MODEMH.DOC - documentation file for MODEMH.


MODEMH is an augmented version of MODEM7A, permitting
handover of control to a remote computer or data terminal. 
It operates by intercepting console i/o at the bios, so that
input can be taken indefferently from the console or the
line, and is output both to line and screen.

The existing MODEM7A facilities are unaffected.	 An 'H'
command hands over control; the remote computer should be in
terminal mode (without echo) to get message and prompt. 
After that the remote operator can feed in MODEMH commands,
cause exit to CPM for other programs, recall MODEMH, and so
on.  Control codes can be sent, but the alternative form with
preceding '^' is available when he doesn't want them to be
recognised by his own computer; in particular, he uses '^E'
if he has put the handed-over computer in terminal mode to
type a message, and wants to leave it again.  He exits his own
terminal mode - for example, to type in his own 'Send' or
'Receive' command - with a genuine 'Ctrl E'.  Similarly, he
can turn YOUR disk-write off with a '^Y' (when he wants to be
off the record!), his own with 'Ctrl Y'.

Typing an 'escape' cancels handover and gets you back in
exclusive control of your machine - UNLESS the remote
operator has crashed it, in which case you do a cold boot in
the same way as when you've crashed it yourself, and start
again.	Note that, although your own keyboard is still active
while you are handed over, it is effective only when the
running program makes a standard (CPM) console i/o call; so
you may have to wait for results.  No message is sent to the
remote operator when you cancel; he may not be listening
anyway, and you can always send him a manual one if he is.

There are no special 'anti-crash' provisions.  I use this
program mainly when I have two computers strapped together
(for transfer of files from one disk format to another) on
the same bench, simply to avoid having to hop between
keyboards.  But it can also be useful over the phone.
(Operator A needs to relieve nature, has back-up copies of 
all his valuable files, and might as well let operator B
carry on meanwhile.  Or an expert gets a novice to hand over
to him to show him how it's done.  Or--)

CONFIGURATION

SORMODH is a version for the Sorcerer; DATMODH is for the
Australian Datamax computer; and I'm going to have to make an
APMODH soon.  The ASM files are tailored for assembly on RMAC
and LINK; they assume you have MODEM.LIB on the same disk in
the same way as for other members of the MODEM7 family.

The sensitive bit of the added program is a 100h block that
needs to be stashed somewhere it won't be overwritten by
other programs the remote operator might want to run.  On the
Sorcerer I have a cunning hole between CPM and the Monitor;
generally, a spot just below CPM will be the best place, or
below the base of DDT (Shh!), ZSID or some other debugger if
you run them.  Put its address at HAVEN in the added equates
near the start of the listing.

Also put the code of an escape character at ESCHAR;
presumably this will be ESC, 1bh, but you can alter it if you
have to.

The remainder of the configuration is as for MODEM7A, EXCEPT
that, because the stashed 100h block needs to be independent,
there are four small subroutines in it, called MINST, MOUST,
MIN and MOUT, that need to be considered as well.  These are
right at the end of the listing; the first two return z-flags
when modem input and output (respectively) are ready, and the
others (respectively) receive or send bytes.  Note also that
if (improbably) you need to insert symbolic addresses when
rewriting these programs, references to these addresses need
to take into account the fact that the program is to be
relocated; the offset is 'HAVEN minus RCAN', and should be
added to jump, call, load and store addresses in the same way
as it is elsewhere in the segment.

If you DO need to rewrite anything here, there are about 30
bytes spare.

Leave me a message on RCPM if you have any troubles, or helpful
suggestions.

				Charles Hamblin