RealView® Developer Kit Assembler Guide 5.4. VFP and condition codes

5.4. VFP and condition codes

In ARM state, 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.

In Thumb^® state on a Thumb-2 processor, you can use an IT instruction to set condition codes on up to four following VFP instructions.

Note

None of the processors supported by this toolkit supports Thumb-2.

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).

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).

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 required to account for unordered results.

The meanings of the condition code mnemonics are shown in Table 5.2.

Table 5.2. 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
`HI`	Unsigned higher	Greater than, or unordered
`LS`	Unsigned lower or same	Less than or equal
`GE`	Signed greater than or equal	Greater than or equal
`LT`	Signed less than	Less than, or unordered
`GT`	Signed greater than	Greater than
`LE`	Signed less than or equal	Less than or equal, or unordered
`AL`	Always (normally omitted)	Always (normally omitted)

Note

The type of the instruction that last updated the flags in the CPSR determines the meaning of condition codes.

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