ARM VERSION 1.2 manual VFP and condition codes

Vector Floating-point Programming

6.4VFP and condition codes

You can use a condition code to control the execution of any VFP instruction. The instruction is executed conditionally, according to the status flags in the CPSR, in exactly the same way as almost all other ARM instructions.

The only VFP instruction that can be used to update the status flags is FCMP. It does not update the flags in the CPSR directly, but updates a separate set of flags in the FPSCR (see FPSCR, the floating-point status and control register on page 6-10).

Note

To use these flags to control conditional instructions, including conditional VFP instructions, you must first copy them into the CPSR using an FMSTAT instruction (see FMRX, FMXR, and FMSTAT on page 6-33).

Following an FCMP instruction, the precise meanings of the flags are different from their meanings following an ARM data-processing instruction. This is because:

•floating-point values are never unsigned, so the unsigned conditions are not needed

•Not-a-Number(NaN) values have no ordering relationship with numbers or with each other, so additional conditions are needed to allow for unordered results.

The meanings of the condition code mnemonics are shown in Table 6-2.

		Table 6-2 Condition codes
Mnemonic	Meaning after ARM data processing instruction	Meaning after VFP FCMP instruction
EQ	Equal	Equal
NE	Not equal	Not equal, or unordered
CS / HS	Carry set / Unsigned higher or same	Greater than or equal, or unordered
CC / LO	Carry clear / Unsigned lower	Less than
MI	Negative	Less than
PL	Positive or zero	Greater than or equal, or unordered
VS	Overflow	Unordered (at least one NaN operand)
VC	No overflow	Not unordered

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