Designer’s
Handbook 21
directives
to
decide
where
to
place
code
and
data
in
logical
memory
and
its
mapping
to
physical
memory
through
a
segment
register
value.
The
directives
are
normally
specified
in
the
BIOS.
However,
the
directives
may
also
be
useful
in
the
user
program
for
certain
tasks
such
as
compiling
a
pilot
BIOS
or
coldloader,
or
special
situations
where
a
user
wants
two
application
coresident
within
a
single
256K
quadrant
of
flash.
Each
origin
statement
has
the
following
syntax:
#<origin type> <origin name> <segment value> <logical address>
<size> apply
All
of
the
arguments
are
required.
<origin type>
may
be
one
of
the
following:
rcodorg, xcodorg, wcodorg,
rvarorg
,
and
wvarorg.
These
origin
types
define
where
to
place
root
code,
extended
code,
watch
code,
root
data,
and
watch
data,
respectively.
The
code
origin
directives
grow
up
(increase)
whereas
the
data
origins
grow
down
(decrease).
Origin
directives
should
be
defined
before
code
or
data
of
the
associated
type
is
compiled.
For
example,
any
data
dec-
larations
or
code
should
appear
after
the
rcodorg and
rvarorg
directives.
Likewise,
xcodorg
statement
should
appear
before
any
code
is
compiled
to
xmem.
<origin name>
creates
an
identifier
associated
with
that
particular
directive.
<segment value>
should
be
an
8-bit
value
segment
value
for
the
origin
segment.
The
physical
address
can
be
calculated
by
shifting
the
<segment value>
to
the
left
by
12
bits
and
adding
the
16-bit
<logical address>
value.
<size>
tells
the
compiler
when
to
issue
an
“out
of
space”
error
for
that
directive
when
the
code
counter
grows
out
of
bounds.
apply
means
that
the
org
statement
will
take
effect
immediately.
This
is
required
for
all
org
statements.
Future
releases
may
allow
different
keywords
to
be
used
in
place
of
the
apply
argument.
Examples
An
typical
example
of
using
the
origin
directives
in
the
BIOS
is
shown
below.
The
dia-
gram
below
shows
how
the
origin
statements
define
the
mapping
between
the
logical
and
physical
address
spaces.
#define DATASEGVAL 0x91
#rvarorg rootdata (DATASEGVAL) 0xc5ff 0x6600 apply // grows down
#rcodorg rootcode 0x00 0x0000 0x6000 apply
#wcodorg watcode (DATASEGVAL) 0xc600 0x0400 apply
#wvarorg watdata (DATASEGVAL) 0xcdff 0x0400 apply // grows down
#xcodorg xmemcode 0xf8 0xe000 0x1a000 apply
// data declarations start here
To Next Page
Loading...
