In 8086 microprocessor, we segment the memory into segments of 64K each because of the 16 bit registers (Since a 20 bit address cannot be stored in the 16 bit register). These segments are categorized as code segment, data segment, stack segment and extra segment. This structure is similar to that of created by a process in operating system. Does that mean each process takes up memory equivalent to 4 segments which will be equivalent to 4*64K in case of 8086 ? And if this is true then by doing some more math we can say that only 4 process will be handled by a 8086 microprocessor at a time (i.e. one of the process will be running state and others would be in block or ready state) since maximum of 16 segments are possible (Total memory size / size of each segment = 1MB/64K = 16).
I have just started studying this and saw this equivalence between process and segments. Does any such connection between the segments of the memory and the memory structure of the process actually exists or it's just my crazy imagination ?
A little history helps. Early UNIX(tm) ran on the Digital pdp minicomputer family. The first circulated versions were V6 & V7, which were exclusive to the pdp-11 family. That family could support a whopping 256K of RAM; but the gp register set (used for address formation) were 16bits wide. There was a limited memory protection scheme in the processor, which permitted the kernel (supervisor) to have a separate address space from user (user); and instructions (addresses generated by pc) to be separate from data (generated by other means). This will probably get edited into the dust by pdp-11 fanbois.
At around this time, intel was rolling what was to become the 8086. Current 8-bit CPUs were already straining at a 64K address space limitation, and were using a concept called bank switching to increase that. In bank switching, some sub-ranges of the 64K address space could be re-pointed into a larger memory bank; so although you could carefully address much more memory. The Hitachi 64180 was one of the CPUs that incorporated this into its silicon; most used external memory controllers.
The 8086 addressing scheme was an amalgamation of these notions. You could produce an Operating System which supported dynamically relocated processes and shared text with up to 64K Instruction + 64K Data. The general idea was you take the segment registers out of the programming model, thus if the OS has to relocate the process, it knows that the process had no saved copy of the old segment value. The commercial OS QNX 1.x, 2.x provided this as a model; the later using the 286 extensions to protect against programs that played with the segment registers.
For programs that didn't care about such subtleties (Lotus 123, ...), you could use the segment registers to effectively create a 2^20 address space on the 8086. It is an ugly programming model in this mode because address formation is A=Seg*16+Base, so Seg=1,Base=0 and Seg=0,Base=16 resolve to the same address.
So, you aren't hallucinating, it was quite intentional, if more than a little half-arsed.
If we talk about Address Space of a process it is the virtual address range which includes static data, stack and heap memory for that particular process. And coming to Process Control Block (PCB) which is a data structure maintained by operating system for each process it manages, where PCB includes a lot of information about the process like process no., process state, program counter, list of open files, cpu scheduling info ...and more.
Now this is the point where I got confused that Address Space is also a memory which stores information about a process and similar thing is done by PCB too. Then how these are connected to each other. I am not able to visualize this in my mind. Why we have these two things existing simultaneously. Isn't it possible to achieve our goal just by using PCB?
Process Address space refer to memory regions the process is using. It typically consists of heap, stack, initialized data, uninitialized data and text. There are mainly two address spaces of a process -- logical and physical.
The PCB is a structure resides in kernel to track the state of process. One of the things PCB contain is memory information. In a typically system, PCB may contain information about pages the process has.
To answer your question, Process Address space is an idea built on top of PCB and many other things (such as page table).
I'm interested in operating systems topic and I have a dummy question. Standard PE executable files are linked to 0x400000. My question is how can operating system load multiply executables with same image base, when virtual memory just maps virtual addresses to physical. Is it storing PDE and PTE index of thread somewhere? Is there some addition to each address before execution starts? How does it work?
Each process gets its own virtual address space, and hence there's no conflict. All virtual address spaces that exist in any one time in the system get mapped into the physical address space. Virtual memory that can't or currently isn't mapped onto a particular physical memory is held in the swap file (swap partition, or alike) — this is called paging.
During thread switches, when the CPU is about to execute a thread from a different process than it was executing so far, the operating system's scheduler informs the CPU (sets the respective registers) about the new virtual address translation table to use. Thus the CPU thinks there's just one virtual address space at the given time, while the operating system can manage many more, one for each process.
Disclaimer: My answer may be a thought of as a bit superficial or imprecise as opposed to the reality. This for the sake of simplicity in respect to the nature of the OPs question. Also, these mechanisms are CPU-dependent and operating system-dependent.
