ACT apricot - Page 293

INTRODUCTION
The BIOS acts in conjunction with MS-DOS 2.0 where
disk I/O is concerned. MS-DOS will call a routine within
the BIOS, passing parameters such as where on the disk to
write the data, how much data to write and where the data
is held in memory. The BIOS will then inform the floppy
disk controller of what has to be done, which will then
perform the actual write, by moving the head to the correct
place on the disk and then sending the actual data to the
disk controller which will write it to the disk.
APPLICATIONS INTEREST
All
applications level programming should only use the
disk handling instructions provided by the language that is
being used or, in exceptional circumstances, the MS-DOS
interrupts can be used to read and write files. Some external
utilities such as DEBUG will allow the user to write
absolute disk sectors. It should be stressed that under no
circumstances must any sectors be changed unless there is a
full knowledge of what is being done. The actual way that
the Apricot uses the disk controllers is sufficient for most
software.
SYSTEMS INTEREST
Most input/output to the disk controller is performed by
the 8089. The 8089 uses a parameter block (located in
memory) which is set up by the BIOS to contain various
pieces of information relating to the relevant disk
operations. Then a signal is sent to the 8089 which reads the
parameter block and takes action on its contents.
It will become apparent if any experimentation is carried
out that all tflephysical details of how the disk is written or
read are handled by the floppy disk controller. The BIOS
passes commands and parameters to the Floppy disk
controller and then invokes the 8089 (as described above).
The 8089 is used in only two of its operation modes, and