How to use Bitwise to extract Page Number and Offset in Language C

Question

I am studying bitwise operators in language C, now i'm developing a paging simulator with FIFO replacement algorithm to operating systems discipline and encountered difficulties with using bitwise.

SIZE PAGE 1024                    // 1024 bits == 128 BYTES
PHYSICAL MEMORY SIZE 10           // 10 bits (PHYSICAL)
VIRTUAL MEMORY SIZE 16            // 16 bits (VIRTUAL)
NUMBER PHYSICAL PAGES 8           // 8 FRAMES
NUMBER OF VIRTUAL PAGES 512

So if we have 16 bits for virtual memory, we have logical addresses from 0 to 65535 ((2^16) -1). With this, we would have 9b for address and 7b for data.

Example with number 546 (0000 0010 0010 0010):

 Page Number  |  Offset
 0000 0010 0  |  010 0010
&1111 1111 1  |  000 0000
 0000 0010 0  |  000 0000
so, using right shift >> I get the value of the address.

The important thing is to consider the page number, the offset will be of no use. I was trying to do with text manipulation, but it did cause problems because not consider the leading zeros, which working with int and bitwise would be correct. I found an example of how to perform the extraction, but it still goes wrong even after making the necessary changes, here is the link: Click Here.

void extract_fields(unsigned int address){ ... }

If someone can help me, because I'm not sure how to apply the bitmask given the address, thank you very much, I believe that it would help a lot of people, since the doubt is something frequent and for academic purposes.

Can this be reduced to something like "I have a number X and want to extract Y and Z using bitwise operations"? — Eugene Sh., Jun 19 '18 at 17:22
Hahahahah, sorry my friend @Eugene Sh., I just wanted to show why I want to do this, I'll simplify the question. — Paulo Pitta, Jun 19 '18 at 17:27
In the example with 546, you need to start with 16 bits `0000 0010 0010 0010`, and chop that into 9-bit and 7-bit chunks. — user3386109, Jun 19 '18 at 17:27

score 2 · Accepted Answer · answered Jun 19 '18 at 17:27

The only thing of importance to solving your problem is how to extract the virtual page number from the given virtual address.

This can be done with a simple bit shift by seven positions to the right (seven is the number of bits needed to address inside the 128-byte page) or with integer division by 128, which drops the remainder:

unsigned int extract_page(unsigned int address) {
    return address >> 7;
}

Use scanf("%u", &address) to read each address from the input file.

How to use Bitwise to extract Page Number and Offset in Language C

1 Answers1