ISO/IEC 14776-412:2006

Information technology - Small computer system interface (SCSI) - Part 412: Architecture model-2 (SAM-2)

The Small Computer System Interface (SCSI) protocol provides an efficient peer-to-peer I/O bus with the maximuim number of hosts and peripherals determined by the bus width (8 or 16). This International Standard specifies common behaviours for SCSI devices.

General Information

• Status: Published
• Publication Date: 26-Oct-2006
• ICS: 35.200 - Interface and interconnection equipment
• Technical Committee: ISO/IEC JTC 1/SC 25 - Interconnection of information technology equipment
• Current Stage: PPUB - Publication issued
• Start Date: 27-Oct-2006
• Completion Date: 30-Jan-2005

Overview

ISO/IEC 14776-412:2006 (SAM-2) - "Information technology - Small Computer System Interface (SCSI) - Part 412: Architecture model-2" - defines the SCSI architecture model and the common behaviours required of SCSI devices. The standard explains the SCSI peer-to-peer I/O bus model (bus widths of 8 or 16 determining maximum hosts and peripherals) and specifies core structural, command and task management concepts used by initiators, targets and multi‑port devices.

Key topics and technical requirements

The standard covers the architectural and behavioural foundations needed for interoperable SCSI implementations, including:

• SCSI architecture model: distributed service model, client–server model, structural model, SCSI domain and service delivery subsystem.
• Device and port models: definitions for initiator, target, initiator/target devices, multi‑port devices, port identifiers and naming.
• Logical units and addressing: logical unit numbers (LUNs), multiple LUN structures (single level, 8‑byte, extended addressing), peripheral and flat space addressing, and well‑known LUN addresses.
• Command model: Execute Command RPC, Command Descriptor Block (CDB) format (operation code and control byte), status codes and status precedence.
• Data transfer and transport services: Data-In / Data-Out delivery, request/response ordering, and transport protocol services supporting command execution.
• Task and task set management: task objects and tags, task lifetimes, abort mechanisms, task management functions (ABORT TASK, ABORT TASK SET, CLEAR ACA, LOGICAL UNIT RESET, TARGET RESET, etc.), and task state transitions.
• Exception handling and special conditions: contingent allegiance (CA/ACA), overlapped commands, unit attention, sense data and autosense, hard and logical unit resets.
• Models for distributed communications and dependent logical units, including the nexus object and task routing.

Applications - who uses this standard

ISO/IEC 14776-412 is essential for:

• Storage device and firmware developers implementing SCSI initiators, targets and multi‑port devices.
• Operating system kernel and device driver engineers ensuring correct command processing and task management.
• SAN/NAS integrators and storage system architects designing interoperable SCSI-based storage networks.
• Test labs and compliance teams validating SCSI conformance and interoperability.
• Embedded systems and appliance vendors implementing SCSI protocols in appliances or RAID/storage controllers.

Practical uses include implementing CDB parsing and status handling, LUN addressing schemes, multi‑port coordination, task abort/reset behavior, and robust error reporting.

Related standards

• Other parts of the ISO/IEC 14776 SCSI series and related SCSI transport specifications (covering protocol mappings, physical transports and additional architecture parts) are typically used together with SAM-2 for full SCSI implementation and conformance.

Standards Content (Sample)

INTERNATIONAL ISO/IEC
STANDARD 14776-412
First edition
2006-10
Information technology –
Small computer system interface (SCSI) –
Part 412:
Architecture model-2 (SAM-2)
Reference number
INTERNATIONAL ISO/IEC
STANDARD 14776-412
First edition
2006-10
Information technology –
Small computer system interface (SCSI) –
Part 412:
Architecture model-2 (SAM-2)
Copyright  2006 ISO/IEC, Geneva   All rights reserved
No part of this publication may be reproduced or utilized in any form or by any means, electronic or
mechanical, including photocopying and microfilm, without permission in writing from the publisher.
International Electrotechnical Commission, 3, rue de Varembé, PO Box 131, CH-1211 Geneva 20, Switzerland
Telephone: [phone] Telefax: [phone] E-mail: [e-mail] Web: www.iec.ch
PRICE CODE
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For price, see current catalogue

