E-8
GUIDELINES FOR WRITING FPU EXCEPTIONS HANDLERS
exception, which is not an explicitly documented behavior of a no-wait instruction. This process
is illustrated in Figure E-3.
Figure E-3 assumes that a floating-point instruction that generates a deferred error (as defined
in the Section E.2.1.1., Basic Rules: When FERR# Is Generated), which asserts the FERR#
pin only on encountering the next floating-point instruction, causes an unmasked numeric ex-
ception. Assume that the next floating-point instruction following this instruction is one of the
no-wait floating-point instructions. The FERR# pin is asserted by the processor to indicate the
pending exception on encountering the no-wait floating-point instruction. After the assertion of
the FERR# pin the no-wait floating-point instruction opens a window where the pending exter-
nal interrupts are sampled.
Then there are two cases possible depending on the timing of the receipt of the interrupt via the
INTR pin (asserted by the system in response to the FERR# pin) by the processor.
Case 1If the system responds to the assertion of FERR# pin by the no-wait floating-point
instruction via the INTR pin during this window then the interrupt is serviced first,
before resuming the execution of the no-wait floating-point instruction.
Case 2If the system responds via the INTR pin after the window has closed then the inter-
rupt is recognized only at the next instruction boundary.
There are two other ways, in addition to Case 1 above, in which a no-wait floating-point instruc-
tion can service a numeric exception inside its interrupt window. First, the first floating-point
error condition could be of the immediate category (as defined in Section E.2.1.1., Basic
Rules: When FERR# Is Generated) that asserts FERR# immediately. If the system delay before
Figure E-3. Timing of Receipt of External Interrupt
Assertion of FERR#
Exception Generating
Floating-Point
Instruction
by the Processor
System
Assertion of INTR Pin
by the System
Case 1
Case 2
Start of the No-Wait
Floating-Point
Instruction
External Interrupt
Sampling Window
Window Closed
Dependent
Delay
