Unsigned Subtract 8 performs four 8-bit unsigned integer subtractions, and writes the results to the destination register. It sets PSTATE.GE according to the results of the subtractions.
It has encodings from the following instruction sets: A32 ( A1 ) and T32 ( T1 ) .
| 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 | 1 | 1 | 0 | 0 | 1 | 0 | 1 | Rn | Rd | (1) | (1) | (1) | (1) | 1 | 1 | 1 | 1 | Rm | ||||||||||||
| cond | |||||||||||||||||||||||||||||||
constant d = UInt(Rd); constant n = UInt(Rn); constant m = UInt(Rm); if d == 15 || n == 15 || m == 15 then UNPREDICTABLE;
| 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 | 1 | 0 | 1 | 0 | 1 | 1 | 0 | 0 | Rn | 1 | 1 | 1 | 1 | Rd | 0 | 1 | 0 | 0 | Rm | |||||||||
constant d = UInt(Rd); constant n = UInt(Rn); constant m = UInt(Rm); // Armv8-A removes UNPREDICTABLE for R13 if d == 15 || n == 15 || m == 15 then UNPREDICTABLE;
For more information about the constrained unpredictable behavior of this instruction, see Architectural Constraints on UNPREDICTABLE behaviors.
| <c> |
| <q> |
| <Rd> |
Is the general-purpose destination register, encoded in the "Rd" field. |
| <Rn> |
Is the first general-purpose source register, encoded in the "Rn" field. |
| <Rm> |
Is the second general-purpose source register, encoded in the "Rm" field. |
if ConditionPassed() then EncodingSpecificOperations(); constant diff1 = UInt(R[n]<7:0>) - UInt(R[m]<7:0>); constant diff2 = UInt(R[n]<15:8>) - UInt(R[m]<15:8>); constant diff3 = UInt(R[n]<23:16>) - UInt(R[m]<23:16>); constant diff4 = UInt(R[n]<31:24>) - UInt(R[m]<31:24>); R[d]<7:0> = diff1<7:0>; R[d]<15:8> = diff2<7:0>; R[d]<23:16> = diff3<7:0>; R[d]<31:24> = diff4<7:0>; PSTATE.GE<0> = if diff1 >= 0 then '1' else '0'; PSTATE.GE<1> = if diff2 >= 0 then '1' else '0'; PSTATE.GE<2> = if diff3 >= 0 then '1' else '0'; PSTATE.GE<3> = if diff4 >= 0 then '1' else '0';
If CPSR.DIT is 1, this instruction has passed its condition execution check, and does not use R15 as either its source or destination:
Internal version only: isa v01_31, pseudocode v2024-03_rel ; Build timestamp: 2024-03-25T10:05
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