ASN-1Step Directives

Directives, similar to command-line options, control ASN-1Step output. Unlike command-line options, directives can be added to the ASN.1 schema files.

Format

Every directive begins with "--<" and ends with ">--". All directives are case-sensitive and have the following format:

  --<[prefix.]directive [absoluteReference] [operand1[, operand2]...[, operandx]]>--

Where

• prefix is the directive type.
• directive is the name of the directive.
• absoluteReference specifies the ASN.1 item(s) the directive will be applied to.
• operand1...operandx is one or more tokens used to configure the directive as outlined in the Directives Reference. Note that operands are mandatory for some directives and optional for others.

Directives at the Local, Module, or Global Level

Local

Directives are specified in-line (old method). Example:

A ::= [1] REAL --<DECIMAL>--

Directives reference a specific item (preferred method). Example:

--<OSS.DECIMAL MyModule.A>--

Module

Directives are specified at the module level, before BEGIN, and do not reference any specific item. Example:

MyModule DEFINITIONS --<OSS.DECIMAL>-- ::= BEGIN ...

Global

Directives are specified outside modules, usually in a directives file, and do not reference any specific item. Example:

--<OSS.DECIMAL>--
MyModule DEFINITIONS ::= BEGIN ...

Contradictory Directives

It is possible to specify contradictory directives, such as OSS.DEFINITE and OSS.INDEFINITE. If this occurs, the last directive specified is used and the previous ones are ignored.

Directive Types

ASN-1Step recognizes three types of directives: Standard, OSS-specific, and OSS-specific legacy-style.

Standard Directives

Syntax

--<ASN1.directive ...>--

Remarks

A standard directive can be placed anywhere in the ASN.1 syntax provided it appears before the referenced item. Standard directives usually have an ASN1 prefix.

Standard directives can also be written as OSS-specific directives with the OSS prefix instead of ASN1. For example,

--<ASN1.Version 2021 Module>--

is equivalent to:

--<OSS.Version 2021 Module>--

Example

--<ASN1.WorkingSet Module1, Module2, Module3>--

OSS-Specific Directives

Syntax

--<OSS.directive...>--

Remarks

OSS-specific directives are used in the ASN.1 input wherever spaces or tabs are allowed: locally, at the module-level, and globally. When used globally, specify the directives before the first module definition.

The OSS prefix should be present when the directive is being specified globally, even though its presence is not mandatory. Likewise, when an OSS-specific directive is used at the module-level, the OSS prefix is not required. However, directives specified at the local-level should not be prefixed with OSS.

The following OSS-specific directives can be used without absolute references and have the same effect as the corresponding legacy-style directives specified globally:

OSS.DECIMAL / OSS.DOUBLE / OSS.MIXED
OSS.DEFAULTVALUE / OSS.NODEFAULTVALUE
OSS.DEFINITE / OSS.INDEFINITE
OSS.EXTENSIBLE / OSS.INEXTENSIBLE
OSS.LONGLONG / OSS.HUGE
OSS.PDU / OSS.NOPDU

Example

--<OSS.DTD XModule.StringType>--

OSS-Specific Legacy-Style Directives

Syntax

--<directive...>--

Remarks

OSS-specific legacy-style directives are the same as non-legacy versions, except the OSS prefix is never used. Most of these directives cannot globally specify that a directive affects only particular type instances. Historically, they were used as inline directives.

These directives can have local, module, or global scope, depending on placement. When specified locally, multiple legacy-style directives can be specified together when separated by vertical bars ('|'). For example, the following two directive specifications are equivalent:

A ::= [1] REAL  --<PDU>--  --<DECIMAL>--

A ::= [1] REAL  --<PDU | DECIMAL>--

Example

A ::= [1] REAL --<DECIMAL>--

Precedence

OSS-specific directives take precedence over OSS-specific legacy-style directives.

Standard directives also take precedence over OSS-specific legacy-style directives.

Precedence is also affected by a directive's scope: local, module, or global. OSS-specific directives specified at the local level take precedence over those at the global level.

