User's Manual

PROGRAMMING NUMERIC APPLICATIONS
The ASM286 version (figure 2-7) defines the external procedure INIT287, which makes the different
initialization requirements of the processor and its emulator transparent to the source code. After defining
the data and setting up the segment registers and stack pointer, the program calls INIT287 and loads
the control word. The computation begins with the next three instructions, which clear three registers
by loading (pushing) zeros onto the stack.
As shown in figure 2-8, these registers remain at the bottom
of the stack throughout the computation while temporary values are pushed
on and popped off the
stack above them.
'
The program uses the CPU LOOP instruction to control its iteration through )CARRA Y; register CX,
which
LOOP automatically decrements, is loaded with N_OF.J(, the number of array elements to be
summed. Register
SI is used to select (index) the array elements. The program steps through )CARRA Y
from back to front,
so SI is initialized to point at the element just beyond the first element to be
processed.
The ASM286 TYPE operator is used to determine the number of bytes in each array element.
This permits changing
)CARRA Y to a long real array by simply changing its definition (DD to DQ)
and reassembling.
Figure
2-8 shows the effect of the instructions in the program loop. on the NPX register stack. The
figure assumes
that the program is in its first iteration, that N_OF.J( is 20, and that }CARRA Y(19)
(the
20th element) contains the value 2.5. When the loop terminates, the three sums are left as the top
stack elements so that the program ends by simply popping them into memory variables.
80287 Emulation
I
The programming of applications to execute on both 80286 and 802~7 is made much easier by the
existence of an
80287 emulator for 80286 systems. The Intel E80287 emulator offers a complete software
counterpart to the
80287 hardware; NPX instructions can be simply emulated in softwar~ rather than
being executed in hardware. With software emulation, the distinction between
80286 arid 80287 systems
is reduced to a simple performance differential (see
Table 1-2 for a performance comparison between
an actual
80287 and an emulator 80287). Identical numeric programs will simply execute more slowly
on 80286 systems (using software emulation of NPX instructions) than on executing NPX instructions
directly. '
When incorporated into the systems software, the emulation of
NPX instructions on the 80286 systems
is completely transparent to the programmer. Applications software needs no special libraries, linking,
or other activity to allow
it to run on an 80286 with 80287 emulation.
To the applications programmer, the development of programs for
80286 systems is the same ~hether
the 80287 NPX hardware is available or not. The full 80287 instruction set is available for use, with
NPX instructions being either emulated or executed directly. Applications programmers need not be
concerned with the hardware configuration of the computer systems
on which their applications will
eventually run.
For systems programmers, details relating to 80287 emulators are described in a later section of this
supplement. An
E80287 software emulator for 80286 systems is contained in the iMDX 364 8086
Software Toolbox, available from Intel and described in the 8086 Software Toolbox Manual.
CONCURRENT PROCESSING WITH THE 80287
Because the 80286 CPU and the 80287 NPX have separate execution units, it is possible for the NPX
to execute numeric instructions in parallel with instructions executed by the CPU. This simultaneous
execution of different instructions
is called concurrency.
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