Understanding Operating Systems
Raza’s Simplified
Chapter 4 MEMORY MANAGEMENT
In this Chapter
Address Binding And Single Absolute Partition
Single Relocatable Partition
Multi Programming
Multiple Partitions
Simple Paging
Simple Segmentation
Segmentation With Paging
Managing Page And Segment Tables
Associative Memory
Inverted Page Table
Swapping
Overlaying
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Introduction While executing one or more programs, OS must exist in memory to supervise the execution. The OS is loaded at the time of booting and remains in memory till the system is shutdown. A part of memory is reserved for OS. The rest of memory is divided into small areas where a process runs in each area. These areas are called partitions. OS memory management is responsible for running parallel processes and protecting each process’ memory from the other processes. It also protects OS memory from user programs. Memory management cannot happen without hardware support. OS must works with hardware to make the memory management a practical thing. That’s why, memory management is hardware and software design issue. And memory management hardware is kept away from any user access.
Address Binding and Single Absolute Partition Logical and Physical Addresses The addresses used in a process are called logical addresses. They are references to the parts of programs or its data. For example, a logical address can be a: ●
Variable
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Function/Procedure name
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File name
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Library name
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Other
When a process is run, it becomes resident in RAM. RAM is called the physical memory, and an address in it is called physical address. Every logical address actually references to a specific physical address The process of translating logical addresses into physical addresses is called address binding. Binding can occur at either compile time or load/runtime
Chapter 4 – Memory management
Fig 4.1 Address binding and single absolute partit
Compile Time Binding This type of binding occurs when the source code is translated into object code. The program may not run immediately. This binding allocates physical memory addresses to every logical address. Due to this reason, the process to execute must be loaded at the specified physical addresses. Such a code is called Absolute code. What will happen if those addresses are not available in physical memory?
Load Time Binding Load time binding is performed when the process is loaded in memory. Such a code is called relocatable code and it can be loaded anywhere in memory.
The overhead involved is that we must specify some mechanism to know which byte in process refers to which byte in memory, i.e. we need to know how to translate a logical address to physical address.
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Adding Protection to the Code In early days of computing, no protection was provided by the OS to processes. The memory was simply divided in two parts, one for OS and the other for a user process. Only one process could run at a time. It was the responsibility of the running user process not to interfere with the OS partition. This limit was not enforced by any means so the processes could ignore it. Mostly, a user process was able to access OS memory and crashes the OS. No hardware support was provided to prevent a process from this type of actions. DOS was built in such a way. Base Register is a hardware register which can provide basic protection to the processes. This register contains the start address of the process run by the user. It is the smallest possible address accessible by the user process. So, when the process generates a physical memory address, it is first compared to the base register. If the give address is less than the base register’s contents, then it is ignored and a trap is signalled. Such a trap is called memory fault trap.
Single Relocatable Partition Single relocatable partition introduces the relocation register. This register also contains start address of the process. But, its contents are added to the process’s generated address. They are not compared. Obviously, the result will be larger than the contents of the relocation register, as memory does not contain negative addresses. Now the process has a logical address space. The logical address space is the collection of all addresses accessible to the process. The compilation of the program takes place assuming that it would be loaded at address 0. Memory management hardware performs the translation from logical to physical address.
Fig 4.2 Trap addressing error
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Multi Programming Single absolute or single relocatable partition schemes are limited. They can run only one process at a time. Such OS are not of much use in today’s interactive computing environment. To run multiple processes at a time and to run them concurrently is called multiprogramming. We need a memory management scheme which would divide the memory into many partitions.
Multiple Partitions These schemes allow more than one processes to reside and run simultaneously. For every process a memory partition is allocated. For such a scheme, we need two registers: ●
Relocation Register
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Limit/size register, this register indicates the end of the process or it contains size of the process.
