msp430 - Need to count number of 1's in binary with assembly

Msp430 - Need to count number of 1's in binary with assembly

Counting the number of 1's (bits set) in a binary number is a common task in assembly programming, including for microcontrollers like MSP430. Here's how you can achieve this using MSP430 assembly language:

Approach

You can use a bitwise operation to count the number of 1's in a binary number. The MSP430 assembly language provides instructions that allow efficient manipulation of bits.

Example Code

Below is an example MSP430 assembly code snippet that counts the number of 1's in a 16-bit binary number:

 .text ; Start of code section countOnes: mov.w R12, R13 ; Copy input number to R13 clr R14 ; Clear R14 (counter for number of 1's) countLoop: ; Check if R13 is zero tst.w R13 jz countDone ; If R13 is zero, jump to countDone ; Check least significant bit (LSB) ; If LSB is 1, increment R14 bit.w #1, R13 jnz incrCount ; Jump if LSB is 1 ; Shift right by one bit ; MSP430 assembly uses "rla" for rotate left and "rra" for rotate right rra.w R13 ; Repeat countLoop jmp countLoop incrCount: ; Increment R14 inc R14 ; Clear the LSB in R13 bic.w #1, R13 jmp countLoop countDone: ; R14 now contains the number of 1's ; R13 contains the original input number (optional) ; Place further instructions here if needed ret

Explanation

  1. Initialization:

    • mov.w R12, R13: Copies the input number (stored in R12 here) to R13.
    • clr R14: Clears R14, which will be used to count the number of 1's.

  2. Counting Process (countLoop):

    • tst.w R13: Tests if R13 is zero. If zero, jumps to countDone.
    • bit.w #1, R13: Checks the least significant bit (LSB) of R13. If 1, jumps to incrCount to increment the count.
    • rra.w R13: Right rotates R13 by one bit, shifting out the LSB.

  3. Incrementing Count (incrCount):

    • inc R14: Increments R14 to count the 1.
    • bic.w #1, R13: Clears the LSB of R13 (which was just counted).
    • jmp countLoop: Jumps back to countLoop to continue counting.

  4. Completion:

    • countDone: After R13 is zero (all bits counted), R14 contains the count of 1's.

  5. Return:

    • ret: Returns from the subroutine (countOnes).

Usage

  • Input: Place the binary number you want to count in R12 before calling countOnes.
  • Output: After calling countOnes, R14 will hold the count of 1's in the binary representation of the number.

Notes

  • This example assumes a 16-bit binary number (R12 and R13 are 16-bit registers). Adjustments may be needed for different sizes.
  • Be mindful of how MSP430 assembly handles shifts and rotates (rra in this case).
  • Ensure you handle edge cases such as all bits being 0 properly.

This code provides a basic framework for counting the number of 1's in a binary number using MSP430 assembly language. Adjustments can be made based on specific requirements or optimizations needed for your application.

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