User's Manual

PROGRAMMING NUMERIC APPLICATIONS
Case 1:
M 0 V I ,
F I L D I
MOV AX,I
FISTP I
Case 2:
F IL D
FWAIT
M 0 V I., 5
F
1ST P
FWAIT
MOV AX,I
Figure 2-9. Synchronizing References to Shared Data
The data-synchronization function of any FW AIT or numeric instruction must be well-documented, as
shown in figure 2-10. Otherwise, a change to the program at a later time may remove the synchronizing
numeric instruction and cause program failure.
High-level languages automatically establish data synchronization and manage it, but there may be
applications where a high-level language may not be appropriate.
For assembly-language programmers,
automatic data synchronization can be obtained using the assem-
bler, although concurrency of execution is lost as a result. To perform automatic data synchronization,
the assembler can be changed to always place a WAIT instruction after the
ESCAPE instruction.
Figure
2-11 shows an example of how to change the ASM286 Code Macro for the FIST instruction to
automatically place aWAIT instruction after the ESCAPE instruction. This Code Macro is included
in the ASM286 source module. The price paid for this automatic data synchronization is the lack of
any possible concurrency between the
CPU and NPX.
Error Synchronization
Almost any numeric instruction can, under the wrong circumstances, produce a numeric error. Concur-
rent execution of the CPU and NPX requires synchronization for these errors just as it does for data
references and numeric instructions. In fact, the synchronization required for data and instructions
automatically provides error synchronization.
However, incorrect data or instruction synchronization may not be discovered until a numeric error
occurs. A further complication
is that a programmer may not expect his numeric program to cause
numeric errors, but in some systems, they may regularly happen.
To better understand these points,
let's look at what can happen when the
NPX detects an error.
T!!e NPX t:'~_!! l'"rfnrm one of two things when a numeric exception occurs:
• The NPX can provide a default fix-up for selected numeric errors. Programs can mask individual
error types to indicate that the
NPX should generate a safe, reasonable result whenever that error
occurs. The default error fix-up activity
is treated by the NPX as part of the instruction causing
the error;
no external indication of the error is given. When errors are detected, a flag is set in the
numeric status register, but
no information regarding where or when is available. If the NPX performs
its default action for all errors, then error synchronization
is never exercised. This is no reason to
ignore error synchronization, however.
2-50