Absolute References

An absolute reference uniquely specifies one or more components in ASN.1 syntax. The notation is similar to that used by many programming languages to access components within named structures and records. The outermost structure is listed first using its identifier followed by a dot ("."). A component of the outermost structure (which could also be a structure) can be listed next. When the desired component is within a nested structure, a dot is added after the name of the containing structure and then the desired component is listed.

You can specify ASN.1 components using:

• A component's identifier.
• An asterisk ("*") to select all rows in a SEQUENCE OF or SET OF array.
• An INTEGER index to designate an element in a SEQUENCE, SET, or CHOICE.
• A dollar sign ("$") followed by a number index to reference a particular CONSTRAINED BY clause.

Example

MyMod DEFINITIONS ::= BEGIN
	Comp1 ::= SET OF SEQUENCE {
			a		INTEGER,
			b		OCTET STRING OPTIONAL,
			c		CHOICE {
						nest1	BOOLEAN,
						nest2	BIT STRING
					}
	}
	Comp2 ::= IA5String
END

In the ASN.1 definition above, the absolute reference for the

• Entire module is MyMod
• SET OF structure is MyMod.Comp1
• SEQUENCE series is MyMod.Comp1.*
• INTEGER is MyMod.Comp1.*.a or MyMod.Comp1.*.1
• OCTET STRING is MyMod.Comp1.*.b or MyMod.Comp1.*.2
• CHOICE is MyMod.Comp1.*.c or MyMod.Comp1.*.3
• BOOLEAN is MyMod.Comp1.*.c.nest1 or MyMod.Comp1.*.c.1
• BIT STRING is MyMod.Comp1.*.c.nest2 or MyMod.Comp1.*.c.2
• IA5String is MyMod.Comp2

Warning

An absolute reference cannot be extended inside of another type reference. For example, in the following ASN.1 definition

MyMod DEFINITIONS ::= BEGIN
     Foo ::= SEQUENCE {
             foo INTEGER
     }
     Bar ::= SEQUENCE {
             bar Foo
     }
END

the absolute reference for the INTEGER is MyMod.Foo.foo, not MyMod.Bar.bar.foo.

Applying Directives to Parameterized Types

When you apply a directive to a parameterized type definition, that directive is applied to every instance of the type. For example, when the OSS.PDU directive is applied to a parameterized type, all instances of this type are treated as PDUs. Likewise, when the ASN1.WorkingSet directive is applied to a parameterized type, all instances of the type are included in the working set.

Instances of Parameterized Types

Applying a directive to an instance of a parameterized type affects only that particular instance.

Example

--<OSS.Truncate Mod.S.f1 2>--
Mod DEFINITIONS ::= BEGIN
    Sof {Type} ::= SEQUENCE OF Type
    S ::= SET {
        f1 [2]Sof{INTEGER},
        f2 [3]Sof{BOOLEAN}
    }
    s S ::= {   f1 { 1, 2, 3},
                f2 {TRUE, FALSE, TRUE}}
END

Pass the notation above through ASN-1Step using

asn1step <filename> -per -test

and you will see that the third element of the f1 field is skipped during decoding, but all elements of the f2 field are decoded.

Types Used as Substitution Types

A directive cannot be applied to any of the following:

• A type used as a substitution type in a parameterized type instance.
• The fields of such a type.
• A type that is a parameter in a parameterized type.

Example

These directives have no effect.

--<OSS.HUGE Test.T3.bt2.at1>--
--<OSS.PDU Test.T3.bt2>--
--<OSS.PDU Test.T2.bt2>--

Test DEFINITIONS ::= BEGIN
   T2 {X} ::= SEQUENCE {at2 BIT STRING, bt2 X OPTIONAL}
   T3 ::= T2 {T1}
   T1 ::= SET {at1 INTEGER, bt1 BOOLEAN}
END

If you change the type for bt2 to a "non-direct" reference to the X parameter (such as SET OF X), the OSS.PDU directives will succeed:

T2 {X} ::= SEQUENCE {at2 BIT STRING, bt2 SET OF X OPTIONAL}

Directives Index

Get detailed information about a directive by clicking on its name.

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