ARM > Introduction to ARM > Compare Instructions

by David Thomas on —

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 of Compare Instructions

  • 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.