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Per my understanding, virtual memory is as follows:
Programs/applications/executables reside in a storage device. Storage device access is much slower than RAM. Hence, programs is copied from storage memory to main memory for execution. Since computers have limited main memory (RAM), when all of the RAM is being used (e.g., if there are many programs open simultaneously or if one very large program is in use), a computer with virtual memory enabled will swap data to the HDD and back to memory as needed, thus, in effect, increasing the total system memory.
As far as I know, most embedded devices do not have disk memory (like smartphones or in car infotainment systems). Code is directly executed from Flash memory. RAM is mainly used as a scratchpad area (local variables, return address etc).
So why do we need virtual memory in embedded systems? (e.g. WinCE and QNX support virtual memory)
Your understanding is completely wrong. You are confusing virtual memory with swapping or page files. There are systems that have virtual memory and no swap or page files and there are systems that swap without virtual memory.
Virtual memory just means that a process has a view of memory that is different from the physical mapping. Among other things, it allows processes to have their own virtual address space.
Storage device access is much slower than RAM. Hence programs is copied from storage memory to main memory for execution. Since computers have limited main memory (RAM), when all of the RAM is being used (e.g., if there are many programs open simultaneously or if one very large program is in use), a computer with virtual memory enabled will swap data to the HDD and back to memory as needed, thus, in effect, increasing the total system memory.
That's swapping (or paging). It has nothing to do with virtual memory except that most modern operating systems implement swapping using virtual memory. Swapping actually existed before virtual memory.
I think you're probably incorrect about these devices running code directly from flash memory. The read speed of flash is pretty low and RAM is very cheap. My bet is that most of the systems you mention don't run code directly from flash and instead use virtual memory to fault code into RAM as needed.
embedded systems, the term itself has a wide range of applications. you could call a small microcontroller with flash program space measured in kbytes or less and ram measured in either bits or bytes (not enough to be kbytes) an embedded system. Likewise a tivo running a full blown operating system on a pretty much full blown computer motherboard (replace tivo with xbox as another example) as an embedded system. So you need to be less vague about your question. virtual memory has little to do with any of that its applications cross those boundaries.
There are many answers above, David S has the best of course that virtual memory simply means the memory address on one side of the virtual memory boundary is different than the physical address that is used on the other side of that boundary. Where, how, why, etc is there a boundary varies.
A popular use for virtual memory, and I might argue a primary use case is for operating systems. One benefit is that for example all applications could be compiled for the same address space, all applications might be compiled such that from the programs perspective they all start at say address 0x8000, and as far as that program when it runs and accesses memory it accesses stuff based on that address. A combination of the hardware and the operating system change that virtual address that the program is using to a physical address. If the operating system allows for multitasking, then each task might think they are in the same address space but the physical addresses are different for each of those tasks. I wont elaborate further on why using an assumed, fixed address space, is a benefit. Another aspect that operating systems use is memory management. Many MMU's will let you segment the memory however. If a user wants to allocate 100 Megabytes of memory the program may access in its virtual address space that 100 meg as if it were linear and in that address space it is linear, but that 100 meg might be broken down into say 4Kbyte chunks that are scattered all about the physical address space, not always likely but certainly technically possible that no two chunks of that physical memory is next to any other chunk of that 100 meg. your memory management doesnt necessarily have to try to keep large physical chunks of memory available for applications to allocate. Note not all MMUs are exactly the same and 4Kbytes is just an example. A third major benefit from virtual address space to an operating system is protection. If the application is bound to the virtual address space, it is often quite easy to prevent that application from touching the memory of any other application or the operating system. the application in this case would operate/execute at a proection level such that all accesses are considered virtual and have to go through a translation to physical, the tables that are used to define that virtual to physical can contain protection flags. If the application addresses a memory address in its virtual space that it has no business accessing, the hardware can trap that and let the operating system take action as to how to handle it (virtualize some hardware, pop up an error and kill the app, pop up a warning and not kill the app but at the same time feed the app bogus data for their transaction, etc).
There are lots of ways this can be used in an embedded system. first off many embedded systems run operating systems, so all of the above, ease of compiling the program for the address space, relative ease of memory management, and protection of the other applications and operating system and other benefits not mentioned. (virtualization being one, being able to enable/disable instruction/data caching on a block by block basis is another)
The bottom line though is what David S pointed out. virtual memory simply means the virtual address is not necessarily equal to the physical address, it can be but doesnt have to be, there is some boundary, some hardware, usually table driven, that translates the virtual address into a physical address. Lots of reasons why you would want to do this, since some embedded systems are indistinguishable from non-embedded systems any reason that applies to a non-embedded system can apply to an embedded system.
