Basic
Disk
Operating
System
(BDOS) Functions
4-20
SET
DMA
ADDRESS
ENTRY
CL:
IAH
DX:
DMA
Offset
FUNCTION
26
SET
DMA
ADDRESS
RETURN
DMA
is
an
acronym
for Direct Memory Address, which is often used in connection
with disk controllers
that
directly access the memory
of
the mainframe
computer
to
transfer
data
to
and
from the disk subsystem. Although many computer systems use
non-DMA
access (i.e., the
data
is
transferred
through
programmed
I/O
opera-
tions), the
DMA
address, has, in
CP/M,
come
to
mean the address
at
which the
128-byte
data
record resides before a disk write
and
after a disk read.
In
the
CP/M-86
environment, the Set
DMA
Address function specifies the offset
of
the read
or
write buffer from the current
DMA
base. Therefore,
to
specify the
DMA
address,
both
a function
26
call
and
a function
51
(Set
DMA
Base) call are required.
The
DMA
address is the value specified by
DX
plus the
DMA
base value, until it is
changed by a subsequent Set
DMA
Address
or
Set
DMA
Base function.
GET
ALLOCATION
ADDRESS
ENTRY
RETURN
CL:
IBH
FUNCTION
27
BX:
ALLOC
Offset
GET
ADDR(ALLOC)
ES: Segment base
An
allocation vector
is
maintained in main memory for each online disk drive.
Various system
programs
use the information provided by the allocation vector
to
determine the
amount
of
remaining storage (see the
STAT
program
in the CP / M-86
User's Guide
for the APC). The
Get
Allocation Address function returns the
segment base
and
the offset address
of
the allocation vector for the currently
selected drive.
The
allocation information may, however, be invalid if the selected
disk has been
marked
read/only.
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