Basic
I/O
System (BIOS) Functions
5-16
SECTOR TRANSLATE
ENTRY
CX: Logical Sector
Number
DX: Translate Table
Offset
FUNCTION
16
SECTRAN
RETURN
This function performs logical
to
physical sector translation to improve the overall
response
of
CP/M-86.
The
CP/M-86
operation system
is
shipped with a "skew
factor"
of
6 for single-sided single-density diskettes
and
3 for double-sided double-
density diskettes. This skew factor allows enough time between sectors for most
programs
to
load
their buffers without missing the next sector.
In
general, SEC-
TRAN
receives a logical sector
number
in CX. This logical sector number may
range from
0
to
n -
I,
where n
is
the number
of
sectors.
SECTRAN
also receives a translate table offset in
DX.
The sector
number
is
used as
an
index into the translate table, with the resulting physical sector
number
in BX.
The
CBIOS for the
APe
includes correct tables for the skew factors in use.
SET
DMA
BASE ADDRESS
ENTRY
•
CX:
DMA
Base
FUNCTION
17
SETDMAB
RETURN
.,
This function sets the
DMA
base address. Register
CX
contains the segment base
for subsequent
DMA
read
or
write operations. The BIOS uses the 128-byte buffer
at
the memory address determined by the
DMA
base
and
the
DMA
offset during read
and
write operations.
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