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| <H2><A NAME="s18">18. Scrolling</A></H2> |
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| <!-- |
| console!scrolling |
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| <!-- |
| scrolling, console |
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| <P>There are two ways to get a screen to scroll. |
| The first, called `hard scrolling', is to leave the text in |
| video memory as it is, but change the viewing origin. This is |
| very fast. |
| The second, called `soft scrolling', involves moving all screen text |
| up or down. This is much slower. |
| The kernel console driver will write text starting at the top of |
| the video memory, continuing to the bottom, then copy the bottom |
| part to the top again, and continue, all the time using hard scrolling |
| to show the right part on the screen. |
| You can scroll back until the top op the video memory by using |
| Shift-PageUp (the grey PageUp) and scroll down again using |
| Shift-PageDown (the grey PageDown), assuming a default keymap. |
| The amount of scrollback is thus limited to the amount of video memory |
| you happen to have and you cannot increase this amount. |
| If you need more scrollback, use some program that |
| buffers the text, like <CODE>less</CODE> or <CODE>screen</CODE> - by using |
| a buffer on disk you can go back to what you did last week. |
| (One can set the amount of scrollback for <CODE>xterm</CODE> by |
| adding a line like <CODE>XTerm*saveLines: 2500</CODE> in <CODE>.Xresources</CODE>.) |
| <P>Upon changing virtual consoles, the screen content of the old VT |
| is copied to kernel memory, and the screen content of the new VT |
| is copied from kernel memory to video memory. Only the visible screen |
| is copied, not all of video memory, so switching consoles means |
| losing the scrollback information. |
| <P>Sometimes, hard scrolling is undesirable, for example when the hardware |
| does not have the possibility to change viewing origin. The first |
| example was a Braille machine that would render the top of video |
| memory in Braille. There is a kernel boot-time option <CODE>no-scroll</CODE> |
| to tell the console driver not to use hard scrolling. |
| See bootparam(7). |
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