Arm® A32/T32 Instruction Set Architecture: Armv8, for Armv8-A architecture profile Documentation | MVN, MVNS (register): Bitwise NOT (register).

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MVN, MVNS (register)

Bitwise NOT (register) writes the bitwise inverse of a register value to the destination register.

If the destination register is not the PC, the MVNS variant of the instruction updates the condition flags based on the result.

The field descriptions for <Rd> identify the encodings where the PC is permitted as the destination register. ARM deprecates any use of these encodings. However, when the destination register is the PC:

The MVN variant of the instruction is an interworking branch, see Pseudocode description of operations on the AArch32 general-purpose registers and the PC.
The MVNS variant of the instruction performs an exception return without the use of the stack. In this case:
- The PE branches to the address written to the PC, and restores PSTATE from SPSR_<current_mode>.
- The PE checks SPSR_<current_mode> for an illegal return event. See Illegal return events from AArch32 state.
- The instruction is undefined in Hyp mode.
- The instruction is constrained unpredictable in User mode and System mode.

It has encodings from the following instruction sets: A32 ( A1 ) and T32 ( T1 and T2 ) .

A1

31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
!= 1111				0	0	0	1	1	1	1	S	(0)	(0)	(0)	(0)	Rd				imm5					stype		0	Rm
cond

MVN, rotate right with extend (S == 0 && imm5 == 00000 && stype == 11)

MVN{<c>}{<q>} <Rd>, <Rm>, RRX

MVN, shift or rotate by value (S == 0 && !(imm5 == 00000 && stype == 11))

MVN{<c>}{<q>} <Rd>, <Rm> {, <shift> #<amount>}

MVNS, rotate right with extend (S == 1 && imm5 == 00000 && stype == 11)

MVNS{<c>}{<q>} <Rd>, <Rm>, RRX

MVNS, shift or rotate by value (S == 1 && !(imm5 == 00000 && stype == 11))

MVNS{<c>}{<q>} <Rd>, <Rm> {, <shift> #<amount>}

d = UInt(Rd);  m = UInt(Rm);  setflags = (S == '1');
(shift_t, shift_n) = DecodeImmShift(stype, imm5);

T1

15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
0	1	0	0	0	0	1	1	1	1	Rm			Rd

T1

MVN<c>{<q>} <Rd>, <Rm> // (Inside IT block)

MVNS{<q>} <Rd>, <Rm> // (Outside IT block)

d = UInt(Rd);  m = UInt(Rm);  setflags = !InITBlock();
(shift_t, shift_n) = (SRType_LSL, 0);

T2

15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
1	1	1	0	1	0	1	0	0	1	1	S	1	1	1	1	(0)	imm3			Rd				imm2		stype		Rm

MVN, rotate right with extend (S == 0 && imm3 == 000 && imm2 == 00 && stype == 11)

MVN{<c>}{<q>} <Rd>, <Rm>, RRX

MVN, shift or rotate by value (S == 0 && !(imm3 == 000 && imm2 == 00 && stype == 11))

MVN<c>.W <Rd>, <Rm> // (Inside IT block, and <Rd>, <Rm> can be represented in T1)

MVN{<c>}{<q>} <Rd>, <Rm> {, <shift> #<amount>}

MVNS, rotate right with extend (S == 1 && imm3 == 000 && imm2 == 00 && stype == 11)

MVNS{<c>}{<q>} <Rd>, <Rm>, RRX

MVNS, shift or rotate by value (S == 1 && !(imm3 == 000 && imm2 == 00 && stype == 11))

MVNS.W <Rd>, <Rm> // (Outside IT block, and <Rd>, <Rm> can be represented in T1)

MVNS{<c>}{<q>} <Rd>, <Rm> {, <shift> #<amount>}

d = UInt(Rd);  m = UInt(Rm);  setflags = (S == '1');
(shift_t, shift_n) = DecodeImmShift(stype, imm3:imm2);
if d == 15 || m == 15 then UNPREDICTABLE; // Armv8-A removes UNPREDICTABLE for R13

For more information about the constrained unpredictable behavior of this instruction, see Architectural Constraints on UNPREDICTABLE behaviors.

Assembler Symbols

<c>	See Standard assembler syntax fields.

<q>	See Standard assembler syntax fields.

<Rd>

For encoding A1: is the general-purpose destination register, encoded in the "Rd" field. Arm deprecates using the PC as the destination register, but if the PC is used:

For the MVN variant, the instruction is a branch to the address calculated by the operation. This is an interworking branch, see Pseudocode description of operations on the AArch32 general-purpose registers and the PC.
For the MVNS variant, the instruction performs an exception return, that restores PSTATE from SPSR_<current_mode>.

For encoding T1 and T2: is the general-purpose destination register, encoded in the "Rd" field.

<Rm>	For encoding A1: is the general-purpose source register, encoded in the "Rm" field. The PC can be used, but this is deprecated.
	For encoding T1 and T2: is the general-purpose source register, encoded in the "Rm" field.

<shift>

Is the type of shift to be applied to the source register, encoded in stype:

stype	<shift>
00	LSL
01	LSR
10	ASR
11	ROR

<amount>	For encoding A1: is the shift amount, in the range 1 to 31 (when <shift> = LSL or ROR) or 1 to 32 (when <shift> = LSR or ASR), encoded in the "imm5" field as <amount> modulo 32.
	For encoding T2: is the shift amount, in the range 1 to 31 (when <shift> = LSL or ROR) or 1 to 32 (when <shift> = LSR or ASR), encoded in the "imm3:imm2" field as <amount> modulo 32.

Operation

if ConditionPassed() then
    EncodingSpecificOperations();
    (shifted, carry) = Shift_C(R[m], shift_t, shift_n, PSTATE.C);
    result = NOT(shifted);
    if d == 15 then          // Can only occur for A32 encoding
        if setflags then
            ALUExceptionReturn(result);
        else
            ALUWritePC(result);
    else
        R[d] = result;
        if setflags then
            PSTATE.N = result<31>;
            PSTATE.Z = IsZeroBit(result);
            PSTATE.C = carry;
            // PSTATE.V unchanged

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