I understand that every process in a computer has an address space, which contains addresses for all the instructions of the process. I also understand that this address is a virtual one - meaning it maps to the physical RAM addresses, and doesn't actually exist.
When a process is to be executed, the CPU sends the MMU the virtual address of the instruction (the page I guess?) that it needs, and then MMU can provide the physical address of it in RAM, which is what the CPU fetches to execute the instruction.
What I don't get is who assigns that virtual address to the process in the first place - how does the CPU know the virtual address? I get that these virtual addresses are generic (I read: Difference between logical addresses, and physical addresses?, and it was very helpful in that aspect), but I don't get who assigns that to begin with. Where even are the page tables stored, and why is the swap space needed?
I've been trying to understand memory mapping for a few days now, mostly through youtube videos, but if anyone knows any good sources I'd really appreciate it!
On older time with virtual address, you have a fixed layout: some data at beginning, the program, the program data, libraries, and than stack. Sometime Kernel had the last bytes reserved. Note: ometime with different layout, but usually same structure on the same OS/version. We had also the possibility to prelink libraries, so you can have quick loading time. Virtual address allow this.
Now it is the kernel that randomize the addresses (to reduce the success rate of buffer overflow attack). So you may have libraries and programs in random places. It is not the MMU, the kernel choose the virtual addresses randomly.
So now you have the virtual address. Now the kernel should assign it to something. If you clone, the virtual address point to the parent process, or when you execute the program, your virtual address point to disk (where the binary reside), and some (the rest, and maybe some guard) are just unassigned, so that program will get a segmentation fault (really a protection exception of CPU). This "physical pointer" could change. Because of swapping the physical address could change, or the pointer could point to different part of disk, but also you may point to old process memory, and then to your modified memory, or to different part of disk.
So virtual addresses are defined by kernel and kernel will handle it (either having a physical address, or an exception, which kernel may check a different source [other process memory, disk]). If the page (virtual address block) has linked a physical address, it is the MMU which do the works (and updating also access count of the page), else it is the kernel (and so much slower).
Related
A page, memory page, or virtual page is a fixed-length contiguous block of virtual memory, described by a single entry in the page table.
I wamna know if kernel memory also can be pagable?
Yes, e.g. on architectures with an MMU every virtual address (user space and kernel space) is translated by the MMU. There is an area where the kernel is directly mapped, i.e. the virtual address is at a fixed offset from their physical address.
When for example a system call needs to access an address in kernel space, the page table of the last process that ran is used. It does not matter which one, since the kernel space is shared between all processes and thus is the same for all.
There is one case where physical addresses are used directly and that is in the boot process before paging is enabled.
As Giacomo Catenazzi mentioned correctly in the comments, these pages are handled differently, e.g. they can not be swapped out.
There is one case where physical addresses are used directly and that is in the boot process before paging is enabled.
On systems with MMU and support for virtual memory there can be
multiple ELFs with the same entry point loaded at the same time as
their entry points are just virtual addresses and are translated to
physical memory address at runtime. For example, on my amd64
machine .text section is always mapped at address 0x00400000 and
_start is always close to that address. But how does it work on systems
without MMU? Many of them probably don't support multitasking at
all. Is it developers' responsibility to pick ELF entry points by
hand so that they don't overlap?
Short story, it does not handled anyhow.
And now there is a long story.
ELF is not executable as is, it's sort of container with executable data. In systems with OS and MMU, OS creates a process and respective MMU page tables for that process. After that read ELF and copy executable code and data sections (BSS) into this newly allocated process memory according to data in ELF file. Once that done, program counter is set to entry point address. You program runs. Hooray.
Important point to highlight is that every process has it's own virtual memory which is mapped into unique physical memory. So virtual addresses could overlap or be the same for different processes, while physical addresses are always different at any given moment.
Now there is a system without MMU. So there is no virtual memory, every process should be placed in unique physical memory region and linked to that precise memory region.
In real life if there is some small system w/o MMU ELF files simply not used. Option 1, all apps linked together into one big binary. Option 2, apps linked separately with unique addresses, executable information is extracted with some 'objcopy' util, and that binaries copied into system for execution. 'OS' should have a list of entry point to start these 'processes'.
In a modern PC, where will
MOV [0x0000], 7
put a 7? Is it the first byte of my RAM, or is it the first byte of the process's address space? Assuming it triggers a memory violation.
You mean assuming it doesn't trigger an access violation? Every process has it's own virtual address space. The first 64kiB are normally kept unmapped, so NULL-pointer accesses actually fault noisily, instead of letting programs silently do Bad Things.
In a user-space process on a typical OS, an absolute address of 0 does refer to the first byte of your process's virtual address space.
