udf

Undefined Instruction

UDF #<imm>

Permanently undefined instruction (generates Undefined Instruction exception).

Details

Undefined Instruction (A32 encoding). This A32 instruction is permanently undefined and raises an Undefined Instruction exception when executed. The 16-bit immediate (imm12 and imm4 concatenated) is available to the exception handler but does not affect execution otherwise. No condition flags are affected.

Pseudocode Operation

UndefinedInstructionException(imm12 ∘ imm4)

Example

UDF #16

Encoding

Binary Layout

1110

01111111

imm12

1111

imm4

Format System

Opcode 0xE7F000F0

Extension A32 (Base)

Operands

imm
ID

Reference (Arm AArch32 ISA)

Instruction Forms

Encoding	Instruction	ISA	Bit pattern
0xE7F000F0	UDF{<c>}{<q>} {#}<imm>	A32	1110 \| 01111111 \| imm12 \| 1111 \| imm4
0xDE00	UDF{<c>}{<q>} {#}<imm>	T32	1101111 \| 0 \| imm8
0xF7F0A000	UDF{<c>}.W {#}<imm>	T32	11110111111 \| 1 \| imm4 \| 10 \| 1 \| 0 \| imm12

Description

Permanently Undefined generates an Undefined Instruction exception. The encodings for UDF used in this section are defined as permanently undefined. However:

Operation

if ConditionPassed() then
    EncodingSpecificOperations();
    UNDEFINED;