Multitasking Provisions; Interfaces - IBM AT 5170 Technical Reference

Multitasking Provisions
The mM Personal Computer AT BIOS contains a feature to
assist multitasking implementation. "Hooks" are provided for a
multitasking dispatcher. Whenever a busy (wait) loop occurs in
the BIOS, a hook is provided for the program to break out of the
loop. Also, whenever BIOS services an interrupt, a corresponding
wait loop is exited, and another hook is provided. Thus a program
may be written that employs the bulk of the device driver code.
The following is valid only in the microprocessor's real address
mode and must be taken by the code to allow this support.
,~
The program is responsible for the serialization of access to
the device driver. The BIOS code is not reentrant.
The program is responsible for matching corresponding wait
and post calls.
Interfaces
There are four interfaces to be used by the multitasking
dispatcher:
Startup
First, the startup code hooks interrupt hex 15. The dispatcher is
responsible to check for function codes of AH= hex 90 or 91.
The "Wait" and "Post" sections describe these codes. The
dispatcher must pass all other functions to the previous user of
interrupt hex 15. This can be done by a IMP or a CALL. If the
function code is hex 90 or 91, the dispatcher should do the
appropriate processing and return by the IRET instruction.
Serialization
It
is up to the multitasking system to ensure that the device driver
code is used serially. Multiple entries into the code can result in
serious errors.
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9-16 Compatibility