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doc up the linux console

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nick black 2020-07-12 02:55:48 -04:00
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TERMS.md
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@ -27,3 +27,55 @@ The following have been established on a Debian Unstable workstation.
| st | `TERM=st-256color` `COLORTERM=24bit` | |
| GNU Screen | `TERM=screen-256colors` | Must be compiled with `--enable-256color`. |
| tmux | | |
## The Linux console
The Linux console supports concurrent virtual terminals, and is manipulated
by userspace via `ioctl()`s. These `ioctl()`s generally fail when applied to
a pseudotty device, as will happen if e.g. invoked upon one's controlling
terminal whilst running in a terminal emulator under X (it is still generally
possible to use them by explicitly specifying a console device, i.e.
`showconsolefont -C /dev/tty0`).
The VGA text console requires the kernel option `CONFIG_VGA_CONSOLE`. A
framebuffer console for VESA 2.0 is provided by `CONFIG_FB_VESA`, while
UEFI-compatible systems can use `CONFIG_FB_EFI`. So long as a framebuffer
driver is present, `CONFIG_FRAMEBUFFER_CONSOLE` will enable a graphics-mode
console using the framebuffer device.
The Linux console can be in either text or graphics mode. The mode can be
determined with the `KDGETMODE` `ioctl()`, and changed with `KDSETMODE`,
using the constants `KD_TEXT` and `KD_GRAPHICS`. Text mode supports a
rectangular matrix of multipixel cells, filled with glyphs from a font,
a foreground color, and a background color. Graphics ("All-Points-Addressable")
mode supports a rectangular matrix of pixels, each with a single color.
Note that both modes require appropriate hardware support (and kernel
configuration options), and might or might not be available on a given
installation. Non-x86 platforms often provide only a framebuffer (graphics)
console.
The kernel text mode loosely corresponds to the 1987 IBM VGA definition. At any
time, the display is configured with a monospace raster font, a palette, and
(when in Unicode mode) a mapping from multibyte sequences to font elements. Up
to 16 colors can be used with a font of 256 glyphs or fewer. Only 8 colors can
be used with fonts having more than 256 glyphs; the maximum font size in any
configuration is 512 glyphs. The keyboard is further configured with a keymap,
mapping keyboard scancodes to elements of the character set. These properties
are per-virtual console, not common to all of them. These limitations are not
typically present on framebuffer consoles.
The following more-or-less standard tools exist:
* `showconsolefont`: show the console font
* `setfont`: load console font
* `fbset`: show and modify framebuffer settings
* `fgconsole`: print name of foreground terminal
* `chvt`: change the foreground terminal
* `deallocvt`: destroy a virtual console
* `dumpkeys`: print all keycodes
* `loadkeys`: load scancode/keycode mapping (the keymap)
* `setkeycodes`: load scancode/keycode mappings one at a time
* `showkeys`: interactively print scancodes
* `kbd_mode`: show or set the keyboard mode
Both `mapscrn` and `loadunimap` are obsolete; their functionality is present
in `setfont`.