arithmetic_and_cpsr_flags

Arithmetic Operations and CPSR Flags in ARM Assembly

ARM assembly language provides a set of instructions for performing arithmetic operations. These operations can also affect the Current Program Status Register (CPSR) flags, which are used for conditional execution of subsequent instructions.

Basic Arithmetic Instructions

Here's a breakdown of the basic arithmetic instructions used in ARM assembly:

• MOV: This instruction moves a value into a register. For example:

  MOV R0, #5

  This line moves the literal value 5 into register R0.

• ADD: This instruction adds two register values and stores the result in a register. For example:

  ADD R2, R0, R1

  This line adds the values in R0 and R1, then stores the result in R2 (R2 = R0 + R1).

• SUB: This instruction subtracts one register value from another and stores the result. For example:

  SUB R2, R0, R1

  This line subtracts the value in R1 from R0 and stores the result in R2 (R2 = R0 - R1).

• MUL: This instruction multiplies two register values and stores the result. For example:

  MUL R2, R0, R1

  This line multiplies the values in R0 and R1 and stores the result in R2 (R2 = R0 * R1).

CPSR Flags and Suffix Instructions

The CPSR flags are affected by certain arithmetic instructions that have an 'S' suffix (e.g., ADDS, SUBS). These flags include:

• N (Negative): Set if the result of an operation is negative.
• Z (Zero): Set if the result is zero.
• C (Carry): Set if there's a carry out or borrow during an operation.
• V (Overflow): Set if there's an overflow from signed operations.

Here are some instructions that affect CPSR flags:

• SUBS: Subtracts like SUB but also updates CPSR flags. It's useful for setting up conditions for subsequent instructions.

  SUBS R2, R0, R1

  This line not only subtracts the value in R1 from R0 but also updates the CPSR flags based on the result.

• ADDS: This instruction adds two register values and updates the CPSR flags based on the result. For example:

  ADDS R2, R0, R1

  This line performs addition (R2 = R0 + R1) and sets the CPSR flags accordingly. If the result is zero, the Z flag is set. If there's a carry out, the C flag is set.

• ADC: This instruction adds two register values along with the carry flag from a previous operation. For example:

  ADC R2, R0, R1

  This line adds the values in R0 and R1 plus the carry flag from CPSR (if it's set). It's useful when performing multi-word arithmetic operations.

Conclusion

Understanding these basic arithmetic instructions and how they interact with CPSR flags is crucial for writing effective ARM assembly code. These concepts allow for conditional execution based on previous operations, which is a powerful feature of ARM architecture.

Apologies for missing those. Let's add explanations for ADDS and ADC:

Extended Arithmetic Instructions with CPSR Flags

These instructions are particularly useful for implementing complex arithmetic operations that depend on the results of previous calculations.

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