Multiple Fixed Partitions Here fixed means that the size of partitions cannot be changed dynamically. The partition size can be equal for all partitions or it may differ from partition to partition. If the partition size is different, then OS needs to store it for every process. If the size of partition is same, then the OS needs to store it only once. A special data structure called partition table stores the process and its allocated
Fig 4.3 Single Relocatable Partition
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partition. This table stores the partition number allocated to the process or the first address of each process. The size of partition is preferably kept a power of 2. When the size of partition is kept a power p of 2, then address translation is simple. The hardware concatenates p 0s to the end of partition number to get the start address of partition. The partition number is obtained from partition table. The limit register sores the size of the partition and it is set at the system boot time. At the time of context switch of the CPU, the new process is loaded into the CPU and the relocation register is also reset. Sometimes it happens that the size of a process is less than the size of its allocated partition. So, some space is wasted. The space wasted within a partition is called internal fragmentation.
To reduce internal fragmentation, different sizes of partitions are provided. The OS must also load the limit register with new value at the context switch. This partition size can be stored in the partition table or it can be calculated from the partition number. OS also selects a partition to load a process. OS cannot load a process on a partition smaller than the size of the process. But, what to do when two large partitions are available and process can be loaded on any of them? Or how to find a correct sized partition suitable for the process? The OS uses partition selection algorithms to select a suitable partition.
Multiple Variable Partitions In this scheme, the OS does not divide the memory before process allocation. Rather it is done dynamically as the processes arrive. The memory is allocated according to the process execution needs. Processes can be loaded anywhere where enough memory is available. Internal fragmentation does not occur. But OS manages much more data about processes and their partitions, including: 1. The exact start address of a process 2. The exact end address of a process
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3. Data about free memory areas When processes are running, they eventually terminate and release their partitions as unused memory areas. After sometime, the memory contains many unused and used areas spread across it randomly/alternatively. This phenomenon is known as checker boarding. The unused space not within any partition is called external fragmentation. Unused areas are also called memory holes.
Checker boarding results in scattered holes. This leaves less ‘contiguous unused memory’ for processes. Hence a process will not load in a smaller partition, although the total unused memory may be larger than needed. However, a process called compaction can be used and applied to organize all holes as a single large hole. It can collect all small holes from memory and place them at a single contiguous location. It requires physically movement of used areas to other areas. So this process is quite costly, as it needs more processing and memory for its own completion.
Recording Allocated and Non-Allocated Memory Bitmap The OS uses a bitmap to keep record which area of memory is in use and which is not in use. Bitmap is a series of 0s and 1s. The OS divides memory in small units. These units are called allocation units. Each allocation unit is associated with a bit in the bitmap. The value of the bit indicates whether the allocation
Fig 4.4 Multiple Variable Partitions
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unit is in use or it is not in use. If we increase the size of allocation unit, then the size of bitmap will reduce. But in this case, if the process size is not the integral multiple of the allocation unit size, memory wastage will increase Linked List This method is used to keep track of free holes in memory. In this scheme, each hole is modified to contain information about its own size and a pointer to the next hole. The OS itself needs a pointer to the first hole in memory. The list can be ordered in any one of the following ways: 1. Memory order 2. Size order 3. Unordered Partition Selection Algorithms If more than one hole in memory can accommodate a single waiting process, then OS needs an algorithm to decide which hole to use. These algorithms include: 1. First fit: Search to find a suitable hole begins from start of memory. The waiting process is loaded into first available hole large enough to contain the process. After loading the process in the hole, some space may be leftover. This leftover space is considered for next selection. Disadvantage: Every time the search is made, the holes at the beginning of the memory are first considered. 2.
Next Fit: It starts searching for the needed hole after the previously allocated hole’s location. So, the search begins not from the beginning of memory. Advantage: The holes in the beginning are not frequently searched.
3. Best Fit: It tries to find the hole exactly equal to the process size. If not found, the next hole large enough to contain the process is used. 4. Worst Fit: It finds the largest hole and loads the process in it.
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Advantage: After loading a process, the leftover space is not enough to accommodate a new process. This problem is much prominent in best fit algorithm. The selection of largest hole in worst fit improves this situation as the leftover space is still enough even to accommodate a new process. Buddy Systems The memory allocation is done as a power of 2. In the beginning, the complete memory is a one single allocation unit. When a process is loaded in memory, the memory is divided in two equal parts. These two parts are called buddies. Then one buddy is again divided in two equal buddies. The division continues until we get a buddy whose size is exactly equal to the size of the process or dividing it further will give a size smaller than the process. When two equal sized consecutive buddies, which were formed from a single buddy are freed, they join back together. For example, if the size of joining buddies is 2K each, then the size of new buddy will be 4K. Maintaining Record of Buddies If the memory size is 2N, then we can have allocation units of sizes from 21 to 2N. The Buddy system needs N lists for free blocks, one list used for each size. The use of separate lists makes it efficient for processing of both free and used allocation units. Buddy systems try to offer a compromise between fixed and variable partitions. However, they suffer from both internal and external fragmentation.
