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README.md

Logical Operations

This code snippet is written in ARM assembly language and demonstrates the use of basic data processing instructions such as MOV, AND, ANDS, ORR, EOR, and MVN.

Code Breakdown

  • MOV R0,#0xFF: This instruction moves the hexadecimal value 0xFF into register R0.
  • MOV R1,#22: Moves the decimal value 22 into register R1.
  • AND R2,R0,R1: Performs a bitwise AND operation between the values in R0 and R1, storing the result in R2.
  • ANDS R2,R0,R1: Similar to AND, but this instruction also updates the condition flags based on the result.
  • ORR R2,R0,R1: Performs a bitwise OR operation between R0 and R1, storing the result in R2.
  • EOR R2,R0,R1: Performs a bitwise Exclusive OR operation between R0 and R1, storing the result in R2.

Using ANDS instead of AND

The ANDS instruction is used instead of AND when we want to update the condition flags based on the result of the operation. Condition flags can affect subsequent instructions depending on their outcomes.

Negation with MVN

  • MVN R0,R0: The MVN (Move Not) instruction performs bitwise negation (NOT) on the value in R0 and stores the result back into R0.

Purpose of the Code Block

The last three instructions:

MOV R0,#0xFF
MVN R0,R0
AND R0,R0,#0x000000FF

These lines set register R0 to 0xFF, then negate it (which would result in 0xFFFFFF00 on a 32-bit system), and finally perform an AND operation with 0x000000FF. The purpose here is to demonstrate negation and then isolate the lower 8 bits of the register by clearing out all other bits.

Code Breakdown with Outputs

  • MOV R0,#0xFF: Moves the hexadecimal value 0xFF (255 in decimal) into register R0.
  • MOV R1,#22: Moves the decimal value 22 into register R1.

Now, let's calculate the results of the bitwise operations:

  • AND R2,R0,R1: The binary representation of R0 (0xFF) is 11111111, and R1 (22) is 00010110. The AND operation will result in 00010110 (22 in decimal) being stored in R2 because only the bits that are set in both R0 and R1 remain set.

  • ANDS R2,R0,R1: This will produce the same result as the AND operation, 00010110 (22 in decimal), but it will also update the condition flags based on this result. For example, if the result is zero, the zero flag will be set.

  • ORR R2,R0,R1: The OR operation sets a bit in R2 if it is set in either R0 or R1. So, 11111111 OR 00010110 will result in 11111111 being stored in R2, which is 255 in decimal.

  • EOR R2,R0,R1: The Exclusive OR operation sets a bit in R2 if it is set in one operand but not both. So, 11111111 EOR 00010110 will result in 11101001 being stored in R2, which is 233 in decimal.

For the negation part:

  • MOV R0,#0xFF: Sets R0 to 255 (11111111).
  • MVN R0,R0: Negates the value in R0. The negation of 11111111 is 00000000 on a 32-bit system since MVN flips all bits.
  • AND R0,R0,#0x000000FF: This AND operation won't change the value because it's essentially ANDing with itself. So, R0 remains as 00000000.