Cohort 23 CS Online Journal

Week 4 Reflection Paging - Virtual address --> Physical address Find the VPN bits (first X bits of a virtual address). Find the offset bits of the virtual address, which are the last Y bits (or, the virtual address with leading VPN bits removed). Find the corresponding PTE (if available). If the most significant bit (MSB) is a 1, it is a valid address. If 0, it is not valid and a physical address cannot be obtained from this virtual address. Find the PFN, which is the last Y bits of the PTE. If one is not listed, the number of pages a physical address can hold determines the PFN bits using the formula: [log2(number of pages)]. Then, combine the PFN and offset in this order to obtain the translated physical address. Swapping Swapping makes use of a few policies to determine what pages to swap in memory: The optimal policy (aka Belady or MIN) replaces pages that are accessed furthest in the future to maximize cache hit rates. This is not very practical because implementing pr...

Week 3 Reflection We learned about virtual memory and how to translate it into physical memory.        The two memory types that we use in our programs is stack memory and heap memory. The stack is managed mostly by the compiler while the heap is long-lived and only allocated/deallocated by the programmer. The heap is allocated near the base of the address space (0) and grows outward, while the stack is allocated at the end of the address space (size_of) and grows downward towards the heap/program code.We used two methods of memory address translation - base and bounds (aka dynamic relocation), and paging.        Base and bounds uses two registers: the base register and the bound (aka limit) register. With these two registers, a program can be translated to physical memory using its virtual address + this base register. However, the free memory between the stack and heap is wasteful and prevents other larger address spaces to fit in desp...

Weekly Reflection, Week 2 I feel a lot more confident in my understanding of C now! From what we've covered in class: CPU scheduling is important to optimizing processing times. Scheduling makes use of two metrics: turnaround time and response time. Turnaround time is how long it takes for a process to complete from when it first entered the schedule. Response time is how long it takes for a process to begin its first run from when it first entered the schedule. The main schedule patterns we've seen are First In, First Out (FIFO and its related schedule Last in First Out or LIFO), Shortest Job First (SJF), Shortest Time-to-Completion First (STCF), and Round Robin (RR). FIFO chooses the first process entered and completes it, then to the second-most process entered, etc. LIFO chooses the last entered process (the tail end) and completes it, then to the second-last process entered, etc. SJF prioritizes jobs with the shortest duration and completes them first.  STCF prioritizes jo...

This Week's Reflection      In this class, we discussed about the basics of operating systems - how they work, why they exist, and what they do differently depending on the task at hand. We discuss memory, hardware and software integrations, user mode vs. kernel mode, and what an operating system abstracts to the user.      The main operation that the OS handles is abstracting multiple processes at once to do work for the user. The OS performs this by virtualizing the CPU via time sharing. The rules that govern how the OS handles time sharing are called policies.      I had trouble remembering what the * and & operators do in C. I get confused where they are used and what differentiates them from each other, like a struct* var, a struct var, and struct *var. Using * before a variable (like *var) is a dereference. When initializing a pointer, use struct *var. This creates a variable of type struct *. The & operator refers to an address...