Additional Information For Writing Your Own Drivers - Radio Shack TRS-80 Operation Manual

3.
The
load address
tells
TRSDOS
where
the data
in this
block
will
be loaded
into
RAM.
Bytes
will
be loaded
serially starting at
the load address.
The
load address
must allow
the entire
block of
program
data
to
load into the
PTC-64.
Comment
Blocks
Comment
blocks consist of the following:
Byte
#
Contents
1
Block
Identifier
(
=
5)
2
Length
—
number
of bytes
in
the
comment
3-end
Comment
Notes:
For comments,
the
block
identifier
always equals binary
"5."
The
length byte gives the
number
of bytes
in the
comment,
i.e.,
the
number
of
bytes
after the
length byte
itself.
This
sum
ranges
from
to
255:
Length Byte Length of
Comment
1
1
2
2
255 255
Trailer
Blocks
Each program
file
ends with
a single
trailer
block
which
tells
the
PTC-64
to
move
the
program
to
its
correct location
and
transfer
control
to
the address
that
follows.
Trailer
blocks consist of the following:
Byte
#
Contents
1
Block
Identifier
(
=
2)
2
Length
(
=
2)
3-4 Transfer address
in
LSB-MSB
sequence
Notes:
1.
For
trailer
blocks, the identifier
must
equal binary
"2."
2.
The
transfer address
is
in
LSB-MSB
sequence.
Additional Information
for
Writing
Your
Own
Drivers
If
you want
to
write
your
own
Printer
Controller drivers, there are certain things that
you
must know:
1.
The PTC-64
is
configured
to
use
Z-80
Mode
1
interrupts.
This
means
that
pressing a
key on
the front
panel causes
a
CALL
to
address
0038H
(if
interrupts are
enabled).
A
STROBE*
from
the
computer
will
generate an
NMI
causing
the
Z-80
to
perform
a
CALL
to
address
0066H. This
may
not
be
disabled.
If
you want
to
prevent
the
computer from
sending data, turn
on
the
FAULT
bit
in Port
1
(Write).
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