INTRODUCTION
The
Apricot Screen driver is
an
advanced
module
that
is
designed
to
handle
all
communications
between
the
CRT
screen
and
the
MS-DOS 2.0 Operating system.
The
software consists of a
number
of modules, and
incl udes
the
following specific areas:
1.
The
CRTC
interface,
which
initializes
the
display
hardware
on
power-up.
This
software is included
in
the
Boot
PROM
as
well
as
the
BIOS.
2. Low-level
routines
to
display a character
in
a
specified
position
on
the
screen.
3.
Cursor
management
routines,
to
keep
track
of
the
cursor
position
and
cursor mode.
4.
Control
code
management
functions,
to
handle
escape sequences
and
Control
codes.
APPLICATIONS INTEREST
Escape
sequence
Management
functi.ons.
The
Screen driver supports
73
different Escape sequences,
not
including
the
ANSI codes.
When
a character 27 decimal
or
IB
hex
is received by
the
Screen Driver, control is
transfered (via a Jump Table)
which
branches to a specific
piece of code depending
on
what
the
next
character
to
be
displayed is. For example,
to
clear
the
VDU screen
the
sequence ESCAPE E has
to
be
sent
to
the
screen driver.
On
receiving
the
ESCAPE code,
the
BIOS
will
wait
until
the
next
character
is
sent
to
be displayed.
When
it
is received (in
this
case
an
"E"), control
will
be transferred to
the
section
of
code
which
deals
with
clearing
the
screen.
When
the
screen
is cleared,
control
is passed
back
to
the
screen driver,
which
can
either
display
the
next
character
or
return
to
the
DOS
for
another
function
to
be
executed.
Table
1 is
an
annotated
list
of all
the
Apricot Escape codes.
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