Simple Paging The physical memory is grouped in small units called page frames. Logical memory is also grouped in small units called pages. The sizes of pages and page frames are equal and fixed. The memory management hardware dictates the size of pages and page frames. While loading a process, its pages are loaded into free page frames and run. The page frames can be non-contiguous.
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Maintaining Record For every process, a page table is maintained. Its primary purpose is to store which page frame is allocated to which page. This table is used when physical address is generated from logical address. For every page in the process, there is one entry/row in the table. So, if the page has 100 entries, then there are 100 pages in the process. First entry tells about first page, second entry tells about second page and so on.
Mechanics The page size is kept to be a power of 2. This reduces complexities at the times of address translations. A logical address contains two type of bits. In the beginning of it, the bits specify the page number. Remaining last bits specify the offset in the page. Offset or displacement is the location within individual pages counted from the beginning of the page as 0. So, if L is the total number of bits in the logical address, and 2P is the size of page, then first L-P bits give the page number and last p bits give the offset.
Translation The translation of logical address to the physical address is simple and straight forward. The page number is extracted from logical address. It is matched in the page table to find its page frame. Then the offset is appended with the page frame number. This all translation remains transparent to the process.
Fig 4.5 Simple Paging
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Multi Levels in Page Tables As the number of pages grows, the size of page table also increases. This is difficult to manage. So, the page table is broken down into small parts, each presenting a level. Now the page number in page table is not a simple page number. It contains more sections. The first part of the page table is used as an index, in top most level table. When the entry in that table is found, it is used to search the next lower level. The second part of the page number is used to search in the second level page table. This process is repeated until an entry in the bottom level table is found. That entry gives the page frame number of the loaded page.
Tracking the Traps Firstly, a size register can be used to determine which logical address would result in an out of bound address trap. Secondly, page table can also do this. It is the most commonly used option. If a process has n pages, then first n entries of its page table are correct. All other entries do not belong to it. So, first n entries in the page table are marked valid, all other are marked as invalid. This is done by placing a valid/invalid bit field in the table. Some systems also offer protection bits as the part of page table. But this scheme is suited for systems using segments.
Fig 4.6 Multi levels in Page Tables
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Simple Segmentation In this scheme the logical memory i.e. the process is divided into smaller parts. These parts are called segments. Their size is kept to be some power of 2. But their size is not fixed. It can vary from segment to segment. The OS does not perform this division. The user or the compiler does it. But hardware sets the maximum size a segment can have.
Keeping Record The size of segments is variable. So the OS cannot predict when and where to divide physical memory to contain a segment. It must maintain the lists of memory holes available for an incoming process. The issues encountered here are similar to the issues faced in multiple variable partitions. A structure called segment table is used to keep track of segments of a process. This table contains the first address of the segment and its size.
Translation An offset/displacement is a location within a segment counted from the start of the segment considering it location 0. If the segment size is 2m and a generated logical address length is L, then first L-m bits in the logical address provide the segment number. Last m bits provide the offset in segment. The segment number and offset is taken from the logical address. The segment table entry is found by comparing the segment number with segment table entries. If the offset is larger than the size of segment, a trap named ‘invalid address fault’ is returned, and the program is terminated. If the offset is correct then the translation is done by summing the segment number and the offset together, hence a physical address is found. This process can be boosted up if the confirmation of size and physical address generation is done in parallel.
Special Features of Segments The user has control on segments. Thus, the user can add many convenient properties to them:
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Fig 4.7 Simple Segmentation
Read Only Segments A read only segment contains data which cannot be changed during processing. To make a segment read only, an extra field called read only bit is included in the segment table. When a write is requested on a segment, then its read only bit is checked. If it is set, meanings the segment is read only, the write operation is denied and a fault is generated.
