I'm studying OS and I have a doubt about physical and logical addresses.
If logical addresses do not exist in real and are only used to indicate physical addresses, why do we use logical addresses at all? Why not directly physical address?
Thanks in advance!
One good example of why an operating system uses a logical address is the concept of virtual memory. For example, a process can be running in Windows which requires 100MB of RAM to execute. Without virtual memory, if this amount of RAM were not available, the process could not run. With virtual memory, the Windows OS can tell the process that the memory it needs is available. However, the OS cannot expose 100MB of physical memory because it does not exist. Instead, the OS will expose 100MB of logical memory. Some or all of this memory may not map to a physical address. Instead, it might map to disk or another location.
Another reason is fragmentation.
Lets say you have 100 MB of memory and the first three processes need 20 MB each. You give them the memory they want, they run and then the second one terminates. You are left with 60 MB of free memory, but any process that wants a sequential address space of 50 MB can't have it.
Using logical addresses gives you that ability.
One of the major benefits of using logical addresses (and in fact, similar thinking is true for most naming abstractions such as domain names in the networking context, file descriptors, etc.) is that programs can be written in a way that is agnostic to the actual layout of physical memory. That is, you can write your program such that data is stored in say, address 0xdeadbeef (this is a logical address), and your program would work just fine (the MMU or a boot loader that performs binary translation would convert the logical address into an appropriate physical address at run-time or at boot time).
In the above scenario, if your program were written to use physical addresses from the get go, you would run the risk of running into conflicts with other processes that use the same address to store data (e.g., other instances of your program).
Related
This is what I understood of logical addresses :
Logical addresses are used so that data on the physical memory do not get corrupted. By the use of logical addresses, the processes wont be able to access the physical memory directly, thereby ensuring that it cannot store data on already accessed physical memory locations and hence protecting data integrity.
I have a doubt whether it was really necessary to use logical addresses. The integrity of the data on the physical memory could have been preserved by using an algorithm or such which do not allow processes to access or modify memory locations which were already accessed by other processes.
"The integrity of the data on the physical memory could have been preserved by using an algorithm or such which do not allow processes to access or modify memory locations which were already accessed by other processes."
Short Answer: It is impossible to devise an efficient algorithm as proposed to match the same level of performance with logical address.
The issue with this algorithm is that how are you going to intercept each processes' memory access? Without intercepting memory access, it is impossible to check if a process has privileges to access certain memory region. If we are really going to implement this algorithms, there are ways to intercept memory access without using the logical address provided by MMU (Memory management unit) on modern cpus (Assume you have a cpu without MMU). However, those methods will not be as efficient as using MMU. If your cpu does have a MMU, although logical address translation will be unavoidable, you could setup a one-to-one to the physical memory.
One way to intercept memory access without MMU is to insert kernel trap instruction before each memory access instruction in a program. Since we cannot trust user level program, such job cannot be delegated to a compiler. Thus, you can write an OS which will do this job before it loads a program into memory. This OS will scan through the binary of your program and insert kernel trap instruction before each memory access. By doing so, kernel can inspect if a memory access should be granted. However, this approach downgrades your system's performance a lot as each memory access, legal or not, will trap into the kernel. And trapping into kernel involves context switching which takes a lot of cpu cycles.
Can we do better? What about do a static analysis of memory access of our programs before we load it into memory so we only insert trap before illegal memory access? However, processes has no predefined execution order. Let's say you have programs A and B. They both try to access the same memory region. Then who should get it with our static analysis? We could randomly assign to one of them. Let's say we assign to B. Then how do we know when will B be done with this memory so we can give to A so it can proceed? Let's say B use this region to hold a global variable, which accessed multiple times throughout its life cycle. Do we wait till the completion of B to give this region to A? What if B never ends?
Furthermore, a static analysis of memory access would be impossible with the present of dynamic memory allocation. If either program A or B tries to allocate a memory region which size depends on user input, then OS or our static analysis tool cannot know ahead of time of where or how big the region is. And thus would not be able to do analysis at all.
Thus, we have to fall back to trap on every memory access and determine if access is legal on runtime. Sounds familiar? This is the function of MMU or logical address. However, with logical address, a trap is incurred if and only if a illegal access has happened instead of every memory access.