14776-412 © ISO/IEC:2006(E) - 3 -
Contents
Page
1 General. 11
1.1 Scope . 11
1.2 Precedence requirements . 11
2 Normative reference . 12
3 Definitions, symbols, abbreviations, and conventions . 13
3.1 Definitions. 13
3.2 Acronyms. 23
3.3 Keywords. 23
3.4 Editorial conventions . 24
3.5 Numeric conventions. 25
3.6 Notation conventions. 25
3.6.1 Hierarchy diagram conventions . 25
3.6.2 Notation for procedures and functions. 26
3.6.3 Notation for state diagrams . 27
4 SCSI Architecture Model. 28
4.1 Introduction. 28
4.2 The SCSI distributed service model . 29
4.3 The SCSI client-server model. 30
4.4 The SCSI structural model . 31
4.5 SCSI domain . 33
4.6 The service delivery subsystem . 33
4.6.1 The service delivery subsystem object. 33
4.6.2 Synchronizing client and server states. 34
4.6.3 Request/Response ordering. 34
4.7 SCSI devices. 35
4.7.1 SCSI initiator device . 35
4.7.2 SCSI target device. 35
4.7.3 SCSI target/initiator device . 37
4.7.4 SCSI port identifier . 37
4.7.5 SCSI task router . 37
4.7.6 SCSI device name. 38
4.7.7 SCSI port name . 38
4.8 Logical units. 39
4.9 Logical unit numbers . 40
4.9.1 Logical unit numbers overview . 40
4.9.2 LUN 0 address. 40
4.9.3 Single level logical unit number structure . 40
4.9.4 Eight byte logical unit number structure . 42
4.9.5 Logical unit addressing method. 44
4.9.6 Peripheral device addressing method . 44
4.9.7 Flat space addressing method . 45
4.9.8 Extended logical unit addressing. 45
4.9.9 Well known logical unit addressing. 47
4.10 Tasks. 48
4.10.1 The task object . 48
4.10.2 Task tags. 48
4.11 The nexus object . 49
4.12 SCSI ports . 50
4.12.1 SCSI port configurations. 50
4.12.2 SCSI devices with multiple ports . 50
4.12.3 Multiple port target SCSI device structure . 51
4.12.4 Multiple port initiator SCSI device structure. 52
4.12.5 Multiple port target/initiator SCSI device structure . 53

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4.12.6 SCSI initiator device view of a multiple port SCSI target device . 53
4.12.7 SCSI target device view of a multiple port SCSI initiator device . 55
4.13 Model for dependent logical units. 56
4.14 The SCSI model for distributed communications . 58
5 SCSI Command Model . 61
5.1 The Execute Command remote procedure . 61
5.2 Command descriptor block (CDB). 62
5.2.1 CDB format. 62
5.2.2 OPERATION CODE byte. 63
5.2.3 CONTROL byte. 64
5.3 Status . 65
5.3.1 Status codes. 65
5.3.2 Status precedence. 66
5.4 SCSI transport protocol services in support of Execute Command. 67
5.4.1 Overview. 67
5.4.2 Execute Command request/confirmation SCSI transport protocol services. 67
5.4.3 Data transfer SCSI transport protocol services . 69
5.4.3.1 Introduction. 69
5.4.3.2 Data-In delivery service . 70
5.4.3.3 Data-Out delivery service . 70
5.5 Task and command lifetimes. 71
5.6 Task management function lifetime. 72
5.7 Aborting tasks. 72
5.7.1 Mechanisms that cause tasks to be aborted . 72
5.7.2 When a SCSI initiator port aborts its own tasks . 72
5.7.3 When a SCSI initiator port aborts tasks from other SCSI initiator ports . 73
5.8 Command processing examples . 73
5.8.1 Unlinked command example . 73
5.8.2 Linked command example. 74
5.9 Command processing considerations and exception conditions. 75
5.9.1 Contingent allegiance (CA) and auto contingent allegiance (ACA) . 75
5.9.1.1 Overview. 75
5.9.1.2 Establishing a CA or ACA. 75
5.9.1.3 Handling tasks when neither CA or ACA is in effect. 77
5.9.1.4 Handling new tasks from the faulted initiator port when CA or ACA is in effect . 77
5.9.1.5 Handling new tasks from non-faulted initiator ports when CA or ACA is in effect . 78
5.9.1.5.1 Commands permitted from non-faulted initiator ports during CA or ACA. 78
5.9.1.5.2 Handling new tasks from non-faulted initiator ports when CA or ACA is in effect . 79
5.9.1.6 Clearing a CA condition. 81
5.9.1.7 Clearing an ACA condition . 81
5.9.2 Overlapped commands . 81
5.9.3 Incorrect logical unit selection . 82
5.9.4 Sense data . 82
5.9.4.1 Sense data introduction. 82
5.9.4.2 Asynchronous event reporting. 83
5.9.4.3 Autosense. 83
5.9.5 Unit Attention condition. 84
5.9.6 Hard reset. 85
5.9.7 Logical unit reset . 85
6 Task management functions. 86
6.1 Introduction. 86
6.2 ABORT TASK. 87
6.3 ABORT TASK SET. 87
6.4 CLEAR ACA . 88
6.5 CLEAR TASK SET . 88
6.6 LOGICAL UNIT RESET. 89
6.7 TARGET RESET . 89
6.8 WAKEUP . 89