As much as folks may want you to believe that a system has a flat address space, it is often an illusion. In a microcontroller for example you might have multiple flash banks and one or more ram banks. Each of these banks has a physical, generally zero based address. Even if there is no mmu or anything else like that there is a place somewhere between the address bus on the processor and the address bus on the flash or ram memory that decodes the address on the processor and uses that to address into the specific memory bank. Often the lower bits match and upper bits are responsible for the bank choices (this is often the case with an mmu as well) so in that sense the processor is living in a virtual address space. (not limited to microcontrollers, this is generally how processors address busses are treated) With microcontrollers depending on a pin being pulled high or low or some other mechanism you might have a chip feature that allows one flash bank to be used to boot the processor or another. You might tie an input pin high and the processors built in bootloader allows you to access and debug the system for example reprogram the application flash. Or perhaps tie that line low and boot the application flash instead of the vendors debugger/boot flash. some chips get even more complicated letting you boot one flash then the program writes a register somewhere instantly changing the memory architecture moving things around, for example allowing ram to be used for the interrupt vector table so your application can be changed after boot rather than a vector table in flash that is not as easy to change at will.
now when you talk about virtual memory as far as swapping to and from a disk, that is a trick often employed by operating systems to give the illusion of having more ram. I mentioned that above under the category of virtualization. virtual memory in the sense that it isnt really there, I have X bytes but will let the software think there are Y bytes (where Y is larger than X) available. The operating system through the virtual tables used by the hardware, manages which memory chunks are tied to physical ram and are allowed to complete as is by the hardware, or are marked as not available in some way, causing an exception to the operating system, upon inspection the operating system determines that this is a valid address for this application, but the data behind this address has been swapped to disk. The operating system then finds through some algorithm another chunk of ram belonging to whomever (part of the algorithm) and it copies that chunk of ram to disk, marks the table related to that virtual to physical as not valid, then copies the desired chunk from disk to ram, marks that chunk as valid and lets the hardware complete the memory cycle.
Not any different than say how vmware or other virtual machines work. You can execute instructions natively on the hardware using virtual memory until such time as you cause an exception, the virtual machine might think you have an xyz network interface and might have a driver that is accessing a register in that xyz network interface, but the reality is you have no xyz hardware and/or you dont want the virtual machine applications to access that hardware, so you virtualize it, you trap that register access, and using software that simulates the hardware you fake that access and let the program on the virtual machine continue. This obviously not the only way to do virtual machines, but it is one way if the hardware supports it, to let a virtual machine run very fast as a percentage of the time it is actually running instructions on the hardware. The slowest way to virtualize of course is to virtualize everything including the processor, every instruction in that case would be simulated, this is quite slow but has its own features (virtualizing an arm system on an x86 or x86 on an arm, xyz on an abc, fill in the blanks). And if that is the type of virtual memory you are talking about in an embedded system, well if the embedded system is for the most part indistinguishable from a non-embedded system (an xbox or tivo for example) then well for the same reasons you could allow such a thing. If you were on a microcontroller, well the use cases there would generally mean if you needed more memory you would buy a bigger microcontroller, or add more memory to the system ,or change the needs of the application such that it doesnt need as much memory. there may be exceptions, but it mostly depends on your application and requirements, a general purpose or general purpose like system which allows for applications or their data to be larger than the available ram, will require some sort of solution. the microcontroller in your keyless entry key fob thing or in your tv remote control or clock radio or whatever normally would not have a need to allow "applications" to require more resources than are physically there.
The more important benefit of using virtual memory is that every process gets its own address space which is isolated from every other process's. That way virtual memory helps keep faults contained and improves security and stability. I should note that it is still possible for two processes to share a bit of memory, to facilitate communication (shared mem IPC).
Also you can do other tricks like conserving memory via mapping shared parts into more than one process's (libc comes to mind for embedded use) address space but only having it once in physical mem. Also this gives it a speed boost, you can even enhance it further the way linux does cheapen fork/clone by only copying the in kernel descriptors and leaving the memory image alone up until the first write access is done with a similar idea.
As a last benefit, in modern systems, it's common to do file I/O via mapping the file into the process space (cf. mmap for example).
It's interesting to note that one can get some of the benefits of "virtual memory" without needing a full-fledged MMU. The hardware requirements can sometimes be amazingly light. The PIC 16C505 has a 5-bit address space and 40 bytes of RAM; addresses 0x10 to 0x1F can map to either of two groups of 16 bytes of RAM. When writing an application which needed to manage two different data streams, I arranged so that all the variables associated with one data stream would be in the first group of 16 "switchable" memory locations, and those associated with the other would be at the corresponding addresses in the second group. I could then use the same code to manage both data streams. Simply set the banking bit one way, call the routine, set it the other way, and call the routine again.
