TMPR4956C datasheet(154/286 Pages) TOSHIBA

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Chapter 8 CPU Exception

8-26

8.21.3

Exception Trap Processing

When a floating-point exception trap is taken, the Cause register indicates the floating-point

coprocessor is the cause of the exception trap.

The Floating-Point Exception (FPE) code is used, and the Cause bits of the floating-point

Control/Status register indicate the reason for the floating-point exception. These bits are, in effect, an

extension of the system coprocessor Cause register.

8.21.4

Flags

A Flag bit is provided for each IEEE exception. This Flag bit is set to a 1 on the assertion of its

corresponding exception, with no corresponding exception trap signaled.

When no exception trap is signaled, floating-point coprocessor takes a default action, providing a

substitute value for the exception-causing result of the floating-point operation. The particular default

action taken depends upon the type of exception. Table 8.21.1 lists the default action taken by the FPU for

each of the IEEE exceptions.

Table 8.21.1 Default FPU Exception Actions

Field

Description

Rounding

Mode

Default Action

Inexact exception

ANY

Supply a rounded result.

Underflow

exception

ANY

Supply a rounded result.

Overflow

exception

Modify overflow values to

∞ with the sign of the

intermediate result.

Modify overflow values to the format’s largest finite

number with the sign of the intermediate result.

Modify negative overflows to the format’s most negative

finite number; modify positive overflows to +

∞

Modify positive overflows to the format’s largest finite

number; modify negative overflows to –

∞

Division by zero

ANY

Supply a properly signed

∞

Invalid operation

ANY

Supply a quiet Not a Number (NaN).

The FPU detects the eight exception causes internally. When the FPU encounters one of these unusual

situations, it causes either an IEEE exception or an Unimplemented Operation exception (E).

Table 8.21.2 lists the exception-causing situations and contrasts the behavior of the FPU with the

requirements of the IEEE Standard 754.

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