Arm® A32/T32 Instruction Set Architecture: Armv8, for Armv8-A architecture profile Documentation | UDF: Permanently Undefined.

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UDF

Permanently Undefined generates an Undefined Instruction exception.

The encodings for UDF used in this section are defined as permanently undefined in the Armv8-A architecture. However:

With the T32 instruction set, Arm deprecates using the UDF instruction in an IT block.
In the A32 instruction set, UDF is not conditional.

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

imm32 = ZeroExtend(imm12:imm4, 32);
// imm32 is for assembly and disassembly only, and is ignored by hardware.

15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
1	1	0	1	1	1	1	0	imm8

imm32 = ZeroExtend(imm8, 32);
// imm32 is for assembly and disassembly only, and is ignored by hardware.

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	1	0	1	1	1	1	1	1	1	imm4				1	0	1	0	imm12

UDF{<c>}.W {#}<imm> // (<imm> can be represented in T1)

imm32 = ZeroExtend(imm4:imm12, 32);
// imm32 is for assembly and disassembly only, and is ignored by hardware.

<c>	For encoding A1: see Standard assembler syntax fields. <c> must be AL or omitted.
	For encoding T1 and T2: see Standard assembler syntax fields. Arm deprecates using any <c> value other than AL.

<q>	See Standard assembler syntax fields.

<imm>	For encoding A1: is a 16-bit unsigned immediate, in the range 0 to 65535, encoded in the "imm12:imm4" field. The PE ignores the value of this constant.
	For encoding T1: is a 8-bit unsigned immediate, in the range 0 to 255, encoded in the "imm8" field. The PE ignores the value of this constant.
	For encoding T2: is a 16-bit unsigned immediate, in the range 0 to 65535, encoded in the "imm4:imm12" field. The PE ignores the value of this constant.

if ConditionPassed() then
    EncodingSpecificOperations();
    UNDEFINED;

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