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How could we implement locks? No matter how we choose to implement them, we must have some hardware support. One possibility for implementing locks on a uniprocessor machine is is to disable interrupts when testing/setting locks. With interrupts disabled on a single processor machine, the processor cannot switch processes, and so we can guarantee that only the active process will have access to the shared data. Another option would be to make use of atomic operations, such as test and set. This type of operation (which usually corresponds to a single atomic assembly instruction) behaves as if it used the following C function, atomically:
int test_and_set(int x) // let x be strictly either 0 or 1.{if (x) { return 1; } else { x=1; return 0; }}
All this needs to be implemented atomically, in hardware. Using this type of atomic operation, one could implement thread lock(l) simply as while test_and_set(l) {
; // do nothing} // spinlock version of thread_lock()and thread unlock(l) simply asl = 0; // we need this to be an atomic clear (or assign) instruction
The assembly instruction test and set can bemade to be atomic acrossmultiple processors. An equivalent option would be an atomic compare and swap assembly instruction. These low-level hardware solutions are then built up into high-level functions, either built into the languages, or in libraries. In general, do not implement your own locking functions, but rather use functions from a tested library. Getting things right can be tricky, and your own solution is also likely to be non-portable.
Determine a parser which is a variant of top-down parsing without backtracking Recursive Descend parser is a variant of top-down parsing without backtracking.
For reading, most operating systems use demand paging. This means that pages are only read from the disk into physical memory when they are needed. In the page table, there is a re
We have talked about blocking locks, which voluntarily yield, and spinlocks, which just spin until they acquire the lock. At ?rst, it seems like spinlocks are verywasteful, and tha
Purpose This is intended to help you develop your understanding of shell scripting in both a Windows and Unix environment. Deliverables For both the first and second op
Q. A CPU scheduling algorithm determined an order for the execution of its scheduled processes. Convinced n processes to be scheduled on one processor how numerous possible differ
Suppose we want to program a Web server; we could easily do so without using concurrency, as follows: while(true) wait connection read from socket and parse url look up url cont
Consider the subsequent resource-allocation policy. Requests as well as releases for resources are allowed at any time. If a request for resources can't be satisfied because the re
What are Tree-structured directories We can generalize the directory structure to a tree of arbitrary height. This permits the user to create their own sub directories and to c
Entry-sequenced file organization The records in an entry-sequenced file are store up in the order they are written into the file. New records are for all time appended to
what are operating system services
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