UDF -- AArch32

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UDF

The encodings for UDF used in this section are defined as permanently UNDEFINED. However:

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

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

Decode for this encoding

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

Decode for this encoding

let imm32 : bits(32) = ZeroExtend{}(imm8); // 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
		S

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
											o1							o2

Encoding

UDF{<c>}{<q>} {#}<imm>

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

Decode for this encoding

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

Assembler Symbols

<c>	For the "A1" variant: see Standard assembler syntax fields. <c> must be AL or omitted.
	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.
	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.
	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.

Operation

if ConditionPassed() then EncodingSpecificOperations(); Undefined(); end;

2025-09_rel_asl1 2026-03-12 12:57:38