Virtual Memory Simulation using python

Overview

• Virtual memory allows a process of P pages to run in F frames, even if F < P. This is achieved by use of a page table, which records which pages are in RAM in which frames, and a page fault mechanism by which the memory management unit (MMU) can ask the operating system (OS) to bring in a page from disk.

Goals

• Simulate working of the OS to maintain a process' use of RAM and the page table.
• The page table must be accessible by MMU and OS

Specifications

The page table has four fields in each page table entry:

• Valid indicating if the page of that index is in RAM.
• Frame giving the frame number of the page in RAM.
• Dirty indicating if the page has been written to.
• Requested which is non-zero only if that page is not in RAM and has been requested by the MMU. In this case it's value is the PID of the MMU.
• Added a fifth attribute in the page table to keep track of the count of page access for finding the victim page using LRU algorithm.

MMU simulator takes several arguments as input

• No. of pages in process
• Reference string
• PID of the OS Process

Example :

• 5,"R0 R1 R1 W3 R0 R2 R2 W4 R0 R2 R2 W4 R0 R2 R2 W4 R0 R1 R1 W3 R0 R1 R1 W3", 10,

OS simulator takes two parameters as input:

• No. of pages in process
• No. of frames allocated to the process

Example:

• (5,3,)

• Os simulator uses least recently used (LRU) algorithm for selecting the victim page

Algorithm

• The Simulator uses threads to run both OS and MMU simultaneously to make the simulation more realistic.

Memory management unit

• Checks if the page is in RAM.
• If not in RAM, writes its PID into the Requested field for that page.
• Simulates a page fault by signalling the OS with a SIGUSR1.
• Blocks until it receives a SIGCONT signal from the OS to indicate that the page has been loaded (load is not done in this project, just simulated by sleep(1) delays).
• If the access is a write access, set the Dirty bit.
• Print the updated page table.

Operating System

• Scan through the page table looking for a non-zero value in the Requested field.
• If a non-zero value is found, it's the PID of the MMU, and indicates that the MMU wants the page at that index loaded.
• If there is a free frame allocate the next one to the page.
• If there are no free frames, we choose the least recently used page as the victim page
• If the victim page is dirty, simulate writing the page to disk by sleep(1) and increment the counter of disk accesses.
• Update the page table to indicate that the victim page is no longer Valid.
• Simulate the page load by sleep(1) and increment the counter of disk accesses.
• Update the page table to indicate that the page is Valid, in the allocated Frame, not Dirty, and clear the Requested field.
• Print the updated page table.
• Send a SIGCONT signal to the MMU to indicate that the page is now loaded.
• If no non-zero Requested field was found, the OS exits the loop.