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The solution in the mid-1960s and 1970s was to use two registers whose contents would be set by the operating system just prior to turning execution over to the user process. One register was called the base or relocation register, and it held the starting address of the memory allocated to the process. The other register was called the limit register, and it held the maximum size of the program. Each time a process accessed memory, the computer hardware (called the memory management unit (MMU)) would check the address to make sure it was less than the limit register contents. If not, the MMU would trap the error and generate an interrupt (an interrupt caused by software is called a trap) that would activate the operating system in kernel mode. Then the OS could deal with the problem, probably by generating a memory protection error and terminating the offending process. On the other hand, in the usual case where the address was within legal range, the MMU hardware would add the contents of the relocation register to the address. The sum would then be the correct physical address for the program executing on this machine at this time, and the access would proceed. Having an MMU with a relocation register meant that a program could be loaded into one part of memory at time 1, and into a different part of memory at time 2. The only change between times would be to the contents of the relocation register. The part of memory concept became more sophisticated over time. Initially, memory was divided into partitions of fixed sizes when the operating system was installed or generated (called a system generation or sysgen ). A program could run in any partition big enough to accommodate it. Later, operating systems gained the feature of being able to support dynamic memory partitions of varying sizes. This complicated the operating system s task of tracking and controlling memory usage, but it increased flexibility. The operating system would maintain a list or bit-map of memory allocations and free memory. When a program became ready to run, the operating system would find a big enough block of memory to accommodate the program, and then dispatch the program to that chunk of memory. In a dynamic multiprogramming environment, with processes of different sizes starting and terminating all the time, it became important to make wise choices about how to allocate memory. It was all too easy to end up with processes scattered throughout memory, with small unallocated blocks of memory between, and no blocks of memory large enough to run another program. For example, a ready program might require 12K of space, and there might be more than that much unused memory, but the unused memory might be scattered widely, with no one block being 12K or larger. Unusable memory scattered between allocated partitions is called external fragmentation. On the other hand, unused memory within an allocated block is called internal fragmentation. Internal fragmentation results because the OS allocates memory in rather large units, usually in pages of 1024 to 4096 memory addresses at a time. Almost always the last page allocated to a process is not entirely used, and so the unused portion constitutes internal fragmentation. Unused memory blocks are called holes, and much research was devoted to finding the most efficient way to allocate memory to processes. The best fit approach sought the smallest hole that was big enough for the new process. As desirable as that approach sounds, it did require the OS to look at every hole, or to keep the holes in order sorted by size, so that it could find the best fitting hole for a new process. Another approach was called first fit, and, as it sounds, first fit accepted the first hole that was large enough, regardless of whether there might be another hole that fit even better. The thought in favor of first fit was that it would execute more quickly. Believe it or not, another contending algorithm was worst-fit. The argument for worst fit was that it would leave the largest remaining hole, and hence might reduce the problem of external fragmentation. As a matter of fact, the research showed worst fit lived up to its name, and was worse than best fit and first fit. TIMESHARING AND SWAPPING Timesharing systems exposed the need to move a partially complete process out of main memory when the process was forced to wait for the impossibly slow (in computer time) user at a terminal. Even an actively typing user might cause the process to wait for the equivalent of 1000 instructions between typed characters. The solution was called swapping. When a process blocked for I/O, it became eligible to be swapped to the disk. If another process were ready to use the memory allocated to the blocked process, the OS simply copied the entire contents of the blocked process s memory, as well as the CPU register contents, to a temporary scratch or swap area on the disk.
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