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MC68060 Software Package
C-18
M68060 USER’S MANUAL
MOTOROLA
M68060FPSP SNAN and OPERR exception handlers, _fpsp_snan and _fpsp_operr, will be
provided for SNAN and OPERR enabled exceptions for the following reasons:
For opclass two pre-instruction exceptions using a single or double source format with
an infinity, denorm, NAN, or zero source operand, the processor does not create the
correct extended-precision value for the FSAVE frame. The MC68060FPSP handlers
convert the value in the FSAVE frame to extended-precision format before passing con-
trol to the user enabled SNAN or OPERR exception handlers (_real_{snan,operr}). No
parameters are passed to the user enabled SNAN or OPERR exception handlers from
the M68060FPSP package since the package provides the illusion that it never existed.
For opclass three post-instruction exceptions, the processor does not store the default
result to the destination memory or integer data register before taking the enabled ex-
ception. The MC68881/882 stored the default result in this scenario. Therefore, to main-
tain compatibility, the M68060FPSP SNAN and OPERR exception handlers calculate
and store the default result before passing control to the user enabled SNAN and OP-
ERR exception handlers (_real_{snan,operr}). No parameters are passed to the user
SNAN or OPERR exception handlers since the M68060FPSP provides the illusion that
it never existed.
A simple pseudo-code diagram for the SNAN and OPERR handlers is provided in the code
sequence shown in Figure C-8.
C.3.2.3.3 Inexact Exception. Opclass zero and two exception instructions taking the inex-
act exception cause pre-instruction exceptions, and opclass three instructions cause post-
instruction inexact exceptions. The processor takes exception vector number forty-nine for
the inexact exception. The FSAVE frame for the exception is valid and contains the source
operand converted to extended precision.
The inexact exception is a maskable exception on the MC68060 for the trap-disabled case.
The floating-point hardware produces the correct result when the inexact exception enable
_fpsp_{snan,operr}() {
if ((opclass == 0) || (opclass == 2)) {
/*
* if src operand is a sgl or dbl
* zero,NAN,denorm, or infinity,
* fix operand in FSAVE frame.
*/
fix_FSAVE_op();
bra.l _real_{snan,operr}();
}
else {/* opclass 3 */
/*
* save default result to memory
* or integer register file.
*/
save_default_result();
bra.l _real_{snan,operr}();
Figure C-8. SNAN/OPERR Exception Handler Pseudo-Code
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