14776-412 © ISO/IEC:2006(E) - 5 -
6.9 Task management SCSI transport protocol services . 90
6.10 Task management function example. 91
7 Task Set Management. 92
7.1 Introduction to task set management . 92
7.2 Controlling task set management . 92
7.3 Task management events . 93
7.4 Task states . 93
7.4.1 Overview. 93
7.4.1.1 Task state nomenclature . 93
7.4.1.2 Suspended information. 93
7.4.2 Enabled task state . 93
7.4.3 Blocked task state . 94
7.4.4 Dormant task state . 94
7.4.5 Ended task state. 94
7.4.6 Task states and task lifetimes . 94
7.5 Task attributes. 95
7.5.1 Simple task. 95
7.5.2 Ordered task. 95
7.5.3 Head of queue task . 95
7.5.4 ACA task. 95
7.6 Task state transitions. 96
7.7 Task set management examples. 97
7.7.1 Introduction. 97
7.7.2 Head of queue tasks. 98
7.7.3 Ordered tasks. 99
7.7.4 ACA task. 100
Annex A (informative)
Identifiers and names for objects . 101
A.1 Identifiers and names overview. 101
A.2 Identifiers and names. 101
A.3 SCSI transport protocol acronyms and bibliography. 103
Annex B (informative)
Terminology mapping . 105
Bibliography . 106

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Tables
1 Single level logical unit number structure for 256 or fewer logical units. 40
2 Single level logical unit number structure for 257 to 16 384 logical units. 41
3 Eight byte logical unit number structure adjustments . 42
4 Eight Byte logical unit number structure . 43
5 Format of addressing fields. 43
6 ADDRESS METHOD field values. 43
7 Logical unit addressing . 44
8 Peripheral device addressing. 44
9 Flat space addressing. 45
10 Extended logical unit addressing . 46
11 LENGTH field values . 46
12 Two byte extended logical unit addressing format. 46
13 Four byte extended logical unit addressing format . 46
14 Six byte extended logical unit addressing format. 46
15 Eight byte extended logical unit addressing format . 46
16 Logical unit extended address methods . 47
17 Well known logical unit extended address format. 47
18 Mapping nexus to SAM-2 identifiers . 49
19 Command Descriptor Block (CDB) Format. 63
20 OPERATION CODE byte . 63
21 Group Code values . 63
22 CONTROL byte . 64
23 Status codes . 65
24 Autosense, CA, and ACA Interactions . 75
25 Blocking and aborting tasks when a CA or ACA is established . 76
26 Task handling when neither CA nor ACA is in effect . 77
27 Handling for new tasks from a faulted initiator port during CA. 77
28 Handling for new tasks from a faulted initiator port during ACA. 78
29 Handling for new tasks from non-faulted initiator ports during CA. 79
30 Handling for new tasks from non-faulted initiator ports during ACA. 80
31 Task Management Functions. 86
32 Task State Nomenclature . 93
33 Task attribute and state indications in examples . 97
34 Dormant task blocking boundary requirements . 99
A.1 Object size and support requirements. 101
A.2 Object identifier size for each SCSI transport protocol. 102
A.3 Object identifier format for each SCSI transport protocol . 102
A.4 Object name size for each SCSI transport protocol . 103
A.5 Object name format for each SCSI transport protocol. 103
B.1 SAM-2 to SAM terminology mapping . 105

