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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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