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Having read this canonical article on TTYs, I'm not sure when kernel-mode TTY emulation (Linux console) is used instead of user-mode TTY emulation (e.g. xterm).

Normally, I'm always using gnome-terminal, xterm, or some other user-mode TTY emulation, so how does kernel-mode TTY emulation fit into this picture? Is Linux console the first TTY that is started, which allows the user to subsequently run e.g. xterm?

Kernel-mode terminal emulation: enter image description here

User-mode terminal emulation (PTY):

enter image description here

  • The kernel mode tty emulation (virtual tty) is only used for the "black" terminals (either vga or frame buffer) that you can access with ctrl-alt-f1, ctrl-alt-f2, etc. They don't even exist on many platforms (neither my router nor my phone has anything like that). Neither pseudo-ttys (as used by xterm, gnome-terminal, ssh, etc) nor actual serial ttys have anything to do with them. – mosvy Jul 11 '19 at 4:59
  • Thanks @mosvy - if these kernel-mode ttys (/dev/tty*) aren't used for anything useful, why do they still exist today? – Shuzheng Jul 11 '19 at 5:04
  • They are useful. They allow you to use linux on a PC-like machine with keyboard and screen without having to install any extra software or use another machine from which to connect via a serial link or network connection. – mosvy Jul 11 '19 at 5:08
  • Userspace terminal emulator don't rely on /dev/ttyX functionality, but pty functionality. – 炸鱼薯条德里克 Jul 11 '19 at 5:12
  • @mosvy - aren't all terminals (tty or pty-based) associated to vga? They are all rendered on the monitor. Or what do you mean by "either vga or frame buffer"? – Shuzheng Jul 11 '19 at 7:59
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Kernel-mode TTY emulation is the /dev/tty* devices that don't involve any X/Wayland programs at all. You use them by pressing Ctrl+Alt+F2 or similar. User-mode TTY emulation is basically anything else that gives you a terminal, and are the /dev/pts/* devices. They're what's created by gnome-terminal, sshd, screen, and basically everything else.

  • Thanks! Is there any reason why all terminal emulation is not moved into userland (PTYs)? – Shuzheng Jul 11 '19 at 5:02
  • @mosvy mentions that kernel-mode tty emulation is only used for "black" terminals that don't exist on many platforms. Why do we see them in /dev/tty* then? – Shuzheng Jul 11 '19 at 5:06
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    @Shuzheng there are no /dev/ttyN devices on my router or phone (both running Linux). – mosvy Jul 11 '19 at 5:12
  • @Shuzheng you may be interested in reading mail-archive.com/dri-devel@lists.freedesktop.org/msg156108.html – sourcejedi Jul 11 '19 at 8:19

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