Attribute byte
An invisible byte at the start of each field on a 3270 screen that says how the field behaves, such as whether you can type in it.
Also written field attribute, MDT, modified data tag, ATTRB, protected field
A 3270 screen is made of fields, and every field starts with an attribute byte. The byte takes up one position on the screen but is never displayed. It tells the terminal how to treat the field that follows.
Attributes include whether the field is protected, so the user cannot type in it, or unprotected for input. They include whether it accepts only numbers, whether it is shown brightly, normally or not at all, which is how password fields hide what you type. They also include the modified data tag, the MDT.
The MDT is set when the user changes a field. When the user presses Enter, the terminal sends back only the fields whose MDT is set. That saves network traffic, but it can surprise programmers: a field the user did not change may not arrive at all, unless the program set its MDT when sending the screen.
In BMS maps, attributes are set with the ATTRB option, such as ATTRB=(UNPROT,NUM,IC). Programs can also change them at run time, for example to highlight a field with an error.
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- BMS mapThe definition of a CICS screen: where each field sits, how long it is, and how it is highlighted.
- 3270 terminalThe classic mainframe display device, and the protocol still used today to talk to z/OS over a network.
- Green screenThe text-only, keyboard-driven interface most mainframe work happens through, named after the green phosphor of early terminals.
- CICS transactionOne unit of online work in CICS: the whole exchange of receiving a request, doing the work, and sending a response.
- PF keysThe function keys F1 to F24, which carry the main actions on a mainframe screen: help, exit, save, page up and down.