Program Status Register (PSR) in ARM Architecture - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to identify the main purpose of the PSR in ARM architecture.

ARM Architecture

The PSR holds the [1] of the processor.

Drag options to blanks, or click blank then click option'

Ainstruction set

Bmemory address

Cstatus flags

Dcache data

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing PSR with memory or cache registers.

Thinking PSR stores instructions.

The Program Status Register (PSR) holds the status flags that indicate the current state of the processor, such as condition flags.

2fill in blank

medium

Complete the code to name a key flag found in the PSR.

ARM Architecture

The [1] flag in the PSR indicates if the last operation resulted in zero.

Drag options to blanks, or click blank then click option'

Azero

Bnegative

Coverflow

Dcarry

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing carry or overflow flags instead of zero.

Confusing negative flag with zero flag.

The zero flag (Z) in the PSR is set when the result of an operation is zero.

3fill in blank

hard

Fix the error in the statement about PSR fields.

ARM Architecture

The PSR contains the [1] field which stores the current processor mode.

Drag options to blanks, or click blank then click option'

Amode

Baddress

Cinstruction

Dcache

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing mode with memory address or cache.

Thinking PSR stores instructions.

The PSR includes a mode field that indicates the current processor mode, such as User or Supervisor.

4fill in blank

hard

Fill both blanks to describe the PSR components.

ARM Architecture

The PSR includes the [1] flags and the [2] field to control processor state.

Drag options to blanks, or click blank then click option'

Acondition

Bmemory

Cmode

Dcache

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing memory or cache with PSR fields.

Confusing condition flags with mode field.

The PSR contains condition flags (like zero, carry) and a mode field that controls the processor's current mode.

5fill in blank

hard

Fill all three blanks to complete the PSR description.

ARM Architecture

The PSR holds [1] flags, a [2] field, and controls [3] execution state.

Drag options to blanks, or click blank then click option'

Acondition

Bmode

Cprocessor

Dmemory

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing memory instead of processor for execution state.

Confusing mode and memory fields.

The PSR holds condition flags, a mode field, and controls the processor's execution state.

Practice

1. What is the primary purpose of the Program Status Register (PSR) in ARM architecture?

easy

A. To hold flags that indicate the results of operations and control processor modes

B. To store the address of the next instruction to execute

C. To manage memory allocation for programs

D. To handle input/output operations directly

Solution

1. Step 1: Understand the role of PSR

   The PSR contains flags like zero, carry, negative, and overflow that reflect the outcome of arithmetic or logical operations.

2. Step 2: Identify PSR's control function

   It also controls processor modes such as user mode or interrupt mode, affecting how the processor behaves.

3. Final Answer:

   To hold flags that indicate the results of operations and control processor modes -> Option A

4. Quick Check:

   PSR = Flags + Mode Control [OK]

Hint: PSR holds flags and modes, not addresses or memory [OK]

Common Mistakes:

• Confusing PSR with program counter
• Thinking PSR manages memory
• Assuming PSR handles I/O directly

2. Which of the following is a correct way to refer to the condition flags in the ARM Program Status Register (PSR)?

easy

A. PC, LR, SP

B. IRQ, FIQ, SVC

C. R0, R1, R2

D. N, Z, C, V

Solution

1. Step 1: Identify condition flags in PSR

   The condition flags in PSR are Negative (N), Zero (Z), Carry (C), and Overflow (V).

2. Step 2: Eliminate unrelated registers and modes

   PC, LR, SP are registers, not flags; IRQ, FIQ, SVC are interrupt modes; R0-R2 are general-purpose registers.

3. Final Answer:

   N, Z, C, V -> Option D

4. Quick Check:

   Flags = N, Z, C, V [OK]

Hint: Remember flags as N, Z, C, V in PSR [OK]

Common Mistakes:

• Mixing registers with flags
• Confusing interrupt modes with flags
• Using general-purpose registers as flags

3. Consider the following ARM assembly snippet that updates the PSR flags after an addition:

ADD R0, R1, R2
CMP R0, #0

What will the Zero (Z) flag in the PSR be if the result in R0 is zero?

medium

A. Cleared to 0

B. Set to 0

C. Set to 1

D. Undefined

Solution

1. Step 1: Understand the CMP instruction effect

   CMP compares R0 with zero and sets the Zero flag if R0 equals zero.

2. Step 2: Analyze the Zero flag behavior

   If R0 is zero, the Zero flag (Z) is set to 1 to indicate equality.

3. Final Answer:

   Set to 1 -> Option C

4. Quick Check:

   R0 == 0 -> Z = 1 [OK]

Hint: CMP sets Z=1 when compared value equals zero [OK]

Common Mistakes:

• Assuming Z flag clears on zero result
• Confusing CMP with ADD effect on flags
• Thinking flags remain unchanged

4. An ARM program tries to update the PSR flags using the instruction:

ADD R0, R1, R2

but the flags are not updated. What is the likely cause?

medium

A. The processor is in user mode where flags can't change

B. The instruction should be ADD{S} R0, R1, R2 to update flags

C. The registers R1 and R2 are invalid

D. The PSR is read-only and cannot be changed

Solution

1. Step 1: Check instruction syntax for flag update

   In ARM, to update PSR flags after arithmetic, the instruction must include the 'S' suffix (e.g., ADDS) to set flags.

2. Step 2: Understand default behavior without 'S'

   Without 'S', the instruction performs addition but does not update the condition flags in PSR.

3. Final Answer:

   The instruction should be ADD{S} R0, R1, R2 to update flags -> Option B

4. Quick Check:

   Use ADDS to update PSR flags [OK]

Hint: Add 'S' suffix to update PSR flags after arithmetic [OK]

Common Mistakes:

• Thinking PSR is read-only
• Assuming all ADD update flags by default
• Blaming invalid registers

5. You want to write an ARM assembly routine that branches to a label error_handler only if the Negative (N) flag in the PSR is set after a subtraction. Which instruction correctly checks this condition?

hard

A. BMI error_handler

B. BNE error_handler

C. BEQ error_handler

D. BVS error_handler

Solution

1. Step 1: Understand the meaning of condition codes

   BNE branches if Zero flag is clear; BEQ if Zero flag is set; BVS if Overflow flag is set; BMI if Negative flag is set.

2. Step 2: Match Negative flag condition to branch

   Since we want to branch when Negative flag (N) is set, the correct instruction is BMI (Branch if Minus).

3. Final Answer:

   BMI error_handler -> Option A

4. Quick Check:

   Negative flag set -> use BMI [OK]

Hint: BMI branches when Negative flag is set [OK]

Common Mistakes:

• Using BNE or BEQ which check Zero flag
• Confusing Overflow flag with Negative flag
• Choosing BVS which checks Overflow