The upper-half tty input routines consist of ttygetc and
ttyread which are the consumers of characters filled into the input buffer.
Ttygetc(devptr)
The routine ttygetc(devptr) performs the following operations:
waits for isem
removes character at the tail of the input queue.
Ttyread(devptr, buff, count)
The routine ttyread (devptr, buff, count) may be used to read count number of chars from the input buffer or all the pending characters (what is the use of this operation?). It does the following:
If count = 0 then all the available characters are copied from the input buffer to buff, and the count of isem is adjusted.
If the count is not zero then as many characters as possible are copied from
the buffer to buf (adjusting count of isem)
The rest are obtained through getc.