explain what happened (C program)

Assume that a system has a 64-bit virtual address with an 8-kb page size. Write a C- program that is passed a virtual address (in decimal) on the command line and have it output the page number and offset for the given address. As an example, your program would run as follows: ./a.out 8219986 The program would output: The address 8219986 contains: Page number: 1003 Offset = 3410 All computation must be done in binary.

#include <stdio.h>
int main(int argc,char *argv[])
{
    unsigned long long int MappedAdress = atoll(argv[1]);
    unsigned long long int page = MappedAdress;
    unsigned long long int offset = MappedAdress;
    //print long long address
    printf("The MappedAdress %llu contains:\n",MappedAdress);
    //shift to right 13 bit to retrive pages, then print
    page = page >> 13;
    printf("Page Number : %llu\n",page);
    //since it's 64-bit opr. Add offset AND 1FFF , 
    offset = offset & 0x1FFF;
    //print the offset
    printf("Offset = %llu\n\n",offset);
    return 0;
}

Can anyone explain to me what just happened here ? and what to shall I do to write such a program but in Java as long as I have no background in C language?

Answers


Can anyone explain to me what just happened here ?

Roughly speaking, you converted an address 64 bit virtual address to a virtual page number and an offset, using the right-shift operator (>>) and the bitwise AND operator (&).

And what to shall I do to write such a program but in Java as long as I have no background in C language?

What you should do is to read your Java textbook or lecture notes to find out about the >> and & operators (for integer operands), and what they do. Then write the equivalent code in Java.

HINTS:

  1. The >> and bitwise & operators mean pretty much the same thing in C and Java.
  2. You should probably also read the lecture notes where your lecturer explained about virtual addresses and how they work.

LEt's go back to the problem statement: given a 64-bit number that represents a byte address on a computer that has 8K size pages, return the page number and offset for that address.

That simply means that you take number / 8K to get the page address and the remainder is tie offset. Of course, that can often be computed faster by using a right shift to get the page number and a mask to get the offset (but do handle leading "1" bit in the 64-bit number).

You don't need the C program to figure out how to break a 64 bit number into the two desired pieces.


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