Introduction to ARM

Compare Instructions

<operation>{cond} Rn,<Operand2>

<operation>

CMP – compare
Flags set to result of (Rn − Operand2).
CMN – compare negative
Flags set to result of (Rn + Operand2).
TST – bitwise test
Flags set to result of (Rn AND Operand2).
TEQ – test equivalence
Flags set to result of (Rn EOR Operand2).

Comparisons produce no results – they just set condition codes. Ordinary instructions will also set condition codes if the “S” bit is set. The “S” bit is implied for comparison instructions.

Examples

CMP r0, #42
Compare R0 to 42.
CMN r2, #42
Compare R2 to −42.
TST r11, #1
Test bit zero.
TEQ r8, r9
Test R8 equals R9.
SUBS r1, r0, #42
Compare R0 to 42, with result.

Remarks

  • CMP is like SUB.
  • CMN is like ADD (subtract of a negative number is the same as add).
  • TST is like AND.
  • TEQ is like EOR (exclusive or of identical numbers gives result of zero).

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