Bit Fields

Some projects build system architecture models to describe how its subsystems and the equipments in these subsystems are to be interconnected. The models identify the interfaces between and within subsystems. Each interface must be fully described, because the control of interfaces is amongst the most critical part of the systems engineering in the project.

Interfaces are described in Interface Control Documents (ICDs), which have different names depending on the types of organisation involved (aerospace, defence, telecommunications and so on), national culture and historical precedent. Typically, the ICDs are documents with names such as:

• Data Exchange Specification (DES)
• Interface Specification (IFS)
• Interface Definition Document (IDD)

No matter what they are called, all such documents need to describe the data exchanges across the interface in considerable detail. This detail is needed to ensure that the equipment and/or software on each side of the interface will work properly when they are connected and so the interface can be proven during subsystem and system integration testing.

In the models, a high-level data definition (also called a DD entry in Cradle) represents the data sent through an interface. This data definition defines the message sent between parts of the system that pass through this particular interface. Each message contains pieces of data that contain other data, and so on, in a hierarchy. To create the actual message to be sent, all of the pieces of data are packed into one or more sequences of bits, called words. The message is therefore a sequence of words.

A bitfield is a table shows how the data in a message are packed into the words that form the message. In the bitfield, each row represents a separate word in the message, each column represents one bit, and collections of columns are labelled to show the bits occupied by each piece of data.

For example:

If there are repeated words in the message, the bitfield will include abbreviations, such as:

There are a number of aspects to specifying bitfields:

• The word size for the message
• How many bits are occupied by each piece of data
• How are individual data elements aligned inside the fields in a message
• Are data elements LSB (least significant bit) or MSB (most significant bit) justified in their field
• Are the pad bits zeroes or ones for each data element
• Should the bitfield should individual data, or groups of data, optionally specified at any level in the data decomposition for the message

Cradle-5.3 introduces support for bitfields inside models and in documents generated by the Document Publisher. This support is:

• New default frames inside data definitions (DD entries) in the Implementation domain
• A new set of consistency checks applied by the Toolset’s Data Dictionary Editor (DDE) and Consistency Checker (CC) that apply to these new bitfield frames and which are only triggered if any of these frames are used
• A new mechanism in the Document Publisher that generates bitfields in the format shown above as tables in the Word documents that it generates

These new facilities have been introduced so that they are transparent to existing data and to all users who are not interested in them.

So, if you create models in the Implementation domain of your project databases, but you are not interested in bitfields, these new facilities will not be triggered. If you want to use the new facilities, then start to use the new frames in data definitions and the new bitfield features will automatically become active.