Python bitmask doesn't give expected output

Question

Im puting a bitmask over a register read value and execute a logical and on the value. what i expect is: clear_reg_val: 00000000000000000100100000000000 My actual output is:

# reg_value is:  00000000000000000100100000001100
# mask_inverted: 11111111111111111111100000000011
# clear_reg_val: 00000000000000000100100000001000
# reg_value is:  00000000000000000100100000001100
# mask_inverted: 11111111111111111111100000000011
# clear_reg_val: 00000000000000000100100000001000

So i don't know why bit 4 of clear_reg_val becomes a '1' instead of a '0' The Python code of the function is added in the screenshot below:

Python function for read modify write

please post the code here, a screenshot is not OK – VPfB Apr 01 '20 at 10:42 — VPfB, Apr 01 '20 at 10:42
I'll do in future, for now my question is answered – Mansur D Apr 02 '20 at 11:28 — Mansur D, Apr 02 '20 at 11:28

score 2 · Accepted Answer · answered Apr 01 '20 at 10:41

The error is in the way you are creating the inverted mask.

You are doing it as a string operation, and converting the string back to an integer using int. You forgot to specify base=2 and the string is treated as a decimal number.

The proper way to invert a 32 bit value is:

MASK32 = 2**32-1

invx = ~x & MASK32

Python bitmask doesn't give expected output

1 Answers1