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VFP Exception Handling

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22.12 Arithmetic exceptions

This section describes the conditions under which the VFP11 coprocessor bounces an arithmetic

instruction based on the potential for the exception. It is the task of the support code to determine

the actual exception status of the instruction. The support code must return either the result and

appropriate exception status bits, or the intermediate result and a call to a user trap handler.

The following sections describe the circumstances when arithmetic exceptions occur:

• FADD and FSUB

• FCMP, FCMPZ, FCMPE, and FCMPEZ on page 22-21

• FMUL and FNMUL on page 22-22

• FMAC, FMSC, FNMAC, and FNMSC on page 22-22

• FDIV on page 22-23

• FSQRT on page 22-23

• FCPY, FABS, and FNEG on page 22-24

• FCVTDS and FCVTSD on page 22-24

• FUITO and FSITO on page 22-24

• FTOUI, FTOUIZ, FTOSI, and FTOSIZ on page 22-24.

22.12.1 FADD and FSUB

In an addition or subtraction, the exponent is initially the larger of the two input exponents. For

clarity, we define the operation as a Like-Signed Addition (LSA) or an Unlike-Signed Addition

(USA). Table 22-7 specifies how this distinction is made. In the table, + indicates a positive

operand, and – indicates a negative operand.

Because it is possible for an LSA operation to cause the exponent to be incremented if the

significand overflows, overflow bounce ranges for an LSA are more pessimistic than they are

for a USA. The LSA ranges are made slightly more pessimistic to incorporate FMAC

instructions. See FMAC, FMSC, FNMAC, and FNMSC on page 22-22.

Underflow bounce ranges for a USA are more pessimistic than they are for an LSA. This is to

accommodate a massive cancellation when the result exponent is smaller than the larger operand

exponent by as much as the length of the significand. The overflow range for a USA is slightly

Table 22-7 LSA and USA determination

Instruction Operand A sign Operand B sign Operation type

FADD++LSA

FADD + – USA

FADD – + USA

FADD––LSA

FSUB + + USA

FSUB + – LSA

FSUB – + LSA

FSUB – – USA

ARM ARM1176JZF-S User Manual

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