The routine ttyoin(iptr), which consumes characters from both the output buffer and the echo buffer, does the following:
If there is a character in the echo buffer, it removes it from the buffer and assigns it to the XBUF register and returns.
If the user has typed a `stop output' command, determined by a user parameter, then the routine disables output interrupts and returns.
If there are no characters in the output buffer, it disables output interrupts and returns.
Otherwise,
it removes the character from the tail of the output buffer and signals
osem
0, 1, or OBMINSP times (what determines how many signals are generated?).