Why is virtual memory needed in embedded systems? [closed]

Question

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)

Answer 1

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.

Answer 2

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.