One of the reasons Virtual Memory exists is so that your device can multitask. It can also act as your RAM does, thus taking the load off of your physical RAM and swapping the load back and forth.
Im interested in understanding how a computer allocates variables for physical memory vs files in virtual memory ( such as on a hard drive ), in terms of how does the computer determine know where to put data. It almost seems random in both memory storage types, but its not because it simply can't put data at a memory address or sector (any location) of a hard drive that's occupied or allocated for another process already. When I was studying how Norton's speed disk ( a program that de-fragments files on hard drives ) on my old W95 system, I noticed from the program's representation of hard drive's data ( a color coded visual map of different data types, e.g. swap files were always first at the top.), consisting of many files spread out all over the hard drive with empty unused areas. In addition some of these areas, I saw what looked like a mix of data and empty space showed a spotty pattern. I want to think its random for that to happen. Like wise, when I was studying the memory addresses of a simple program I wrote in C, I noticed that each version of my program after recompiling it after changes - showed different addresses for segments and offsets. I was expecting the computer to use the same address when I recompiled it. Sometimes the same address would be used, other times it was different. Again, I want to think its random also for memory locations to be chosen by programs. I thought that memory allocation or file writing was based on the first empty space available, written in a contiguous manner.
So my question is, I want to know how and what is it in the logic works of a common computer, that decides where it writes its data in such a arbitrary manner for either type of location (physical RAM or Dynamic )? What area of computer science (if not assembly language) would I need to study that would explains this, almost random behavior?
Thanks in Advance
Something broader and directly from computer science would be a linked list. http://en.wikipedia.org/wiki/Linked_list
Imagine if you had a linked list and simply added items to the end, these items might live linearly in memory or disk or whatever somewhere. But as you remove some items in the middle of the list by having say item number 7 point at item number 9 eliminating item number 8. As with memory allocation for allocs or virtual memory or hard drive sector allocation, etc how fast you fragment your storage has to do with the algorithm you use for allocating the next item.
file systems can/do use a link list type scheme to keep track of what sectors are tied to a single file. it is fast and easy to use the link list but deal with fragmentation. A much slower method would be to have no fragmentation but be constantly copying/moving files around to keep them on linear sectors.
malloc() allocation schemes and MMU allocation schemes also fall under this category. Basically any time you take something, slice it up into fractions and put a virtual interface in front of those fractions to give the appearance to the programmer/user that they are linear. Malloc() (not counting the virtual memory via the MMU) is the other way around allocating a number of linear chunks of those fractions to meed the alloc need, and having an alloc/free scheme that attempts to keep as many large chunks available, just in case, a bad malloc system is one where you have half of your memory free but the maximum malloc that works without an out of memory error is a malloc of a small fraction of that memory, say you have a gig free and can only allocate 4096 bytes.
You should look at virtual memory and TLB (translation lookaside buffer) or paging.
It is not trivial to implement virtual memory and paging. The performance of your whole system depends on it. If it's not done properly your system will thrash.
It is early morning here so Wikipedia will have to do for now: https://en.m.wikipedia.org/wiki/Translation_lookaside_buffer
EDIT:
Those coloured spots you saw in your defrag were chunks on your HDD. Each chunk is of some specified size. Depending on how fragmented your HDD is, you might have portions of your HDD that look like this:
*-*-***-***-*
where * means full, and - means empty
This (above) could be part of one application/file or multiple files; I will assume one file is split across those to simplify my example. At the end of each * there is a pointer to the next location where the next * chunk is (this is called a linked list). The more fragmented your HDD is (or memory) the more of these pointers to next chunk you will have. This in turn uses more space for next pointers instead of using space for data and the result is more overhead when reading that data. If this is a file on disk, you will have multiple seeks (which are bad because they're slow) if your data is not grouped together (locality principle). When you use defrag, it moves and groups all chunks together (as best as it can).
*-*-***-***-*
becomes
*********----
The OS decides paging and virtual memory addressing (and such). TLB is a hardware (a cache) that aids this process (it maps physical memory to virtual memory addresses for fast look up). The CPU communicates with the TLB via MMU
To answer your questions
You should study operating systems.
Yes the locations where to place your files on HDD are decided by the OS. If you deleted a file and download it again, there is no guarantee it will be placed in the same location-most likely not.
A nice summary of all these components and principles I mentioned here work: Click Here. It's a ppt with slides from a Real Time Operating Systems book (if I'm not mistaken the same exact one I used)