It is simulated by the OS to programs as if they were using physical memory. The need of the extra layer (logical address) is necessary for data-integrity purposes. You can make the analogy of logical addresses as the language of OS for addresses because without this Mapping, OS would not be able to understand what are the "actual" addresses allowed to any program. To remove this ambiguity, logical address mapping is required so that the OS know what logical address maps to what physical addressing and whether that physical address location is allowed to that program. It performs the "integrity checks" on logical addresses and not on physical memory because you can check the integrity by changing the logical address and do manipulations but you cant really do the same on physical memory because it would affect the already running processes using the memory.
Also I would like to mention that the base register and limit register are loaded by executing privileged instructions and privileged instructions are executed in kernel mode and only operating system has access to kernel mode and therefore CPU cannot directly access the registers. I hope I helped a little :)
There are some things that you need to understand.
First of all a CPU is unable to access the physical memory directly. In order to calculate the physical address a CPU needs a logical address. Logical address is then used compute the physical address. So this is the basic need of logical addresses to access physical memory. Without logical address you cannot access it. This conversion is necessary. Suppose if there is a system which do not follow virtual/logical addresses, that system will become highly vulnerable to hacker or intruder as they can access physical memory directly and manipulate the useful data on any location.
Second thing, when a process runs, CPU generates logical address in order to load that process on main memory. Now the purpose of this logical address here is, the memory management. The size of registers are very less as compared to the actual size of process. So we need to relocate the memory in order to obtain the optimum efficiency. MMU (Memory Management Unit) comes into play here. Physical memory is calculated by MMU using the logical address. So logical addresses are generated by processes and MMU access physical address based on that logical address.
This example will make it clear.
If data is stored on address 50, base register holds the value 50 and offset holds 0. Now, MMU shifts it to address 100, this would be reflected in logical address as well. Offset becomes 100-50=50. So, now if data is needed to be retrieved via logical address, it goes to base address 50 and then see the offset i.e. 50, it goes to address 100 and access data. Logical address keeps the record of the data where it has been moved. No matter how many address locations that data change, it will be reflected in logical address and hence this logical address give accessibility to that data whatever physical address it holds now.
I hope it helps.
I want to know that why the CPU generates logical addresses and then maps them into Physical addresses with the help of memory manager? Why do we need them.
Virtual addresses are required to run several program on a computer.
Assume there is no virtual address mechanism. Compilers and link editors generate a memory layout with a given pattern. Instruction (text segment) are positioned in memory from address 0. Then are the segments for initialized or uninitialized data (data and bss) and the dynamic memory (heap and stack). (see for instance https://www.geeksforgeeks.org/memory-layout-of-c-program/ if you have no idea on memory layout)
When you run this program, it will occupy part of the memory that will no longer be available for other processes in a completely unpredictable way. For instance, addresses 0 to 1M will be occupied, or 0 to 16k, or 0 to 128M, it completely depends on the program characteristics.
If you now want to run concurrently a second program, where will its instructions and data go to memory? Memory addresses are generated by the compiler that obviously do not know at compile time what will be the free memory. And remember memory addresses (for instructions or data) are somehow hard-coded in the program code.
A second problem happens when you want to run many processes and that you run out of memory. In this situations, some processes are swapped out to disk and restored later. But when restored, a process will go where memory is free and again, it is something that is unpredictable and would require modifying internal addresses of the program.
Virtual memory simplifies all these tasks. When running a process (or restoring it after a swap), the system looks at free memory and fills page tables to create a mapping between virtual addresses (manipulated by the processor and always unchanged) and physical addresses (that depends on the free memory on the computer at a given time).
Logical address translation serves several functions.
One of these is to support the common mapping of a system address space to all processes. This makes it possible for any process to handle interrupt because the system addresses needed to handle interrupts are always in the same place, regardless of the process.
The logical translation system also handles page protection. This makes is possible to protect the common system address space from individual users messing with it. It also allows protecting the user address space, such as making code and data read only, to check for errors.
