Assembly Language Programming(3) CPSR FLAGS

✅ Code Explanation (ARMv7 Assembly)

.global _start
_start:
    MOV R0, #6      ; Store 6 in R0
    MOV R1, #7      ; Store 7 in R1
    SUBS R2, R0, R1 ; R2 = R0 - R1, updates CPSR flags

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✅ Meaning of SUBS R2, R0, R1

• SUB (Subtract operation): R2 = R0 - R1

• S (Set flags): Updates CPSR (Current Program Status Register) based on the result.

• Unlike SUB, which only performs subtraction, SUBS updates the CPSR flags accordingly.

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✅ Relationship with CPSR (Current Program Status Register)

The CPSR stores status flags that reflect the result of the previous operation.

Major Flags in CPSR

✅ Execution of SUBS

MOV R0, #6      ; R0 = 6
MOV R1, #7      ; R1 = 7
SUBS R2, R0, R1 ; R2 = R0 - R1 = 6 - 7 = -1

• R2 = 6 - 7 = -1

• Since the result is -1, the N (Negative) flag is set to 1.

• The result is not zero, so Z (Zero) flag remains 0.

• Borrow occurs (since 6 is smaller than 7), so C (Carry) flag is set to 0.

• No overflow occurs, so V (Overflow) flag remains 0.

✅ CPSR Flag Status After Execution

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✅ Summary

• SUBS R2, R0, R1 performs R2 = R0 - R1 and modifies the CPSR flags.

• If the result is negative, the N flag is set to 1.

• If the result is zero, the Z flag is set to 1; otherwise, it remains 0.

• The C flag is set based on whether a borrow occurs in the subtraction.

• The V flag is set if an overflow occurs.

Since SUBS updates CPSR flags, it can influence conditional execution using instructions like BEQ, BNE, BMI, etc. 🚀