doc up the linux console

TERMS.md

@@ -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`.