Masking, clearing registers and encryption

Overview

Recall questions on why a bitwise operation covers a whole register at once, extracting the upper nibble, setting and inverting chosen bits, clearing a register, testing for an even number, XOR encryption worked in binary, extracting a colour component, and the weakness of a short repeating key.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Why can a bitwise operation be applied to a whole register in a single instruction rather than one bit at a time?

   Model answer

   Every bit position has its own logic gate and all of them operate at once, so the whole word is processed within one machine cycle.

2. 2

   Which operation and which mask extract the upper four bits of a byte, and what must follow?

   Model answer

   The byte is ANDed with 11110000 in binary, and the result is then shifted right four places so that the nibble sits in the low bits.

3. 3

   Calculate 11010110 AND 11110000, giving the result in binary and in denary.

   Model answer

   The result is 11010000 in binary, which is 208 in denary.

4. 4

   Which operation and mask force bit 3 of a byte to 1 without disturbing any other bit?

   Model answer

   The byte is ORed with 00001000 in binary, which sets that bit while leaving every bit paired with a 0 unchanged.

5. 5

   Which operation and mask invert the lower nibble of a byte while leaving the upper nibble alone?

   Model answer

   The byte is XORed with 00001111 in binary, because XOR with 1 inverts a bit while XOR with 0 reproduces it.

6. 6

   State the quickest way a processor can force a register to zero using a logical operation, and explain why it works.

   Model answer

   The register is XORed with itself, because every bit is then paired with a copy of itself and XOR of two equal bits is 0.

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