Solutions HW#2 Problem 1 The "degree of multiprogramming" is the maximum number of processes that may be supported by a uniprocessor at any given time. Discuss some factors that must be considered in determining the degree of multiprogramming for a particular system. The number of resources that are available, ie the number of actively waiting processes. Size of your Main Memory will controll how many process you can keep in quick mem. Ulimatly it will be how your processes are scheduled and how the resources are used, so a good long-term, mid-term and short-term scheduler will control the degree of multiprogramming that your system will provide. Problem 2 When is a batch processing the preferred strategy for work to be done by the computer? When is timesharing the preferred strategy? Batch. Jobs with similar needs are batched together and run through the computer as a group by an operator or automatic job sequencer. Performance is increased by attempting to keep CPU and I/O devices busy at all times through buffering, off-line operation, spooling, and multiprogramming. Batch is good for executing large jobs that need little interaction; it can be submitted and picked up later. Time sharing. This systems uses CPU scheduling and multiprogramming to provide economical interactive use of a system. The CPU switches rapidly from one user to another. Instead of having a job defined by spooled card images, each program reads its next control card from the terminal, and output is normally printed immediately to the screen. Problem 3 Below is a list of fields from a classic process control block. In a modern process model with threads and processes, say whether each field should be implemented in the process or thread control block.
a.User name - PCB - all like threads have same UID, no need to replicate in all threads. b.Stack bottom TCB each thread has it's own stack bottom c.Resources blocking me TCB in User-level threads it doesn't really matter, you will block anyway. But in Kernel level threads, each thread could be blocking on different resources. d.Primary memory allocated to me PCB each process needs to know the memory that it has and can then divide that among the threads. e.Files allocated to me TCB each thread may open it's own files
f.Execution state TCB each thread has it's own PC register and it's own execution state.
Problem 4
When a new process moves from the ready state to the running state, each register in the CPU must be set either to initial values or to the values the process had when it was last interrupted. Explain why the PC register is typically the last register loaded for the new process. PC (program counter) points to the next instruction to be executed. If you replace this with another value before saving all the other registers and loading in all new registers, then you begin to execute the instruction of the newly loaded PC register before you have stored and loaded all state of the CPU.
Problem 5 Explain why all thread in a processes block when one of the thread blocks in a user space thread implementation. Why is this not the case in a kernel thread implementation? User-level threads are viewed by the kernel as one process, all the thread managament is done by the user. Since when a typical one thread process blocks on IO, it would be waiting for the IO to return, it is better to switch that process out of the CPU and let another process make some progress. In ULT this is not true, we could simply set another thread from that process into the CPU and it could make progress, but since the kernel views that process and a single threaded process, it puts the entire process on the blocked/waiting queue and waits for the IO to return before moving it back onto the ready queue. Problem 6 What are the differences between a trap and an interrupt? What is the use of each function? Answer: An interrupt is a hardware-generated change-of-flow within the system. An interrupt handler is summoned to deal with the cause of the interrupt; control is then returned to the interrupted context instruction. and A trap is a software-generated interrupt. An interrupt can be used to signal the completion of an I/O to obviate the need for device polling. A trap can be used to call operating system routines or to catch arithmetic errors. Problem 7 MS-DOS provided no means of concurrent processing. Discuss three major complications that concurrent processing adds to an operating system. Answer: 1._ A method of time sharing must be implemented to allow each of several processes to have access to the system. This method involves the preemption of processes
that do not voluntarily give up the CPU (by using a system call, for instance) and the kernel being reentrant (so more than one process may be executing kernel code concurrently). 2.Processes and system resources must have protections and must be protected from each other. Any given process must be limited in the amount of memory it can use and the operations it can perform on devices like disks. 3.Care must be taken in the kernel to prevent deadlocks between processes, so processes aren't waiting for each other's allocated resources. Problem 8 Describe the actions a kernel takes to context switch between processes. Answer: In general, the operating system must save the state of the currently running process and restore the state of the process scheduled to be run next. Saving the state of a process typically includes the values of all the CPU registers in addition to memory allocation. Context switches must also perform many architecture-specific operations, including flushing data and instruction caches. Problem 9 Protecting the operating system is crucial to ensuring that the computer system operates correctly. Provision of this protection is the reason for dual-mode operation, memory protection, and the timer. To allow maximum flexibility, however, you should also place minimal constraints on the user. The following is a list of instructions that are normally protected. What is the minimal set of instructions that must be protected? a. Change to user mode. b. Read the clock. c. Turn off interrupts. d. Change to monitor mode. e. Read from monitor memory. f. Set value of timer. g. Write into monitor memory. Answer: The minimal set of instructions that must be protected are: a. Change to monitor mode. b. Read from monitor memory. c. Write into monitor memory. d. Turn off interrupt. e. Set value of timer Problem 10 Suppose you are asked to develop an OS for a personal computer with an expected market life of 5 years. What might be the relative importance of the following requirements?
a. c.
Performance Correctness
b. Protection and security
d. Maintainability e. f. Commercial factors Standards and Open systems
Which of these requirements would be different for an operating system for a large mainframe with a market life of 20 years
5 year PC a Little importance, most people don't push there systems very hard and good hardware can mask bad performance. b Very important in the day of the internet. c It is only around for 5 years, as long as it works, does it matter how correct it is? Not really. d It is only around for 5 years, it doesn't matter if we can maintain a system like that. e Look at windows, this is huge. f Look at windows, not a big deal. But look at linux, open source has made it what it is. As for standards, this is always important when developing software, and a system is only as good as the things that people want to run on it. 10 year MF a Very important, mainframes compute huge amounts of data, the more you can squeeze out the better. b Not as important, assuming you aren't connected to the WAN and you trust the people who are using it. c Important, you can't expect people to be debugging your problems for the next 20 years. d Important, need to keep it running for 20 years, new hardware, new software, etc. e You are only selling a few of these to people who already know they need them, so commercial factor is not as big here. f This is important if you want the mainframe to be easily maintainable for the next 20 years.