Intel 80286 User Manual

OVERVIEW OF NUMERIC PROCESSING
DENORMALIZED OPERAND
If an instruction attempts to operate on a denormal, the NPX reports the denormalized operand excep-
tion. This exception allows users to implement in software an option of the proposed
IEEE standard
specifying that operands must be prenormalized before they are used.
NUMERIC OVERFLOW AND UNDERFLOW
If the exponent of a numeric result is too large for the destination real format, the 80287 signals a
numeric overflow. Conversely, if the exponent of a result
is too small to be represented in the destina-
tion format, a numeric underflow
is signaled. If either of these exceptions occur, the result of the
operation
is outside the range of the destination real format.
Typical algorithms are most likely to produce extremely large and small numbers in the calculation of
intermediate, rather than final, results. Because of the great range
of the temporary real format
(recommended
as the destination format for intermediates), overflow and underflow are relatively rare
events in most 80287 applications.
INEXACT RESULT
If the result of an operation is not exactly representable in the destination format, the 80287 rounds
the number and reports the precision exception. For example, the fraction
113 cannot be precisely repre-
sented in binary form. This exception occurs frequently and indicates
that some (generally acceptable)
accuracy has been lost; it is provided for applications
that need to perform exact arithmetic only.
HANDLING NUMERIC ERRORS
When numeric errors occur, the NPX takes one of two possible courses of action:
• The
NPX can itself handle the error, producing the most reasonable result and allowing numeric
program execution to continue undisturbed.
• A software exception handler can be invoked by the
CPU to handle the error.
Each of the six exception conditions described above has a corresponding flag bit in the 80287 status
word and a mask bit in the 80287 control word.
If an exception is masked (the corresponding mask bit
in the control word = 1), the 80287 takes an appropriate default action and continues with the compu-
tation.
If the exception is unmasked (mask=O), the 80287 asserts the ERROR output to the 80286 to
signal the exception and invoke a software exception handler.
The
NPX reports ari exception by setting the corresponding flag in the NPX status word to 1. The
NPX then checks the corresponding exception mask in the control word to determine if it should
"field" the exception (mask= 1), or if it :siguill the to the CPU to 2. soft'.l!f!.!e
exception handler (mask=O).
If the mask is set, the exception is said to be masked (from user software), and the NPX executes its
on-chip masked response for that exception.
If the mask is not set (mask=O), the exception is unmasked,
and the NPX performs its unmasked response. The masked response always produces a standard result,
then proceeds with the instruction. The unmasked response always traps to a software exception handler,
allowing the
CPU to recognize and take action on the exception. Table 1-17 gives a complete descrip-
tion
of all exception conditions and the NPX's masked response.
1-34
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Intel 80286 Specifications

General

Processor Type	Microprocessor
Clock Speed	6 MHz to 25 MHz
Transistor Count	134, 000
Addressable Memory	16 MB
Instruction Set	x86
Architecture	x86
Data Bus Width	16-bit
Address Bus Width	24-bit
Operating Modes	Real mode, Protected mode
Process Technology	1.5 µm
Voltage	5V
Introduced	1982
Package	68-pin PGA