User's Manual

SYSTEM-LEVEL NUMERIC PROGRAMMING
8086/87/88/186 MACRO ASSEMBLER Test for presence of a NUIIOr;cs Chip. Rev;slon 1.0
PAGE
LOC OBJ
0039 EB0790
D03C
003C EB0490
003F
003F EB0190
004Z
D04Z
LINE
60
61
6Z
63
64
65
66
67
68
69
70
71
7Z
73
74
75
76
77
78
79
80
81
ASSEHBL Y COMPLETE, NO ERRORS FOUND
SOURCE
An 80387 Is present. If denormol e.cepttons are used for an 8087/Z87,
they "",t be mosked. Th. 80387 will automatically normalize denormol
operands faster than an exception handler can.
j"" founct387
set up for no NPX
j"" e.tt
found_87 _Z87:
.et up for 87/Z87
j"" exit
found 387:
- set up
for 387
,
•• tt.
code end.
end start,ds.:dgroup, •• :dgroup:8at
Figure 3-1. Software Routine to Recognize the 80287 (Cont'd.)
Configuring the Numerics Environment
Once the 80286 CPU has determined the presence or absence of the 80287 NPX, the 80286 must set
either the
MP or the EM bit in its own machine status word accordingly. The initialization routine can
either
• Set the MP bit in the 80286 MSW to allow numeric instructions to be executed directly by the
80287 NPX component
• Set the EM bit in the 80286 MSW to permit software emulation of the 80287 numeric instructions
The Math Present (MP) flag of the 80286 machine status word indicates to the CPU whether an 80287
NPX is physically avaiiabie in the system. The MP flag controls the function of the WAIT instruction.
When executing
aWAIT instruction, the 80286 tests only the Task Switched (TS) bit if MP is set; if
it finds
TS set under these conditions, the 'CPU traps to exception #7.
The Emulation Mode (EM) bit of the 80286 machine status word indicates to the CPU whether NPX
functions are to be emulated. If the CPU finds EM set when it executes an ESC instruction, program
control
is automatically trapped to exception #7, giving the exception handler the opportunity to emulate
the functions of an
80287. The 80286 EM flag can be changed only by using the LMSW (load machine
status word) instruction (legal only at privilege ievei
0) ami examiut;d with the aid of th~ SMSW (~t0re
machine status word) instruction (legal at any privilege level).
The EM bit also controls the function of the WAIT instruction.
If the CPU finds EM set while ~xecut
ing a WAIT, the CPU does not check the ERROR pin for an error indication.
For correct
80286 operation, the EM bit must never be set concurrently with MP. The EM and MP
bits of the 80286 are described in more detail in the 80286 Operating System Writer's Guide. More
3-4