Apparently you found some "canned" code someone else wrote.

The real questions would be, "Do you know how checksums work and how to code them? Do you know what this code does? Do you know how to use it?"

I'll bet you still can't honestly answer that with a "yes."

Do you know what
{
checksum += *start_addr;
}
does?

Is

if (checksum != 0x80)

a valid test for all possible code within the "range?"

Why is that the case?

Is

for (; len > 0; len--, start_addr++)

a valid 'C' statement?

Why is that the case?

Are you aware that some of the code you've "snagged" is irrelevant to the checksum process?

Why is that the case?

Maybe that's why you don't know what the role of the timers is!

Have you read the textbook at all? Have you managed to learn, at least, a mimimal amount about 'C'?

RE