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APPLICATION ARCHITECTURE
transfered to
any
point
in
a segment. (The
selector
part
of
the
far
pointer
replaces
the
value in the CS register while the offset part
replaces the value in
EIP)
A full set
of
condi-
tional Jump instructions, which branch based
on the value
of
a status flag,
is
also available;
these instructions can also transfer to locations
up to 2
32
bytes away.
Procedures
and
functions (subroutines)
can
be invoked with the Call instruction
and
a
called routine
can
return
with the
Return
instruction. As with
Jumps,
Calls within a
code
segment have
near
pointer
operands
which specify a new value for the Instruction
Pointer, while Calls to a different code segment
have far
pointer
operands
that
change
the
CS
register in
addition
to
EIP. Call instructions
push
the
address
of
the
following instruction
onto
the
stack
and
then
load
the
Instruction
Pointer
(and
the
CS register, if
the
transfer
is
to a different segment). The Return instruction
pops
the saved value(s) from the
stack
into
Erp,
and
CS, if applicable. Calls
can
be
indefinitely nested
and
recursive, subject only
to the size
of
the
stack.
For
controlling loops, the 80386 provides the
Loop
and
conditional
Loop instructions in
addition
to
conditional
Jumps.
The
loop
instructions use
the
ECX
register as a repeti-
tion counter; they decrement
ECX
and
termi-
nate the loop when the register's value becomes
zero. The conditional
Loop
instructions termi-
nate a loop prematurely when a flag takes a
specified value. While
the
Loop instructions
are designed
for
"bottom
of
loop"
testing,
adding
a
Jump
If
ECX
Zero
instruction
I RELATIVE
ADDRESSES
...
"'f-----
INCREASING SIGNIFICANCE
79
71
3 0
(G::::J
I I I I I
PACKED
DECIMAL
Q
...
'S-IG-N--------MAGNITUDE-----------'
31
23 0
...
11--.
............
________ 1 SHORT REAL
l'
•
II
MAGNITUDE----....J
BIASED EXPONENT
SIGN
63
51
0
LJIIL-_-LI
____________
---JILONG
REAL
~
II
MAGNITUDE
t BIASED EXPONENT
SIGN
79 62 0
..
I
&..1
___
--11
...
11
_________________
---11
TEMPORARY REAL
tL.1
_--,-_---ll
L.I
-------
MAGNITUDE
l t
____
BIASED EXPONENT
L-.-----SIGN
Figure 2-8. Numeric Coprocessor Data Type Storage Examples
2-9