Logical translation is also a prerequisite for implementing virtual memory. In an virtual memory system, each process's address space is constructed in secondary storage (ie disk). Pages within the address space are brought into memory as needed. This kind of system would be impossible to implement if processes with large address spaces had to be mapped contiguously within memory.
I know that user program generates logical addresses.Suppose there is a small code snippet in C .When address is printed,the addresses are virtual addresses.My question is where are those addresses fetched from?where exactly do the allocated values and variables stay?At main memory or secondary memory?If main memory then why there is physical address?
User mode programs only see logical addresses. Only the operating system (kernel mode) sees physical memory.
My question is where are those addresses fetched from?
Those are the logical addresses assigned by the program loader ad linker.
where exactly do the allocated values and variables stay?At main memory or secondary memory?
In a virtual memory system, it may be in main memory or secondary storage.
If main memory then why there is physical address?
It is a logical address that is mapped to a physical address using page tables.
I am kind of new to the computer architecture and Operating System,
but I will try to answer as much as I can. As far as I have
understood about the logical address (Which I still have trouble
understanding, about where it is fetched from or where it is stored.
I mean these addresses (numbers) gotta be stored somewhere,
otherwise CPU can't generate it by itself, right?), these addresses
are assigned by CPU or a processor and depends on the CPU
architecture. Each process is assigned a virtual/logical
address. And this logical address is translated to physical address
by Memory Management Unit of CPU (MMU).
Where exactly do the allocated values and variables stay? As user3344003 said, it may be in main memory or secondary storage.
If main memory then why there is physical address? The reason lies
in the concept of Virtual Memory. Each process has its own virtual
address and a page table. Process's logical address are mapped
through this page table to the Physical memory (RAM). Whatever that
logical address is, it gets mapped to Physical address. If the
Physical memory gets full, then OS evicts some of the less used
or unused process to Secondary storage and puts the needed process
in the RAM. That way multiple process can run at the same time.
Every process assumes that they have all the space in RAM just for
themselves. If not for virtual memory, then physical memory would be
full and process might crash and may shut down the OS as well.
Hope it helps. I am still learning, if my understanding about Logical address and virtual memory is wrong then Please comment.
While understanding the concept of Paging in Memory Management, I came through the terms "logical memory" and "physical memory". Can anyone please tell me the diff. between the two ???
Does physical memory = Hard Disk
and logical memory = RAM
There are three related concepts here:
Physical -- An actual device
Logical -- A translation to a physical device
Virtual -- A simulation of a physical device
The term "logical memory" is rarely used because we normally use the term "virtual memory" to cover both the virtual and logical translations of memory.
In an address translation, we have a page index and a byte index into that page.
The page index to the Nth path in the process could be called a logical memory. The operating system redirects the ordinal page number into some arbitrary physical address.
The reason this is rarely called logical memory is that the page made be simulated using paging, becoming a virtual address.
Address transition is a combination of logical and virtual. The normal usage is to just call the whole thing "virtual memory."
We can imagine that in the future, as memory grows, that paging will go away entirely. Instead of having virtual memory systems we will have logical memory systems.
Not a lot of clarity here thus far, here goes:
Physical Memory is what the CPU addresses on its address bus. It's the lowest level software can get to. Physical memory is organized as a sequence of 8-bit bytes, each with a physical address.
Every application having to manage its memory at a physical level is obviously not feasible. So, since the early days, CPUs introduced abstractions of memory known collectively as "Memory Management." These are all optional, but ubiquitous, CPU features managed by your kernel:
Linear Memory is what user-level programs address in their code. It's seen as a contiguous addresses space, but behind the scenes each linear address maps to a physical address. This allows user-level programs to address memory in a common way and leaves the management of physical memory to the kernel.
However, it's not so simple. User-level programs address linear memory using different memory models. One you may have heard of is the segmented memory model. Under this model, programs address memory using logical addresses. Each logical address refers to a table entry which maps to a linear address space. In this way, the o/s can break up an application into different parts of memory as a security feature (details out of scope for here)
In Intel 64-bit (IA-32e, 64-bit submode), segmented memory is never used, and instead every program can address all 2^64 bytes of linear address space using a flat memory model. As the name implies, all of linear memory is available at a byte-accessible level. This is the most straightforward.
