making tops20 EMACS more efficient.

[Christopher C. Stacy <CSTACY @ MIT-MC>]

I don't know what people have been proposing for TOPS-20, but here is
the documentation for the system call that ITS provides to make TECO
more efficient:

ECHOIN: echo characters and store them in a buffer, 
        until a break character

	arg 1	TTY channel number
	arg 2	Byte pointer to buffer
	arg 3	Number of characters left in buffer
	arg 4	Address of break table
	arg 5	Address of TECO buffer block (or zero)

	The second and third arguments are counted out as characters
	are stored.  When the call returns due to a break character, the
	count will still be greater than zero.

	ECHOIN tells the system to echo characters and insert them in
	the buffer specified by the second and third arguments without
	requiring the user program to be executed.  This is more
	efficient and gets visibly faster response time.  Echoing
	stops when either the count is exhausted or a break character
	is input.  When this happens, the ECHOIN returns.  If it returns
	because of a break character, then the break character
	is available for normal input.	Non-break characters following
	the break character are not handled.

	Break characters are specified by the break table,
	a four-word block pointed to by the fourth argument.  Each of
	the four words says, for 32 characters, whether they break
	or not.  In the first word, the sign bit is for SAIL code 0,
	and bit 1.5 is for SAIL code 37.  The sign bit of the second
	word is for Space, and bit 1.5 is for ?.  And so on.
	Characters with Control or Meta set (including ASCII control
	characters on non-Meta keyboards) are always break characters.

	To prevent timing errors, ECHOIN will return immediately if
	there are characters in the input buffer already.

	For use by TECO, ECHOIN can update the block of counters which
	TECO uses to remember the extent of the buffer.  The fifth
	argument should point to this block, the "Buffer block", which
	is seven words long.  When a character is inserted, the first
	two words of the buffer block are not changed, the next four
	are incremented, and th next one is decremented.  Passing the
	buffer block to ECHOIN allows ECHOIN to insert characters
	directly into TECO's buffer as an indivisible operation.
	Note that the insertion of the characters themselves is still
	controlled by the byte pointer and count arguments.
	However, in the future this may change; when ECHOIN is made
	able to handle operations besides insertion, it will use the
	buffer block for updating the buffer (which may involve deletion)
	but will still insert all characters it processes down the byte
	pointer.  This way, TECO will have both an updated buffer and
	a list of the commands which the user typed.