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Memory Mapped I/O Most computers of the 1960s, 1970s, and 1980s had a special set of instructions for I/O, and many computers still do. With such architecture, reading from a buffer on an interface requires a different instruction than reading from a memory location. In one computer architecture, for instance, the I/O instruction to read from the interface in slot 7 is LIA 7, load into the A-register from slot 7. The memory reference instruction to read from memory location 7 is LDA 7, load into the A-register from memory location 7. The instructions LIA and LDA are different. LIA is a privileged instruction, available only in privileged mode, and LDA is not; any program can execute LDA. In the 1980s the idea of memory-mapped I/O gained currency. With memory-mapped I/O, certain memory locations are reserved for reference to the control registers and buffers of the I/O interfaces. Communicating with the controllers becomes as easy as writing to and reading from memory. Memory-mapped I/O offers several advantages. First, since I/O uses the same instructions as memory references, I/O programs (drivers) can be written entirely in a high-level language. Second, protection against issuing I/O commands in user mode can be effected using the memory protection hardware already required for enforcing bounds on a user program s access to memory. No special mechanism for detecting the privileged status of I/O commands is required. A disadvantage of memory-mapped I/O becomes apparent when the computer design includes separate buses for memory and I/O. Since the controllers cannot see the transfers to memory, some mechanism is necessary to intercept reads and writes to I/O mapped-memory locations and pass them off to the I/O bus. Another disadvantage of memory-mapped I/O is that most modern computers, in order to speed access to memory, use local cache memory to hold the contents of recently accessed memory locations. A cache holds a copy of what is in memory in a location closer to the CPU. If a driver needs to read the control flag register to learn if the interface is busy, and the contents of the memory location are cached, the driver may never see the device become available. Some additional mechanism must be added to provide for selective caching a mechanism that avoids caching of I/O mapped addresses. Some computers use both special I/O instructions and memory mapping. For instance, the PC architecture uses memory mapping for the transfer of buffers to I/O controllers, and also uses I/O instructions to write to the control registers of the interfaces.
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PROCESSES AND SCHEDULING The conceptualization of a running program has been refined as the field of computing has matured. Early on, one spoke of running a job, or program. The program would be loaded on the computer, and it would run to completion. Later, computer scientists made the distinction between a program and a process. The program is the set of instructions that will be executed when the program runs, and a process is an instance of a running program. The difference is that the process maintains the state information about that execution of the program; it has certain values for variables, for example, and a program location counter value at any particular moment, and a certain section of the computer memory assigned to it. In a multiprogramming environment (the name multiprogramming emerged before the widespread use of the term process, for in today s language one would call it multiprocessing ), the distinction between program and process becomes important. Two instances of an editor program, for example, may be simultaneously operating on different files. Therefore, two processes with different state information are executing the same program. It s important for the operating system to maintain the distinction between the two processes. Later still, the concept of a process was broken into two concepts: the process and the thread. The process is the set of resources assembled for the execution of the program (files, memory allocation, etc.), and the thread is a line of execution within the program. Before programmers had access to threads, tasks that required multiple lines of execution would be written as separate processes that communicated with one another somehow (shared memory, messages, shared file, etc.). However, as we will show, setting up a new process is a fairly high-overhead task for an operating system, and the communication of messages between processes can itself require a significant programming effort and entail additional operating system overhead. These problems, which were encountered frequently, motivated the
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