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PROGRAMMING NUMERIC APPLICATIONS
and SET$REAL$MODE procedures as
well
as arithmetic expressions. SAVE$REAL$STATUS saves
the
80287
state
(registers,
status,
and control words, etc.) on
the
CPU's
stack.
RESTORE$REAL$STATUS reloads the state information; the preempting task must invoke this
procedure before terminating
in
order
to
restore the 80287
to
its state
at
the time the running task
was
preempted. This enables the preempted task
to
resume execution from the point of its preemption.
ASM286
The ASM286 assembly language provides programmmers with complete access to all of the facilities
of the
80286 and 80287 processors.
The programmer's
view
of the 80286/80287 hardware
is
a single machine with these resources:
160
instructions
12
data types
• 8 general registers
• 4 segment registers
• 8 floating-point registers, organized
as
a stack
DEFINING DATA
The ASM286 directives shown
in
table
2-16
allocate storage
for
80287 variables and constants.
As
with other storage allocation directives, the assembler associates a type with any variable defined with
these directives. The type value
is
equal to the length of the storage unit
in
bytes (10 for DT, 8 for
DQ, etc.). The assembler checks the type of any variable coded in
an
instruction
to
be certain that it
is
compatible with the instruction. For example, the coding FIADD ALPHA
will
be
flagged as an
error if
ALPHA's type
is
not 2 or
4,
because.integer addition
is
only
available for word and short
integer data types. The operand's type also tells the assembler which machine instruction
to
produce;
although
to
the programmer there
is
only an FIADD instruction, a different machine instruction
is
required for each operand type.
On occasion it
is
desirable
to
use
an
instruction with
an
operand that
has
no
declared type. For example,
if register
BX
points to a short integer variable, a programmer may want
to
code FIADD [BX]. This
can
be
done
by
informing the assembler of the operand's type
in
the instruction, coding FIADD DWORD
PTR [BX]. The corresponding overrides for the other storage allocations are WORD PTR, QWORD
PTR, and TBYTE PTR.
Directive
Interpretation
Data Types
DW
Define Word Word integer
DD
Define Doubleword
Short integer, short real
DQ Define Quadword
Long integer, long real
DT Define Tenbyte
Packed decimal, temporary real
2-40