Date   : Thu, 05 May 1983 10:36:17-PDT (Thu)
From   : ihnp4!spanky!burl!duke!mcnc!unc!bch@ucb-vax.arpa
Subject: Re: vt180 & mboot3

I have gotten a version of Modem working on the VT-180 but have used
a slightly different approach than that reported earlier.  Rather
than check the status of the port by attacking the magic control block
location in the bios, I installed an interrupt trap between the
RST location and the system interrupt handler.  This gives me first
crack at incoming data from the communication line before the vt-180
BIOS can get its hands on it.  As it turns out, the device handler
for the communication line likes to ignore certain characters (like
NUL) which can thoroughly upset file transfer.  This method gives
me everything and allows me to sense disconnection, parity errors,
overruns and framing errors as well.

As part of INITMOD:

...
       LHLD    00011H          ;load interrupt handler address
       SHLD    INTHAND+1       ;store in jump location
       LXI     H,RXINT         ;load trap routine address
       DI                      ;be careful
       SHLD    00011h          ;save in RST10 JMP address
       EI                      ;let it go
...

Now, in an independent module:

RXINT  PUSH    A               ;save register
       IN      059H            ;read status port
       STA     COMSTAT         ;save value
       ANI     02H             ;is this our interrupt?
       JZ      RXRET           ;no
       IN      058H            ;get incoming character
       STA     COMBYTE         ;save it
RXRET  POP     A               ;restore accumulator
INTHAND JMP    $-$             ;exit through system
COMSTAT        DB      0               ;result of status port read
COMBYTE        DB      0               ;result of data port read


Your recieve data handler can then treat the locations COMSTAT and
COMBYTE as if they were memory mapped IO, though your transmit
data routine must use ported IO.

Clearly, there are a number of more complicated things one can do
with this approach, including building a ring buffer to buffer
incoming data until something as slow as Microsoft Basic can get to
it.

                               Good Luck,
                                       Byron