Filter
Top Products
THE
80286
INSTRUCTION SET
xb: a simple byte memory variable without a base or index register. MOY instructions between AL
and memory have this optimized form if
no
indexing
is
required.
..
xw: a simple word memory variable without a base or index register. MOY instructions between AX
and memory have this optimized form if
no
indexing
is
required.
Clocks
This column gives the number of clock cycles
that
this form of the instruction .takes to execute. The
amount of time for each clock cycle
is
computed
by
dividing one microsecond by the number of
MHz
at
which the 80286
is
running. For example, a lO-MHz 80286 (with the
eLK
pin connected to a
20-MHz crystal) takes
100 nanoseconds for each clock cycle.
Add one clock to instructioris that use the base plus index plus displacement form of addressing. Add
two clocks for each 16-bit memory based operand reference located
on
an odd physical address. Add
one clock for each wait state added to each memory read. Wait states inserted in memory writes or
instruction fetches do not necessarily increase execution time.
The clock counts establish the maximum execution rate of the 80286. With
no
delays in bus cycles,
the actual clock count
of
an 80286 program will average
5-10%
more than the calculated cl()ck count
due to instruction sequences that execute faster than they can be fetched from memory.
Some instruction forms give two clock counts, one unlabelled and one labelled. These counts indicate
that
the instruction has two different clock times for two different circumstances. Following are the
circumstances for each possible label:
mem: The instruction has an operand that can either
be
a register or a memory variable. The unlabelled
time
is
for the register; the mem time
is
for the memory variable. Also, one additional clock cycle
is
taken for indexed memory variables for which all three possible indices (base register, index register,
and displacement) must be added.
noj: The instruction involves a conditional jump or interrupt. The unlabelled time holds when the
jump
is
made; the
noj
time holds when the jump
is
not made.
pm:
If
the instruction takes more time to execute when the 80286
is
in Protected Mode. The unlabelled
time
is
for Real Address Mode; the pm time
is
for Protected Mode.
Description
This
is
a concise description of
the.
operation performeulur ili;5 fviill
Gf
the
:~~tr!!ct!0!!.
MOT"
rletails
are given in the
"Operation" section that appears later in this chapter.
Flags Modified
This
is
a list of the flags that are set to a meaningful value by the instruction.
If
a flag
is
always set to
the same value by the instruction, the value
is
given
("=0"
or
"=
I")
after the flag name. .
8-6