UDF Permanently Undefined Permanently Undefined generates an Undefined Instruction exception. The encodings for UDF used in this section are defined as permanently UNDEFINED. 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 ) . 1 1 1 0 0 1 1 1 1 1 1 1 1 1 1 1 UDF{<c>}{<q>} {#}<imm> let imm32 : bits(32) = ZeroExtend{}(imm12::imm4); // imm32 is for assembly and disassembly only, and is ignored by hardware. 1 1 0 1 1 1 1 0 UDF{<c>}{<q>} {#}<imm> let imm32 : bits(32) = ZeroExtend{}(imm8); // imm32 is for assembly and disassembly only, and is ignored by hardware. 1 1 1 1 0 1 1 1 1 1 1 1 1 0 1 0 UDF{<c>}{<q>} {#}<imm> UDF{<c>}.W {#}<imm> let imm32 : bits(32) = ZeroExtend{}(imm4::imm12); // imm32 is for assembly and disassembly only, and is ignored by hardware. <c> For the "A1" variant: see Standard assembler syntax fields. <c> must be AL or omitted. <c> For the "T1" and "T2" variants: see Standard assembler syntax fields. Arm deprecates using any <c> value other than AL. <q> See Standard assembler syntax fields. <imm> For the "A1" variant: 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. <imm> For the "T1" variant: 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. <imm> For the "T2" variant: 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(); end; 2026-03-26 20:27:25 2026-03_rel