Sharing the Segments Read only segments can be shared among many processes to reduce memory usage. This is especially helpful when two processes are running from the same program code. Another use of shared segments is to load a subroutine library. Its read only access is given to the processes. But this situation requires much care as the same segment addresses will be utilized by many processes. Three types of addressing are used in such situations: 5. Relative addressing 6. Indirect addressing 7. Direct addressing
Relative Addressing This type of addressing gets the offset from the program counter and generates the required address.
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Indirect Addressing Indirect addressing uses a register which specifies the correct segment to use.
Direct Addressing It uses segment and offset of the required address.
Segmentation with Paging Segmentation and paging both have their advantages. Segmentation provides security and sharing. Paging provides efficiency. If we combine both, we get dual advantages. To do this, a segment is divided into equal sized pages and a page table is maintained to keep record of pages in segment. Normally a segment number and offset is contained in a logical address in a segmented system. But in segmentation with paging, the segment offset has two parts. First is the page number second is page offset. Segment table tells about the address of the page table in the segment. To get page table entry, the page number bits in the logical address are added to the page table address. Then, the page offset is appended to the page frame number found in the page table to get its physical address. Note that this time the process will be divided in segments. Every segment will be divided in pages. Thus every segment will need its own page table.
Managing Page and Segment Tables Page tables are maintained in paged systems and segment tables are maintained in segmented systems. These tables are consulted by the OS when a logical address
Fig 4.8 Simple Segmentation Paging
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needs to be translated into a physical address. Every process has its own page and segment table. A special register for memory management is used and it contains reference to the current process’s table. At the time of context switch, this register is also updated to refer to the table of loaded process.
Associative Memory We also call it translation look aside buffer. This memory is especially useful when the size of page table is very large, i.e. the number of pages is high. It increases the search performance. Associative memory is a subset of page table containing most recently used pages and their related information only. It is designed in the way that every element in it can be searched in parallel to other elements. So if 1 microsecond is taken to search one element, then to search 10, 20 or 25 elements 1 microsecond is needed. So, it is extremely fast than page table. Every element in it is given a number to identify it called an index. The OS gives the search value to associative memory and gets the index.
Searching Process When a page is searched, the page table and associative memory are used together. If the page is in associative memory it is directly used and search in page table is stopped. Since the associative memory is too fast, therefore this process completes quickly. But if the associative memory does not have the page information, then OS waits for data from page table. When page table provides the information, it is used for translation. Associative memory is then updated and this new information is also stored in it by replacing some old one. It will improve future searches. To measure the performance of associative memory, hit ratio is used. Hit ratio is the ratio of accesses found in associative memory to the accesses not found in associative memory. The system gives good performance if the hit ratio is high.
Precautions At the time of context switch, the old entries in the associative memory cannot be used for the new process. The OS does it by marking them invalid.
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Fig 4.9 Associative Memory
Second way is that associative memory assigns a field of process id and when an access is done, the process id should match the current process to use an entry.
Inverted Page Table If the paging systems are using large virtual address space and associative memory, the inverted page table minimizes the size of page table. It works in the reverse order. Its entry contains a page frame and page allocated to it. When a page is not found in associative memory, the inverted page table is consulted. If hashing is used then the procedure speeds up.
Swapping In swapping a process is taken away from memory and stored on disk.
It is done when the RAM runs short and we need space to load some new processes or data. This swapped out process is placed at a specific location on disk. A process can be selected from ready or blocked queues for swapping. When there is space available in memory, the process is returned. Swapping is done by medium term scheduler called swapper. Swapping increases the number of processes using CPUs i.e. the
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degree of multiprogramming. In other words, CPU can handle more processes in a given interval and its idle time is reduced.
Overlaying In this scheme, the programmer defines two or more than two segments of a program. They need not be in memory simultaneously to execute. The OS loads one overlay segment or the other and executes it. So the physical memory needed is much less than the program without overlaying. Segments can execute alternatively. The only disadvantage is that the programmer is heavily involved. It is not easy and even not reliable for a programmer to divide a process in this way. Programmer’s knowledge and experience does counts a lot. A simple approach is that we do not involve the programmer in making parts of the process and let the OS decide what to do when memory is less than the process size. Such a scheme is virtual memory.
Fig 4.10 Overlaying
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