Anatomy of a mainframe screen
A mainframe screen is a fixed grid, usually 24 lines of 80 characters, divided into fields. Some fields are for you to type in and the rest are protected. Almost every panel you meet follows the same layout: a title, a message area, a command line, the body and a list of function keys.
A grid of fields, not a page of text
A 3270 display is a grid of character positions. The application divides it into fields. A field is either *protected* (labels, titles, data you may only read) or *unprotected* (places where you can type). When you press Tab, the cursor jumps to the next unprotected field and skips everything else. That is how you can tell where you are allowed to type.
Data Set List Utility INVALID DSNAME Option ===> _ Dsname Level . . . VSAHA1.WORK.* Volume serial . . (body: fields to fill in, or a list to act on) F1=Help F2=Split F3=Exit F7=Backward F8=Forward F9=Swap F12=Cancel 4A MA a 03/015
Field attributes you will notice
- Highlighted or coloured fields usually mean input, or something important.
- Hidden fields, such as the password on a logon panel, accept typing but never display it.
- Numeric-only fields refuse letters. The keyboard locks if you try.
- Some fields are pre-filled with the value you used last time. Type over it, or clear it with Erase EOF.
Line commands and the command line
Lists such as a dataset list, a member list or a job list have a narrow input column on the left of each row. A letter typed there is a line command, and it acts on that row only. Text on the Command ===> line is a primary command and acts on the whole panel. Mixing the two up is the most common beginner slip. Typing D on the command line does something completely different from typing D beside a dataset.
On a dataset list, where would you type a command that should act on just one dataset: the command line, or the column beside that dataset?
Show a hint
Lists have a narrow column on the left of each row.
Show the solution
In the line command column beside that dataset. The command line acts on the whole panel.
Examples are for learning. Run commands and jobs only on a system you are authorised to use, such as a training or test system, and never on production without approval.
Common mistakes
The keyboard locks and nothing you type is kept. Press Reset, then use Tab to jump to a real input field.
A letter beside a list row acts on that row. The same letter on the command line may mean something else entirely, or be rejected.
The short message tells you why your last action failed. Read it before trying again.
What you will see at work
- Every product (ISPF, SDSF, IOF, CICS screens, scheduler panels) uses this same layout, so learning it once pays off everywhere.
- Colleagues will say 'put an S beside it'. That means a line command in the left-hand column.
- Ask which screen size your team uses. Screenshots in runbooks are often taken at a larger size than the default.
Key terms
Check your understanding.
Take this lesson's quiz and save your progress. Free.