for a bit field) by software. Values other than 0 produce unpredictable results.

Should-Be-Zero-or-Preserved (SBZP)

Must be written as 0, or all 0s for a bit field, by software if the value is being written without having been previously read, or if the register has not been initialized. Where the register was previously read on the sameprocessor, since the processor was last reset, the value in the field should be preserved by writing the value that was previously read.

Hardware must ignore writes to these fields.

If a value is written to the field that is neither 0 (or all 0s for a bit field), nor a value previously read for the same field on the same processor, the result is unpredictable.

UNKNOWN

An unknown value does not contain valid data, and can vary from moment to moment, instruction to instruction, and implementation to implementation. An unknown value must not be a security hole. unknown values must not be documented or promoted as having a defined value or effect.

UNK/SBZP

unknown on reads, Should-Be-Zero-or-Preserved on writes.

In any implementation, the bit must read as 0, or all 0s for a bit field, and writes to the field must be ignored.

Software must not rely on the field reading as 0, or all 0s for a bit field, and must use an SBZP policy to write to the field.

UNK

Software must treat a field as containing an UNKNOWN value.In any implementation, the bit must read as 0, or all 0s for a bit field. Software must not rely on the field reading as zero.

UNPREDICTABLE

The behavior cannot be relied upon. unpredictable behavior must not represent security holes. unpredictable behavior must not halt or hang the processor, or any parts of the system. unpredictable behavior must not be documented or promoted as having a defined effect.

Unsigned data types

Represent a non-negative integer in the range 0 to +2^N?1, using normal binary format.

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