Power ISA/Machine State Register

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The Machine State Register, or MSR, is a Branch Facility register usually defined in Book III in of the Power ISA. Among other uses, the MSR contains information about the privilege level the thread is running at. When referring to a setting in the Machine State Register, the code of the setting is in subscript directly after the abbreviation MSR. For example, documentation will refer to the Hypervisor State bit as MSRHV.


Machine State Register Bits
Bit Code Name Defined
2.07 3.0B
0 SF Sixty-Four-Bit Mode Yes No Yes
1 Tags Active (TA)[note 1] Yes Yes Yes
2 Reserved No No No
3 HV Hypervisor State Yes No Yes
4 Reserved No No No
5 SLE Split Little Endian Yes No Yes[note 2]
6:28 Reserved No No No
29:30 TS Transaction State Yes No Yes
31 TM Transactional Memory Available Yes No Yes
32 CM Computation Mode No Yes No
33 Reserved No No No
34 Implementation-dependant No Yes No
35 GS Guest State No Yes No
36 Implementation-dependant No Yes No
37 UCLE User Cache Locking Enable No Yes No
38 VEC Vector Available Yes Yes[note 3] Yes
39 Reserved No No No
40 VSX VSX Available Yes Yes Yes
41 S Secure Mode[1] No No No
42:45 Reserved No No No
46 CE Critical Enable No Yes No
47 Reserved No No No
48 EE External Interrupt Enable Yes Yes[note 4] Yes
49 PR Problem State Yes Yes Yes
50 FP Floating-Point Available Yes Yes Yes
51 ME Machine Check Interrupt Enable Yes Yes[note 5] Yes
52 FE0 Floating-Point Exception Mode 0 Yes Yes Yes
53 SE Single-Step Trace Enable Yes Yes[note 6] Yes[note 7]
54 BE Branch Trace Enable Yes Yes[note 8] Yes[note 7]
55 FE1 Floating-Point Exception Mode 1 Yes Yes Yes
56 Reserved No No No
57 Reserved No No No
58 IR Instruction Relocate Yes Yes[note 9] Yes
59 DR Data Relocate Yes Yes[note 10] Yes
60 Implementation-dependant No Yes No
61 PMM Performance Monitor Mark Yes Yes Yes
62 RI Recoverable Interrupt Yes No Yes
63 LE Little-Endian Mode Yes No Yes
  1. Undocumented; see The PowerPC AS Tagged Memory Extensions and The Talos II, Blackbird POWER9 systems support tagged memory.
  2. Power ISA version 3.0B defines this bit as something set in hardware to be zero, and warns against changing it.
  3. Power ISA version 2.07 Book III-E defines this as SPV - SP/Embedded Floating-Point/Vector Available
  4. Power ISA version 2.07 Book III-E calls this External Enable
  5. Power ISA version 2.07 Book III-E calls this Machine Check Enable
  6. Power ISA version 2.07 Book III-E defines this as Implementation-dependent
  7. 7.0 7.1 Power ISA version 3.0B defines bits 53 and 54 together as TE - Trace Enable - and defines having both bits set as reserved
  8. Power ISA version 2.07 Book III-E defines this as DE - Debug Interrupt Enable
  9. Power ISA version 2.07 Book III-E defines this as IS - Instruction Address Space
  10. Power ISA version 2.07 Book III-E defines this as DS - Data Address Space


Bit 0 - Sixty-Four-Bit Mode (SF)

0 The thread is in 32-bit mode
1 The thread is in 64-bit mode

Bit 3 - Hypervisor State (HV)

0 The thread is not in hypervisor state
1 The thread is in hypervisor state if MSRPR = 0

Bit 5

Split Little Endian (SLE)

0 Instruction and data storage use the same endian-ness
1 Instruction storage accesses use the value specified by MSRLE. Data storage accesses use the opposite.

Power ISA version 3

Power ISA version 3 does not define bit 5 so much as warn that, "software must ensure that this bit contains 0". It then reassure that the hardware will initialise it to 0.

Bit 49 - Problem State (PR)

0 The thread is in a privileged state
1 The thread is in problem state

Bit 63 - Little-Endian Mode (LE)

0 The thread is in Big-Endian mode
1 The thread is in Little-Endian mode.

Multi-bit States

Privilege State

The privilege state of a thread is a property of multiple bits in the MSR; however, not all of these states are available on all Power processors; for example, the Ultravisor state was first officially announced for POWER9's DD2.3 stepping. Additionally, at time of release the official ISA documentation still does not fully describe the Ultravisor state.[1]

0 0 0 Privileged
0 0 1 Problem
0 1 0 Hypervisor
0 1 1 Problem
1 0 0 Privileged
1 0 1 Problem
1 1 0 Ultravisor
1 1 1 Reserved


MSRSLE MSRLE Data Instruction
0 0 Big-Endian Big-Endian
0 1 Little-Endian Little-Endian
1 0 Little-Endian Big-Endian
1 1 Big-Endian Little-Endian