Basic Disk Operating System (BDOS) Functions
4-32
SET
DMA
BASE
ENTRY
CL: 33H
DX: Base
Address
RETURN
FUNCTION
51
SET
DMA
BASE
The
Set
DMA
Base function sets the base register for subsequent
DMA
transfers.
The
word
parameter
in
DX
is
a
paragraph
address
and
is
used with the
DMA
offset
to
specify the address
of
a 128-byte buffer
area
for the disk read
and
write functions.
Note
that
upon
initial
program
loading, the default
DMA
base is set
to
the address
of
the user's
data
segment (the initial value
of
DS)
and
the
DMA
offset is set to
0080H, which provides access
to
the default buffer in the base page.
GET
DMA
BASE
ENTRY
RETURN
CL: 34H
FUNCTION
52
BX:
DMA
Offset
GET
DMA
BASE
ES:
DMA
Segment
The
Get
DMA
Base function returns the current
DMA
Base Segment address in ES,
and
the current
DMA
Offset in BX.
BDOS
Memory Management and Program Functions
Memory
is allocated in two distinct ways
under
CP
IM-86.
The first
is
through
a
static allocation
map,
located within the BIOS,
that
defines the physical
memory
which
is
available
on
the host system.
In
this way, it is possible to
operate
CP/M-86
in a memory configuration which
is
a mixture
of
up
to
eight non-contiguous areas
of
RAM
or
ROM
along with reserved, missing,
or
faulty memory regions.
In
a simple
RAM-based system with contiguous memory, the static
map
defines a single region,
usually
started
at
the end
of
the BIOS
and
extending
up
to
the
end
of
available
memory.
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