14776-412 © ISO/IEC:2006(E) - 7 -
Figures
Page
0 SCSI document structure. 9
1 Requirements precedence. 11
2 Example hierarchy diagram . 25
3 Example state diagram . 27
4 Client-Server model . 29
5 SCSI client-server model . 30
6 SCSI I/O system and domain model. 31
7 Overall SCSI domain model. 32
8 SCSI domain model . 33
9 Service delivery subsystem model. 33
10 SCSI initiator device model. 35
11 SCSI target device model . 36
12 SCSI target/initiator device model. 37
13 Logical unit model . 39
14 Eight Byte logical unit number structure adjustments . 42
15 SCSI device functional models . 50
16 Multiple port target SCSI device structure model. 51
17 Multiple port SCSI initiator device structure model . 52
18 Multiple port target/initiator SCSI device structure model . 53
19 SCSI target device configured in a single SCSI domain. 54
20 SCSI target device configured in multiple SCSI domains . 54
21 SCSI target device and SCSI initiator device configured in a single SCSI domain. 55
22 Dependent logical unit model. 56
23 Example of hierarchical system diagram . 57
24 Protocol service reference model . 58
25 Protocol service model. 59
26 Request-Response ULP transaction and related LLP services . 60
27 Model for Data-In and Data-Out data transfers. 69
28 Command processing events . 73
29 Linked command processing events. 74
30 Task management processing events . 91
31 Example of Dormant state task behavior . 94
32 Task states. 96
33 Head of queue tasks and blocking boundaries (example 1) . 98
34 Head of queue tasks and blocking boundaries (example 2) . 98
35 Ordered tasks and blocking boundaries . 99
36 ACA task example . 100

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INFORMATION TECHNOLOGY -
SMALL COMPUTER SYSTEM INTERFACE (SCSI) -
Part 412: Architecture Model - 2 (SAM-2)
FOREWORD
1) ISO (International Organization for Standardization) and IEC (International Electrotechnical Commission) form the specialized system for
worldwide standardization. National bodies that are members of ISO or IEC participate in the development of International Standards.
Their preparation is entrusted to technical committees; any ISO and IEC member body interested in the subject dealt with may participate
in this preparatory work. International governmental and non-governmental orgnizations liaising with ISO and IEC also participate in this
preparation.
2) In the field of information technology, ISO and IEC have established a joint technical committee, ISO/IEC JTC 1. Draft International
Standards adopted by the joint technical committee are circulated to national bodies for voting. Publication as an International Standard
requires approval by at least 75 % of the national bodies casting a vote.
3) The formal decisions or agreements of IEC and ISO on technical matters express, as nearly as possible, an international consensus of
opinion on the relevant subjects since each technical committee has representation from all interested IEC and ISO member bodies.
4) IEC, ISO and ISO/IEC Publications have the form of recommendations for international use and are accepted by IEC and ISO member
bodies in that sense. While all reasonable efforts are made to ensure that the technical contenct of IEC, ISO and ISO/IEC Publications is
accurate, IEC or ISO cannot be held responsible for the way in which they are used or for any misinterpretation by any end user.
5) In order to promote international uniformity, IEC and ISO member bodies undertake to apply IEC, ISO and ISO/IEC Publications transpar-
ently to the maximum extent possible in their national and regional publications. Any divergence between any ISO/IEC Publication and
the corresponding national or regional publication should be clearly indicated in the latter.
6) ISO and IEC provide no marking procedure to indicate their approval and cannot be rendered responsible for any equipment declared to
be in conformity with an ISO/IEC Publication.
7) All users should ensure that they have the latest edition of this publication.
8) No liability shall attach to IEC or ISO or its directors, employees, servants or agents including individual experts and members of their
technical committees and IEC or ISO member bodies for any personal injury, property damage or other damage of any nature whatso-
ever, whether direct or indirect, or for costs (including legal fees) and expenses arising out of the publication of, use of, or reliance upon
this ISO/IEC publication or any other IEC or IEC/ISO publications.
9) Attention is drawn to the normative references cited in this publication. Use of the referenced publications is indispensable for the correct
application of this publication.
10) Attention is drawn to the possibility that some of the elements of this International Standard may be the subject of patent rights. ISO and
IEC shall not be held responsible for identifying any or all such patent rights.
International Standard ISO/IEC 14776-412 was prepared by subcommittee 25: Interconnection of information
technology equipment, of ISO/IEC joint technical committee 1: Information technology.
This International Standard has been approved by vote of the memeber bodies, and the voting results may be
obtained from the address given on the title page.