With paging enabled, there's no way even for the kernel to use physical addresses directly. To write to a given physical address, would have to create a page table entry mapping that physical page to a virtual page (or find an existing mapping), invlpg to make sure the TLB isn't caching a stale entry, and then use that virtual address.
it depends on the system architecture. Every architecture provides an instruction set and a memory layout. Furthermore it depends on the operating system you use. E.g. Real Time Operating systems often do not provide Virtual Memory.
greets
What is the purpose of logical address? Why should CPU generate logical address? it can directly access relocatable register base address and limit to exe a process. Why should MMU make a mapping between logical and physical address?
Why?
Because this gives the Operating System a way to securely manage memory.
Why is secure memory management necessary?
Imagine if there was no logical addressing. All processes were given direct access to physical addresses. A multi-process OS runs several different programs simultaneously. Imagine you are editing an important letter in MS Word while listening to music on YouTube on a very recently released browser. The browser is buggy and writes bogus values to a range of physical addresses that were being used by the Word program to store the edits of your letter. All of that information is corrupt!
Highly undesirable situation.
How can the OS prevent this?
Maintain a mapping of physical addresses allocated to each process and make sure one process cannot access the memory allocated to another process!
Clearly, having actual physical addresses exposed to programs is not a good idea. Since memory is then handled totally by the OS, we need an abstraction that we can provide to processes with a simple API that would make it seem that the process was dealing with physical memory, but all allocations would actually be handled by the OS.
Here comes virtual memory!
The need of logical address is to securely manage our physical memory.
Logical address is used to reference to access the physical memory location.
A logical address is generated so that a user program never directly access the physical memory and the process donot occupies memory which is acquired by another process thus corrupting that process.
A logical address gives us a surety that a new process will not occupy memory space occupied by any other process.
In execution time binding, the MMU makes a mapping from logical address to physical address because in this type of binding:
logical address is specifically referred to as virtual address
The address actually has no meaning because it is there to illusion the user that it has a large memory for its processes. The address actually bear meaning when mapping occurs and they get some real addresses which are present in physical 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.
So first the CPU will generate the logical address then the MMU of Operating system will take over and do the mapping.
The binding of instruction and data of a process to memory is done at compile time, load time or at execution time. Logical address comes into picture, only if the process moved during its execution time from one memory segment to another. logical address is the address of a process, before any relocation happens(memory address = 10). Once relocation happened for a process(moved to memory address = 100), just to redirect the cpu to correct memory location=> memory management unit(MMU), maintains the difference between relocated address and original address(100-10 = 90) in relocation register(base register acts as relocation register here) . once CPU have to access data in memory address 10, MMU add 90(value in relocation register) to the address, and fetch data from memory address 100.
Consider the following CPU instruction which takes the memory at address 16777386 (decimal) and stores it in Register 1:
Move &0x010000AA, R1
Traditionally programs are translated to assembly (machine code) at compile time. (Let's ignore more complex modern systems like jitting).
However, if this address allocation is completed statically at compile time, how does the OS ensure that two processes do not use the same memory? (eg if you ran the same compiled program twice concurrently).
Question:
How, and when, does a program get its memory addresses assigned?
Virtual Memory:
I understand most (if not all) modern systems use Memory Management Units in hardware to allow for the use of virtual memory. The first few octets of an address space being used to reference which page. This would allow for memory protection if each process used different pages. However, if this is how memory protection is enforced, the original question still persists, only this time with how page numbers are assigned?
EDIT:
CPU:
One possibility is the CPU can handle memory protection by enforcing that a process id be assigned by the OS before executing memory based instructions. However, this is only speculation, and requires support in hardware by the CPU architecture, something I'm not sure RISC ISAs would be designed to do.
With virtual memory each process has separate address space, so 0x010000AA in one process will refer to different value than in another process.
Address spaces are implemented with kernel-controlled page tables that processor uses to translate virtual page addresses to physical ones. Having two processes using the same address page number is not an issue, since the processes have separate page tables and physical memory mapped can be different.
Usually executable code and global variables will be mapped statically, stack will be mapped at random address (some exploits are more difficult that way) and dynamic allocation routines will use syscalls to map more pages.
(ignoring the Unix fork) The initial state of a processes memory is set up by the executable loader. The linker defines the initial memory state and the loader creates it. That state usually includes memory to static data, executable code, writeable data, and the stack.
In most systems a process can modify the address space by adding pages (possibly removing them as well).
[Ignoring system addresses] In virtual (logical) memory systems each process has an address space starting at zero (usually the first page is not mapped). The address space is divided into pages. The operating system maps (and remaps) logical pages to physical pages.
Address 0x010000AA in one process is then a difference physical memory address in each process.