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The Termcap Library

1.5.2 When There Is Not Enough Padding

There are several common manifestations of insufficient padding.

• Emacs displays `I-search: ^Q-' at the bottom of the screen.

  This means that the terminal thought its buffer was getting full of display commands, so it tried to tell the computer to stop sending any.

• The screen is garbled intermittently, or the details of garbling vary when you repeat the action. (A garbled screen could be due to a command which is simply incorrect, or to user option in the terminal which doesn't match the assumptions of the terminal description, but this usually leads to reproducible failure.)

  This means that the buffer did get full, and some commands were lost. Many changeable factors can change which ones are lost.

• Screen is garbled at high output speeds but not at low speeds. Padding problems nearly always go away at low speeds, usually even at 1200 baud.

  This means that a high enough speed permits commands to arrive faster than they can be executed.

Although any obscure command on an obscure terminal might lack padding, in practice problems arise most often from the clearing commands `cl' and `cd' (see section 3.7 Clearing Parts of the Screen), the scrolling commands `sf' and `sr' (see section 3.5 Scrolling), and the line insert/delete commands `al' and `dl' (see section 3.8 Insert/Delete Line).

Occasionally the terminal description fails to define `sf' and some programs will use `do' instead, so you may get a problem with `do'. If so, first define `sf' just like `do', then add some padding to `sf'.

The best strategy is to add a lot of padding at first, perhaps 200 msec. This is much more than enough; in fact, it should cause a visible slowdown. (If you don't see a slowdown, the change has not taken effect; see section 2.5 When Changes in the Data Base Take Effect.) If this makes the problem go away, you have found the right place to add padding; now reduce the amount until the problem comes back, then increase it again. If the problem remains, either it is in some other capability or it is not a matter of padding at all.

Keep in mind that on many terminals the correct padding for insert/delete line or for scrolling is cursor-position dependent. If you get problems from scrolling a large region of the screen but not from scrolling a small part (just a few lines moving), it may mean that fixed padding should be replaced with position-dependent padding.