Intel 80286 User Manual

OVERVIEW OF NUMERIC PROCESSING
Table 1-9 shows how the instruction set deals with unnormal operands. Note that the unnormal may
be the original operand or a temporary created by the 80287 from a denormal.
ZEROS AND PSEUDO ZEROS
The value zero in the real and decimal integer formats may be signed either positive or negative,
although the sign of a binary integer zero
is always positive. For computational purposes, the value of
zero always behaves identically, regardless of sign, and typically the fact that a zero may be signed
is
transparent to the programmer. If necessary, the FXAM instruction may be used to determine a zero's
sign.
The zeros discussed above are called true zeros; if one of them
is loaded or generated in a register, the
register
is tagged zero. Table 1-10 lists the results of instructions executed with zero operands and also
shows
how a true zero may be created from nonzero operands.
Only the temporary real format may contain a special class of values called pseudo zeros. A pseudo
zero
is an unnormal whose significand is all zeros, but whose (biased) exponent is nonzero (true zeros
have a zero exponent). Neither
is a pseudo zero's exponent all ones, since this encoding is reserved for
infinities and NANs. A pseudo zero result will be produced if
two unnormals, containing a total of
more than 64 leading zero bits in their significands, are multiplied together. This
is a remote possibility
in most applications, but it can happen.
Table 1-9. Un normal Operands and Results
Operation
Addition/subtraction
Multiplication
Division (unnormal dividend only)
FPREM (unnormal dividend only)
Division/FPREM (unnormal
divisor)
Compare/FTST
FRNDINT
FSQRT
FST, FSTP (short/long real
destination)
FSTP (temporary real destination)
FIST, FISTP, FBSTP
FLD
FXCH
Transcendental instructions
Result
Normalization of operand with larger abosolute
value determines normalization of result.
If either operand
is unnormal, result is unnormal.
Result is unnormal.
Result if normalized.
Signal invalid operation.
Normalize
as much as possible before making
comparison.
Normalize as much
as possible before rounding.
Signal invalid operation.
If value
is above destination's underflow bound-
ary, then signal invalid operation; else signal
underiiow.
Store
as usual.
Signal invalid operation.
Load
as usual.
Exchange
as usual.
Undefined; operands must be normal and are not
checked.
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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