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Basic I/O System (BIOS) Functions
5-14
SET DMA OFFSET ADDRESS
ENTRY
CX: DMA Offset
FUNCTION 12
SETDMA
RETURN
This function sets the DMA Offset Address. Register CX contains the DMA (Direct
Memory Address) offset for subsequent READ or WRITE operations. For exam-
ple, if CX is 80H when SETDMA is called, all subsequent READ operations read
their data into 80H through OFFH offset from the current DMA segment base, and
all subsequent WRITE operations get their data from that address, until the next
calls to SETDMA and SETDMAB occur. This data is then transferred to the DMA
buffer addressed by the SETDMA and SETDMAB commands.
READ SELECTED SECTOR
ENTRY
FUNCTION 13
READ
RETURN
AL: Return Code
OH,OFFH
Assuming the drive has been selected, the track has been set, the sector has been set,
and the DMA offset and segment base have been specified, the READ subroutine
attempts to read based on these parameters by calling R_ W _COMMON for
diskette I/O or R_ W _COMMONHD for hard disk I/O. It returns zero in register
AL if the read is successful. If the read is not successful, it returns OFFH in register
AL.
This routine first calculates the physical sector address from the logical track
address given by the BDOS. It then determines whether the requested record
already exists in the work buffer. If the record is not in the buffer, the routine calls
the appropriate routine to read from the disk medium to a buffer. R_ W _COM-
MON reads 128 bytes from a single density diskette, and 256 bytes from double
density diskette. R_ W _COMMONHD reads 8K bytes (multisector read) from the
disk to the work buffer. Finally, the requested record is moved from the work buffer
to the user-specified DMA address.
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