schema/Fields/ConstantField.json

{
    "license": {
        "copyright": "Copyright (c) 2010-2026 Arm Limited or its affiliates. All rights reserved.",
        "info": "This document is Non-confidential and licensed under the BSD 3-clause license."
    },
    "additionalProperties": false,
    "info": [
        [
            "This field represents a `RO` constant field with a specific value. For example a 4",
            "bit constant field `'0101'`."
        ]
    ],
    "properties": {
        "_meta": {
            "$ref": "../Meta.json"
        },
        "_type": {
            "enum": [
                "Fields.ConstantField"
            ]
        },
        "name": {
            "see": "../Fields/Field.json",
            "type": [
                "string",
                "null"
            ]
        },
        "rangeset": {
            "see": "../Fields/Field.json",
            "$ref": "../Rangeset.json"
        },
        "value": {
            "info": [
                "The constant value of this ConstantField, which can be:",
                [
                    " - A single `Value` (or string) such as `'0000'`",
                    " - An IMPLEMENTATION DEFINED value, which:",
                    "       - can be open ended to any value that fits within the size of this field.",
                    "       - or restricted to a set of allowed constraints defined by the architecture."
                ]
            ],
            "oneOf": [
                {
                    "$ref": "../Values/Value.json"
                },
                {
                    "$ref": "../Values/ImplementationDefined.json"
                }
            ],
            "examples": [
                {
                    "_type": "Values.Value",
                    "value": "'00'",
                    "meaning": "A defined constant value with a custom meaning"
                },
                {
                    "_type": "Values.ImplementationDefined",
                    "constraints": {
                        "'00'": "A possible value chosen by an implementation.",
                        "'01'": "A second possible value chosen by an implementation."
                    }
                }
            ]
        },
        "access": {
            "default": null,
            "$ref": "../Traits/HasFieldAccess.json#/definitions/access",
            "info": [
                [
                    "Constant fields are by nature `RO` in an implementation. ",
                    "This means that once implemented, the value cannot be changed. However in some cases,",
                    "software reads may return another value."
                ],
                [
                    "For example, a constant value with the value of `1` may have a ",
                    "Read-As-Zero accessor (RAZ), meaning that even though the value is set to ",
                    "`1` software must interpret it as `0`."
                ],
                "Note:",
                [
                    " - The write counterpart of the access must always be `WI` as ",
                    "   `ConstantFields` are not writable.",
                    " - If the value of access is set to null (or omitted), then ",
                    "   `RO/WI` is implied."
                ]
            ]
        },
        "description": {
            "$ref": "../Traits/HasDescription.json#/definitions/description",
            "see": "../Fields/Field.json"
        }
    },
    "required": [
        "value",
        "rangeset"
    ],
    "title": "ConstantField",
    "type": "object",
    "examples": [
        {
            "_type": "Fields.ConstantField",
            "name": "C1",
            "rangeset": [
                {
                    "_type": "Range",
                    "start": 0,
                    "width": 2
                }
            ],
            "description": {
                "_type": "Description",
                "after": null,
                "before": "A constant field that does not change its value during runtime"
            },
            "value": {
                "_type": "Values.Value",
                "value": "'00'",
                "meaning": null
            }
        },
        {
            "_type": "Fields.ConstantField",
            "name": "C2",
            "rangeset": [
                {
                    "_type": "Range",
                    "start": 0,
                    "width": 2
                }
            ],
            "description": {
                "_type": "Description",
                "after": null,
                "before": "Constant value with possible architecture values which must be configured during an implementation"
            },
            "value": {
                "_type": "Values.ImplementationDefined",
                "constraints": {
                    "_type": "Valuesets.Values",
                    "values": [
                        {
                            "_type": "Values.Value",
                            "value": "'00'",
                            "meaning": "A possible value chosen by an implementation."
                        },
                        {
                            "_type": "Values.Value",
                            "value": "'01'",
                            "meaning": "A second possible value chosen by an implementation."
                        }
                    ]
                }
            }
        }
    ],
    "$schema": "http://json-schema.org/draft-04/schema#"
}