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Basic I/O System (BIOS) Functions
5-10
GET I/O MAPPING BYTE
ENTRY
FUNCTION 19
GETIOB
RETURN
AL: IOBYTE
This function returns the current value of the logical to physical input/output
device byte (IOBYTE) in AL. This eight-bit value is used to associate physical
devices with CP/M-86's four logical devices.
SET I/O MAPPING BYTE
ENTRY
CL: 10BYTE
FUNCTION 20
SETIOB
RETURN
This function uses the value in CL to set the value of the IOBYTE stored in the
BIOS.
Disk I/O Subroutines
This section describes the disk I/O subroutines, functions 8 through 18.
Disk I/O is always performed through a sequence of calls to the various disk access
subroutines. The subroutine sets up the disk number to access, the cylinder (for
hard disk), track, and sector on the disk, and the direct memory access (DMA)
offset and segment addresses involved in the I/O operation. After all these parame-
ters have been set, the subroutine makes a call to the READ or WRITE function.
The READ or WRITE function, in turn, calls either the R_ W _COMMON (for
diskette) or R_ W _COMMONHD (for hard disk) routine to perform the actual
I/O operation. Note that there is often a single call to SELDSK to select a disk
drive, followed by a number of read or write operations to the selected disk before
selecting another drive for subsequent operations. Similarly, there may be a call to
set the DMA segment base and a call to set the DMA offset followed by several calls
which read or write from the selected DMA address before the DMA address is
changed. The track and sector subroutines are always called before the READ or
WRITE operations are performed.
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