Finally we get to Virtual Memory. This is a feature of the CPU facilitated by the MMU, totally unseen to user-level programs, and managed by the kernel. It allows physical addresses to be mapped to virtual addresses, organized as tables of pages ("page tables"). When virtual memory ("paging") is enabled, tables can be loaded into the CPU, causing memory addresses referenced by a program to be translated to physical addresses transparently. Page tables are swapped in and out on the fly by the kernel when different programs are run. This allows for optimization and security in process/memory management (details out of scope for here)
Keep in mind, Linear and Virtual memory are independent features which can work in conjunction. If paging is disabled, linear addresses map one-to-one with physical addresses. When enabled, linear addresses are mapped to virtual memory.
Notes:
This is all linux/x86 specific but the same concepts apply almost everywhere.
There are a ton of details I glossed over
If you want to know more, read The Intel® 64 and IA-32 Architectures Software Developer Manual, from where I plagiarized most of this
I'd like to add a simple answer here.
Physical Memory : This is the memory that is actually present and every process needs space here to execute their code.
Logical Memory:
To a user program the memory seems contiguous,Suppose a program needs 100 MB of space in memory,To this program a virtual address space / Logical address space starts from 0 and continues to some finite number.This address is generated by CPU and then The MMU then maps this virtual address to real physical address through some page table or any other way the mapping is implemented.
Please correct me or add some more content here. Thanks !
Physical memory is RAM; Actually belongs to main memory. Logical address is the address generated by CPU. In paging,logical address is mapped into physical address with the help of page tables. Logical address contains page number and an offset address.
An address generated by the CPU is commonly referred to as a logical address, whereas an address seen by the memory unit—that is, the one loaded into the memory-address register of the memory—is commonly referred to as a physical address
The physical address is the actual address of the frame where each page will be placed, whereas the logical address is the address generated by the CPU for each page.
What exactly is a frame?
Processes are retrieved from secondary memory and stored in main memory using the paging storing technique.
Processes are kept in secondary memory as non-contiguous pages, which implies they are stored in random locations.
Those non-contiguous pages are retrieved into main Memory as a frame by the paging operating system.
The operating system divides the memory frame size equally in main memory, and all processes retrieved from secondary memory are stored concurrently.
An operating system/computer architecture question here. I was reading about caches, about how virtually indexing the cache is an option to reduce address translation time. I came across the following:
"Virtual cache difficulties include:
Aliasing
Two different virtual addresses may have the same physical address."
I can't think of a scenario when this can occur. It's been a while since my O/S days and I'm drawing a blank.
Could someone provide an example? Thanks
Two processes might have a shared mapping. E.g., in Unix, executable code is typically mapped into a region shared between all processes that execute the same program. (In fact, a single process might have several mappings of the same underlying memory, e.g. when it mmap's the same file twice.)
I believe that the executable sections of programs can possibly be shared between processes--thus being mapped twice.
For example: if you load two instances of vim, there will be two processes. Both process will likely map to the same executable code in physical memory.
shmat() is a typical example of same physical address being mapped as two different virtual address in two different processes.
If you do pmap -x pid_A .
you will you see the virtual mem map for process A similarly for Process B.
Actual Phy mem is not exposed to the user-space program.
Now SayProcess A and B share a shared memory segment and shared memory pointer be sh_mem_ptr_A and Sh_mem_ptr_B.
If you print these pointers their address(virtual) will be different.
Because Sh_mem_ptr_A is a part of memory map of Process A, Similarly sh_mem_ptr_B for Process B.
Kernel maintains the maaping of Virtual-to- phy addr. By page table and offset.
Higher bits map to the page table and offset maps to offset in the page table. So If you notice the Lower order bits of sh_mem_ptr_A and sh_mem_ptr_B they will be same(but may not be true always).
Also each process is allocated 4GB of virtual space (in 32 bit system), out of which 1 GB (depends upon Os to Os) is mapped for OS. Since OS is common for all processes, so the lower 1GB of virtual addresses are common for all the process, which are mapped to same OS physical pages.