Memory management - Revision history

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'''Memory management''' is the name for the hardware in a [[Central Processing Unit|CPU]] which controls which areas in physical [[main memory]] a given [[user]] [[process]] is allowed to use. In other words, it controls what the process' [[address space]] looks like.

This hardware is needed for two reasons. First, in a [[time-sharing]] [[operating system]]s, to prevent one user's erroneous (or hostile) program from interfering with the memory of another user (or the OS itself); i.e. to confine it to that area of main memory to which it had been assigned. Second, it is useful in the allocation of main memory, particularly in [[virtual memory]] systems, where memory is allocated in [[page]]s of equal size, which makes allocation of physical memory much simpler - unlike in earlier [[base and bounds]] memory management.

Early [[International Business Machines|IBM]] [[System/360]] machines exposed the actual physical memory addresses to users (hence the ubiquitous use of BALR instructions to load [[index register]]s), although the protection mechanism in their case was not a bounds register, but [[protection key]]s on each block of memory. Also, the need for an [[adder]] in the addressing logic was not done away with entirely - to prevent the need to re-link a program each time it was loaded, indexing registers were used extensively, needing an adder there.

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 This hardware is needed for two reasons. First, in a [[time-sharing]] [[operating system]]s, to prevent one user's erroneous (or hostile) program from interfering with the memory of another user (or the OS itself); i.e. to confine it to that area of main memory to which it had been assigned. Second, it is useful in the allocation of main memory, particularly in [[virtual memory]] systems, where memory is allocated in [[page]]s of equal size, which makes allocation of physical memory much simpler - unlike in earlier [[base and bounds]] memory management.
-Early [[International Business Machines|IBM]] [[System/360]] machines exposed the actual physical memory addresses to users (hence the ubiquitous use of BALR instructions to load [[index register]]s), although the protection mechanism in their case was not a bounds register, but [[protection key]]s on each block of memory. Also, the need for an [[adder]] in the addressing logic was not done away with entirely - to prevent the need to re-link a program each time it was loaded, indexing registers were used extensively, requiring an adder there.
+Early [[International Business Machines|IBM]] [[System/360]] machines exposed the actual physical memory addresses to users (hence the ubiquitous use of BALR instructions to load [[index register]]s), although the protection mechanism in their case was not a bounds register, but [[protection key]]s on each block of memory. Also, the need for an [[adder]] in the addressing logic was not done away with entirely - to prevent the need to re-[[linking|link]] a program each time it was loaded, indexing registers were used extensively, requiring an adder there.
 [[Category: CPU Basics]]

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	Early [[International Business Machines\|IBM]] [[System/360]] machines exposed the actual physical memory addresses to users (hence the ubiquitous use of BALR instructions to load [[index register]]s), although the protection mechanism in their case was not a bounds register, but [[protection key]]s on each block of memory. Also, the need for an [[adder]] in the addressing logic was not done away with entirely - to prevent the need to re-link a program each time it was loaded, indexing registers were used extensively, requiring an adder there.		Early [[International Business Machines\|IBM]] [[System/360]] machines exposed the actual physical memory addresses to users (hence the ubiquitous use of BALR instructions to load [[index register]]s), although the protection mechanism in their case was not a bounds register, but [[protection key]]s on each block of memory. Also, the need for an [[adder]] in the addressing logic was not done away with entirely - to prevent the need to re-link a program each time it was loaded, indexing registers were used extensively, requiring an adder there.
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		+	[[Category: CPU Basics]]

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-'''Memory management''' is the name for the hardware in a [[Central Processing Unit|CPU]] which controls which areas in physical [[main memory]] a given [[user]] [[process]] is allowed to use. In other words, it controls what the process' [[address space]] looks like.
+'''Memory management''' is the name for the hardware (sometimes called the '''Memory Management Unit''', or '''MMU''') in a [[Central Processing Unit|CPU]] which controls which areas in physical [[main memory]] a given [[user]] [[process]] is allowed to use. In other words, it controls what the process' [[address space]] looks like.
 This hardware is needed for two reasons. First, in a [[time-sharing]] [[operating system]]s, to prevent one user's erroneous (or hostile) program from interfering with the memory of another user (or the OS itself); i.e. to confine it to that area of main memory to which it had been assigned. Second, it is useful in the allocation of main memory, particularly in [[virtual memory]] systems, where memory is allocated in [[page]]s of equal size, which makes allocation of physical memory much simpler - unlike in earlier [[base and bounds]] memory management.
-Early [[International Business Machines|IBM]] [[System/360]] machines exposed the actual physical memory addresses to users (hence the ubiquitous use of BALR instructions to load [[index register]]s), although the protection mechanism in their case was not a bounds register, but [[protection key]]s on each block of memory. Also, the need for an [[adder]] in the addressing logic was not done away with entirely - to prevent the need to re-link a program each time it was loaded, indexing registers were used extensively, needing an adder there.
+Early [[International Business Machines|IBM]] [[System/360]] machines exposed the actual physical memory addresses to users (hence the ubiquitous use of BALR instructions to load [[index register]]s), although the protection mechanism in their case was not a bounds register, but [[protection key]]s on each block of memory. Also, the need for an [[adder]] in the addressing logic was not done away with entirely - to prevent the need to re-link a program each time it was loaded, indexing registers were used extensively, requiring an adder there.