Python bit-wise operations, making changes to existing variable

Question

I'm setting a value (PID) in the 13 bits allocated as per the mask.. i'm trying to set a flag on bit 4 as well as keeping the original value in the 13 bit mask..

Thoughts??

header =    0x0

pid_mask =  0x1fff
TP_mask =   0x2000
PID =       0x1FFe #8190
TP =        0x1

header = ((header & pid_mask) | (PID << 8))

print(bin(header))
print(hex(header))
print(int(header))

header = ((header & TP_mask) | (TP << 12))


print(bin(header))
print(hex(header))
print(int(header))

And this is my output

This part is good (first print statements)

0b111111111111000000000
0x1ffe00
2096640

This part I expect to be the above + the additional bit flip (second print statements)

0b11000000000000
0x3000
12288

Update

I was being a total Muppet, thanks to @Lesiak for all his help

Here is the updated code

def createHeader(pid,tei,pusi,tranportPriority,tsc,afc,cc):

header = pid << 8

if tei == 1:
    header = header | (0x1 << (22 + 1))
    print(bin(header))
if pusi == 1:
    header = header | (0x1 << (21 + 1))
    print(bin(header))
if tranportPriority == 1:
    header = header | (0x1 << (8 + 13))
    print(bin(header))


    header = header | (tsc << 6)
    print(bin(header))
    header = header | (afc << 4)
    print(bin(header))
    header = header | (cc << 0)
    print(bin(header))

print(hex(header))

return header


header = (createHeader(pid,1,1,1,0x1,0x1,0x2))

The only problem I have is a have to slice off the last byte header[1:4] as it returns 4 bytes not 3. Not sure why....

Answer 1

You are constructing a new packet from scratch. There is no sense in using a bitmask for that. Just place the appropriate chunks of the packet in their respective places. To determine the right place, check your protocol definition and calculate the number of bits in the fields fill the less-significant bits.

PID =       0x1FFe #8190
TP =        0x1

header = PID << 8

print(bin(header))
print(hex(header))
print(int(header))

header = header | (TP << (8 + 13))


print(bin(header))
print(hex(header))
print(int(header))

You should use bitmask in the inversed scenario: you received a packet and want to extract data out of it.

Update

How I arrived to 8 bit shift for PID and 21 for TP? Look at the protocol definition: There are 8 bits that go after PID (they are on less significant bits)

• 4 for Continuity Counter
• 2 for Adaptation field control
• 2 for Transport scrambling control

For TP:

• 8 bits from the list above
• 13 bits for PID