Question $2$: Memory Management $($Max Mark: $20/100)$

It is sometimes useful to know which pages in a user program$$s virtual address space

have been accessed $($read or write or both$).$ This assignment requires you to write a

system call for xv$6$ that checks which page in a process$$s virtual address space has

been accessed. Note that a good understanding of Task $6\mathrm{.}2$ will be helpful. The kernel

function ptableprint$()$ could be useful for debugging the code for this assignment.

Therefore you may want to use the same copy of xv$6$ that you have used for Task $6\mathrm{.}2$ for

this question.

Task: Write a system call named pageAccess$()$ that will modify a bitmap that

indicates which of the $64$ physical memory pages have been accessed. The function

prototype of this system call for user programs will be:

int pageAccess$($char$*$ buf, unsigned int npages, unsigned int$*$ bmp$)$;

This function should return a negative value if unsuccessful $($for any reason$).$ Any other

returned value indicates success.

buf points to the start address of the virtual address space that needs to be checked

for access.

npages gives the number of pages that should be examined. It should be not larger than

$64.$

bmp is a pointer to an unsigned integer that acts as a bitmap which indicates if a page

has been accessed. Each bit of the unsigned integer corresponds to a page. Since an

unsigned integer is $64$ bits in size, npages is limited to $64.$

$2024$ AUT $4$

As an example, if pages $1,2,$ and $30$ have been accessed, the lower $32$ bits of this

integer should have $1$s only for bits $1,2$ and $30($the rest are $0$s$),$ giving a decimal value

of $230+22+21=1073741830($hexadecimal $$40000006)$ as shown below.

This function should return a negative value if unsuccessful $($for any reason$).$ Any other

returned value indicates success.

An example test program for your system call has been provided in the file

pgaccess$\_$test.c$.$ This program should be compiled as a user program in xv$6.$ It also

serves as an example of how the system call is to be used. The bitmap should be set to

the above value for this example test program. But you should check that your system

call returns the values correspondingly if other pages have been accessed.

A skeleton code of your kernel function of this system call, sys$\_$pageAcess$(),$ can be

found in the file sys$\_$pageAccess.c$.$ Note that sys$\_$pageAcess$()$ does not take any

arguments. This is because the call to pageAccess is made in the user program $($in user

mode$).$ Therefore, these arguments cannot be passed directly to a kernel function $($in

kernel mode$)$ in the usual way. Instead, you will need to access the arguments in

sys$\_$pageAcess$()$ using argaddr and argint as demonstrated in the skeleton code.

With the bitmap pointer, it is easier to store it in a variable in this function and then copy

it to the user space before returning using copyout$().$ The code to do this has also

been provided. The remaining code to implement this kernel function will need to be

supplied by you