Question

Sample 1: Logícal Operations. Check the examples done in class during the lecture. How you isolate a specific field How you set fields to 0 How you mask fields How you complement fields -> xori xor s .......

Answer 1

Isolate a specific field:

Lets say we have a two byte word a=x14B8. By performing an AND operation with x00F0, we get x00B0. In the result, B has been isolated from the initial word.

By Shift operations we can not achieve that , A shift left followed by shift right cannot do that.

Set Fields to 0:

• Using AND operation on DATA with 0FE3Fh  we can Set destination field to zero.

Masking:

A bit mask is used to mask out unwanted bits in a byte, and just leave the ones you need. For example, suppose you have a byte value, and you want to find the value of the lowest hexadecimal digit. To do this you need to take the value of the lower 4 bits, ignoring the higher 4 bits.

For the AND function, we know that:

• ANDing anything with 0 is always 0, ie A & 0 = 0.
• ANDing anything with 1 leaves the value unchanged, ie A & 1 = A.

We can think of bitwise AND as a masking function. Here, we are using input B as a mask:

Bit 7 6 5 4 3 2 1 0 01111100 Byte A 00001111 Mask 00001100 Result

In this case:

• For the low 4 bits, where the mask bits are 1, the bit values in A are passed through to the result.
• For the high 4 bits, where the mask bits are 0, the bits in the result are forced to 0

Complement Field:

The bitwise complement is also called as one's complement operator since it always takes only one value or an operand. It is a unary operator.

Bitwise complement operator is denoted by symbol tilde (~).

When we perform complement on any bits, all the 1's become 0's and vice versa.

If we have an integer expression that contains 0000 1111 then after performing bitwise complement operation the value will become 1111 0000.