NEC CP/M-86 System Reference Manual page 88

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.