Restricting IO Accesses
This section is a copy of the section entitled “IO Port Anarchy” on page 32 from the chapter entitled “Multitasking Problems” on page 31. It is repeated here to eliminate the need to turn back to that section. It states the nature of the IO-related problem that must be dealt with in VM86 (as well as other) programs.
Assume that the currently executing task needs to initiate a disk access.
To do this, it must program the disk controller's IO registers with the information defining the disk command type (e.g., disk read), the cylinder number, the head (or surface) number, the start sector number and the number of sectors to be transferred. This is accomplished by executing a series of OUT instructions that cause ...