14776-412 © ISO/IEC:2006(E) - 9 -
INTRODUCTION
General
The set of SCSI standards consists of this International Standard and the SCSI implementation standards (see
SCSI standards family).
The set of SCSI standards specifies the interfaces, functions and operations necessary to ensure interoperability
between conforming SCSI implementations. This standard is a functional description. Conforming implementations
may employ any design technique that does not violate interoperability.
SCSI standards family
Figure 1 shows the relationship of this standard to the other standards and related projects in the SCSI family
standards.
Device-Type Specific Command Sets
Shared Command Set (for all device types)
SCSI Transport Protocols
Interconnects
Figure 0 — SCSI document structure
Figure 0 shows the general applicability of the documents with respect to one another. Figure 0 does not imply a
relationship such as a hierarchy, protocol stack or system architecture.
The functional areas identified in Figure 0 characterize the scope of standards within a group as follows:
Architecture Model: Defines the SCSI systems model, the functional partitioning of the SCSI standard set and
requirements applicable to all SCSI implementations and implementation standards.
Device-Type Specific Command Sets: Implementation standards that define specific device types including a
device model for each device type. These standards specify the required commands and behaviour that is specific
to a given device type and prescribe the requirements to be followed by a SCSI initiator device when sending
commands to a SCSI target device having the specific device type. The commands and behaviours for a specific
device type may include reference commands and behaviours that are shared by all SCSI devices.
Shared Command Set: An implementation standard that defines a model for all SCSI device types. This standard
specifies the required commands and behaviour that is common to all SCSI devices, regardless of device type,
and prescribes the requirements to be followed by a SCSI initiator device when sending commands to any SCSI
target device.
SCSI Transport Protocols: Implementation standards that define the requirements for exchanging information so
that different SCSI devices are capable of communicating.
Architecture Model
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Interconnects: Implementation standards that define the communications mechanism employed by the SCSI
transport protocols. These standards may describe the electrical and signaling requirements essential for SCSI
devices to interoperate over a given interconnect.
For references, see bibliography.
The SCSI Architecture Model-2 (SAM-2) standard is divided into seven clauses and two annexes:
Clause 1 Scope
Clause 2 Normative references
Clause 3 Definitions, symbols and abbreviations
Clause 4 SCSI architecture model
Clause 5 SCSI command model of the SCSI architecture
Clause 6 Task management functions common to SCSI devices
Clause 7 Task set management capabilities common to SCSI devices
Annex A Identifier and name definitions of the SCSI transport protocols
Annex B Terminology mapping
Bibliography
14776-412 © ISO/IEC:2006(E) - 11 -
INFORMATION TECHNOLOGY -
SMALL COMPUTER SYSTEM INTERFACE (SCSI) -
Part 412: Architecture Model - 2 (SAM-2)
1 General
1.1 Scope
This part of ISO/IEC 14776 defines a reference model that specifies common behaviours for SCSI devices and an
abstract structure that is generic to all SCSI I/O system implementations.
This International Standard specifies generic requirements that pertain to SCSI implementation standards and
implementation requirements.
1.2 Precedence requirements
An implementation requirement specifies behaviour in terms of measurable or observable parameters that apply to
an implementation. Examples of implementation requirements defined in this International Standard are the com-
mand descriptor block format and the status values to be returned upon command completion.
Generic requirements are transformed to implementation requirements by an implementation standard. An
example of a generic requirement is the hard reset behaviour specified in 5.9.6.
SCSI Architecture Model - 2
SCSI Implementation SCSI Implementation SCSI Implementation
Standard Standard Standard
Key:
Generic Implementation
SCSI
Requirements Requirements
Implementation
Figure 1 — Requirements precedence
As shown in figure 1, all SCSI implementation standards shall reflect the generic requirements defined herein. In
addition, an implementation claiming SCSI compliance shall conform to the applicable implementation require-
ments defined in this standard and the appropriate SCSI implementation standards. In the event of a conflict
between this document and other SCSI standards under the jurisdiction of ISO/IEC JTC 1/SC 25, the requirements
of this standard shall apply.
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2 Normative reference
The following referenced document is indispensable for the application of this document. For dated references,
only the edition cited applies. For undated references, the latest edition of the referenced document (including any
amendments) applies.
ISO/IEC 14776-452, Information technology – Small Computer System Interface (SCSI) – Part 452: Primary Com-
mands-2 (SPC-2)
For all other references, see bibliography.

14776-412 © ISO/IEC:2006(E) - 13 -
3 Definitions, symbols, abbreviations and conventions
3.1 Definitions
For the purpose of this international standard the following definitions apply.
3.1.1
ACA command
command performed by a task with the ACA attribute (see 3.1.4, 4.10, and 7.5.4
...