Basic
I/O
System (BIOS) Functions
GET
MEM
DESC
TABLE
OFFSET
ENTRY
FUNCTION
18
GETSEGB
RETURN
BX:
MR
T Address
This
function returns the address
of
the
Memory
Region
Table
(MRT)
in BX.
The
returned
value
is
the offset
of
the table relative
to
the
start
of
the
operating
system.
The
table defines
the
location
and
extent
of
physical
memory
which is available
for
transient programs.
Memory
areas reserved for
interrupt
vectors
and
the
CP
1M
-86
operating
system
are
not
included in the
MRT.
The
Memory
Region
Table
takes the following form.
MRT:
0:
1:
n:
8-bit
R-Cnt
R-Base
I R-Length 1
R-Base
I R-Length 1
R-Base
R-Length
16-bit 16-bit
R-Cnt
is
the
number
of
Memory
Region Descriptors (equal
to
n + 1 in the
diagram
above),
and
R-Base
and
R-Length give the
paragraph
base
and
length
of
each
physically contiguous
area
of
memory. Again,
the
reserved interrupt locations,
normally
0-3FFH,
and
the
CP/M-86
operating
system are
not
included in this
map,
because the
map
contains regions available
to
transient programs.
If
all
memory
is
contiguous, the
R-Cnt
field
is
1
and
n
is
0, with only a single
Memory
Region
Descriptor
which defines
the
region.
Chapter
7 describes
the
layout
and
construction
of
the
disk
parameter
tables
referenced by the various subroutines in the
BIOS.
5-17
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