Find out the value of the NZCV flags of the following instructions. Assume each instruction runs individually, i.e., these instructions are not part of a program. ADD r3, r0, r2; S... Find out the value of the NZCV flags of the following instructions. Assume each instruction runs individually, i.e., these instructions are not part of a program. ADD r3, r0, r2; SUBS r3, r0, r0; ADDS r3, r0, r2; LSL r3, r0, #1; LSRS r3, r1, #1.
Understand the Problem
The question is asking to evaluate the NZCV flags (Negative, Zero, Carry, Overflow) of various assembly language instructions based on given register values. The instructions provided are arithmetic and logical operations, and the flags need to be determined based on the results of these operations.
Answer
1. ADD r3, r0, r2: N=0, Z=0, C=0, V=0 2. SUBS r3, r0, r0: N=0, Z=1, C=1, V=0 3. ADDS r3, r0, r2: N=0, Z=0, C=0, V=0 4. LSL r3, r0, #1: N=0, Z=0, C=1, V=0 5. LSRS r3, r1, #1: N=0, Z=1, C=0, V=0
Answer for screen readers
Results for Each Instruction:
- ADD r3, r0, r2: N = 0, Z = 0, C = 0, V = 0
- SUBS r3, r0, r0: N = 0, Z = 1, C = 1, V = 0
- ADDS r3, r0, r2: N = 0, Z = 0, C = 0, V = 0
- LSL r3, r0, #1: N = 0, Z = 0, C = 1, V = 0
- LSRS r3, r1, #1: N = 0, Z = 1, C = 0, V = 0
Steps to Solve
-
Identify Register Values
Given:
- $r0 = 0xFFFF,FFFF$ (Hexadecimal)
- $r1 = 0x0000,0001$
- $r2 = 0x0000,0000$
-
ADD r3, r0, r2
Calculate the result of the operation:
$$ r3 = r0 + r2 = 0xFFFF,FFFF + 0x0000,0000 = 0xFFFF,FFFF $$
Determine the flags:
- N (Negative): Since the result is positive (not the highest bit set), $N = 0$
- Z (Zero): The result is not zero, $Z = 0$
- C (Carry): No carry occurred, $C = 0$
- V (Overflow): No overflow occured, $V = 0$
-
SUBS r3, r0, r0
Calculate the result:
$$ r3 = r0 - r0 = 0xFFFF,FFFF - 0xFFFF,FFFF = 0x0000,0000 $$
Determine the flags:
- N (Negative): Result is zero, $N = 0$
- Z (Zero): The result is zero, $Z = 1$
- C (Carry): The subtraction did not produce a borrow, $C = 1$
- V (Overflow): No overflow, $V = 0$
-
ADDS r3, r0, r2
Calculate the result:
$$ r3 = r0 + r2 = 0xFFFF,FFFF + 0x0000,0000 = 0xFFFF,FFFF $$
Determine the flags (same as the first ADD):
- N = 0, Z = 0, C = 0, V = 0
-
LSL r3, r0, #1
Left shift $ r0 $ by 1:
$$ r3 = r0 << 1 = 0xFFFF,FFFF << 1 = 0xFFFF,FFFE $$
Determine the flags:
- N (Negative): Still positive, $N = 0$
- Z (Zero): Not zero, $Z = 0$
- C (Carry): The highest bit shifted out, $C = 1$
- V (Overflow): No overflow, $V = 0$
-
LSRS r3, r1, #1
Right shift $ r1 $ by 1:
$$ r3 = r1 >> 1 = 0x0000,0001 >> 1 = 0x0000,0000 $$
Determine the flags:
- N = 0 (result is zero)
- Z = 1 (result is zero)
- C = 0 (no carry)
- V = 0 (no overflow)
Results for Each Instruction:
- ADD r3, r0, r2: N = 0, Z = 0, C = 0, V = 0
- SUBS r3, r0, r0: N = 0, Z = 1, C = 1, V = 0
- ADDS r3, r0, r2: N = 0, Z = 0, C = 0, V = 0
- LSL r3, r0, #1: N = 0, Z = 0, C = 1, V = 0
- LSRS r3, r1, #1: N = 0, Z = 1, C = 0, V = 0
More Information
The NZCV flags indicate the result of arithmetic and logical operations in assembly language. They are crucial for managing control flow and decision-making in programs.
Tips
- Misinterpreting the flags: Ensure each flag corresponds correctly to the result.
- Not accounting for the operation type: For example, SUBS affects the flags differently due to the subtraction operation.
- Confusing signed and unsigned operations when evaluating flags.
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