Ziatech Corporation ZT 8809A, Руководство по эксплуатации

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Numeric Data Processor (8087)
INTERRUPT/NUMERIC ERRORS
Two courses of action are possible when a numeric error occurs: The
NDP can handle the error itself, allowing numeric program execution
to continue undisturbed; or host software can manage it. In order to
have the 8087 handle a numeric error, set its associated mask bit in
the NDP control word. Each numeric error may be individually
masked.
The NDP has a default fixup action defined for all possible numeric
errors when they are masked. The default actions were carefully
selected for their generality and safety. For example, the default fixup
for the precision error is to round the result using the rounding rules
currently in effect. If the invalid error is masked, the NDP generates a
special value called indefinite as the result of any invalid operation.
Any arithmetic operation with an indefinite operand always generates
an indefinite result. In this manner, the result of the original invalid
operation propagates throughout the program wherever it is used.
When a questionable operation, such as multiplying an unnormal
value by a normal value, occurs, the NDP signals this occurrence by
generating an unnormal result.
The required response by host software to a numeric error depends on
the application. The needs of each application must be understood
when deciding how to treat numeric errors. There are three basic
approaches to a numeric error:
1. No response required. Let the NDP perform the default fixup.
2. Stop everything, severe fault detected in system requiring imme-
diate action.
3. Interrupt only lower priority operations.
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