Calculating a checksum
Name:
Date:
Class:
A
Quick Questions
Shade in the A or B circle for each question.
1. How is a simple checksum calculated for a block of data?
2. Which calculation keeps a total inside one byte?
3. A block contains the bytes 200, 100 and 50. What is the checksum?
4. A block contains the bytes 10, 20, 30 and 40. What is the checksum?
5. The bytes 255 and 1 are added. What checksum do we get?
B
Match the Pairs
Draw a line connecting each item on the left to its match on the right.
| Left column | Right column | |
|---|---|---|
| 1checksum = sum(bytes) % 256 | Achecksum = total MOD 256 | |
| 2350 % 256 | BThe bytes are added but the total is never reduced. 350 is greater than 255, so it does not fit in one byte; it needs % 256 to give 94. | |
| 3The first step of a checksum is to add the values of all the bytes in the block together. | CThe total is divided by 255 instead of 256. One byte holds 256 different values, so the modulus must be 256 (the correct answer is 94). | |
| 4If the total is more than 255, only the low byte is kept by taking the remainder after dividing by 256. | Dtotal = 12 + 30 + 8 + 50 | |
| 5checksum = (200 + 100 + 50) % 255 | EAdd up every byte in the block, then keep the remainder after dividing by 256 so the checksum fits in one byte. | |
| 6checksum = 200 + 100 + 50 | FGives 94, because 350 is too big for one byte and 350 - 256 leaves 94. |
C
Definitions
checksum = sum(bytes) % 256
D
Fill in the Blanks
Complete each line of code by filling in the missing parts.
1.
checksum = ___(bytes) % 256
2.
checksum = sum(bytes) _ 256
3.
checksum = sum(bytes) % ___
4.
bytes = [200, 100, 50]
total = sum(bytes)
checksum = _____ % 256
5.
# 200 + 100 + 50 = 350
checksum = 350 % 256
print(checksum) # __
E
Spot the Error
Shade in the letter of the line that contains an error.
Question 1
A
data = [200, 100, 50]
B
total = len(data)
C
checksum = total % 256
D
print(checksum)
Question 2
A
a = 200
B
b = 100
C
total = a * b
D
checksum = total % 256
Question 3
A
total = 0
B
for b in [200, 100, 50]
C
total += b
D
print(total % 256)
Question 4
A
data = [120, 80, 100
B
total = sum(data)
C
print(total % 256)
Question 5
A
data = [255, 1, 2]
B
total = sum(data)
C
checksum = total // 256
D
print(checksum)
Answer Key
Calculating a checksum
Section A: Quick Questions
Q1
A — Add up all the byte values and keep the result within one byte
Q2
A — total % 256
Q3
B — 94
Q4
A — 100
Q5
B — 0
Section B: Match the Pairs
Q1
checksum = sum(bytes) % 256 → Add up every byte in the block, then keep the remainder after dividing by 256 so the checksum fits in one byte.
Q2
350 % 256 → Gives 94, because 350 is too big for one byte and 350 - 256 leaves 94.
Q3
The first step of a checksum is to add the values of all the bytes in the block together. → total = 12 + 30 + 8 + 50
Q4
If the total is more than 255, only the low byte is kept by taking the remainder after dividing by 256. → checksum = total MOD 256
Q5
checksum = (200 + 100 + 50) % 255 → The total is divided by 255 instead of 256. One byte holds 256 different values, so the modulus must be 256 (the correct answer is 94).
Q6
checksum = 200 + 100 + 50 → The bytes are added but the total is never reduced. 350 is greater than 255, so it does not fit in one byte; it needs % 256 to give 94.
Section C: Definitions
Q1
checksum = sum(bytes) % 256 — Add all the byte values in the block, then keep the remainder after dividing by 256 so the result fits in one byte. Example: 200+100+50=350, 350 mod 256 = 94.Section D: Fill in the Blanks
Q1
checksum = sum(bytes) % 256Q2
checksum = sum(bytes) % 256Q3
checksum = sum(bytes) % 256Q4
bytes = [200, 100, 50]
total = sum(bytes)
checksum = total % 256Q5
# 200 + 100 + 50 = 350
checksum = 350 % 256
print(checksum) # 94Section E: Spot the Error
Q1
B — len(data) only counts how many bytes there are (3). A checksum needs the sum of the byte values, so it should be total = sum(data).
Q2
C — A checksum is found by adding the bytes, not multiplying them. It should be total = a + b.
Q3
B — A for loop header must end with a colon. It should be for b in [200, 100, 50]:
Q4
A — The list is missing its closing square bracket. It should be data = [120, 80, 100].
Q5
C — // gives the whole-number quotient, not the remainder. The checksum is the remainder after dividing by 256, so use %.