RealView Compilation Tools Assembler Guide: 5.4. Condition codes

5.4. Condition codes

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

In ARM state, except for the instructions that are common to both VFP and NEON, you cannot use a condition code to control the execution of NEON instructions.

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

The only VFP instruction that can be used to update the status flags is VCMP. It does not update the flags in the APSR 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 APSR using a VMRS instruction (see VMRS and VMSR).

Following an VCMP 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 required
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 5.4.

Table 5.4. Condition codes

Mnemonic	Meaning after ARM data processing instruction	Meaning after VFP VCMP instruction
`EQ`	Equal	Equal
`NE`	Not equal	Not equal, or unordered
`CS or HS`	Carry set or Unsigned higher or same	Greater than or equal, or unordered
`CC or LO`	Carry clear or 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 APSR determines the meaning of condition codes.

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