Autosave: 2024-07-08 07:00:04

zk/Memory_Management_Unit.md

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+---
+title: Memory_Management_Unit
+tags: [memory, Linux]
+created: Monday, July 08, 2024
+---
+
+# Memory_Management_Unit
+
+## Related notes

zk/VirtualMemory.md

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+---
+tags:
+  - memory
+  - Linux
+---
+
+# Virtual memory and the Memory Management Unit
+
+## What is virtual memory?
+
+Virtual memory is an abstraction of physical memory capacity and allocation that
+is accessible to user space. The kernel handles physical memory allocation and
+presents this to user space as a simplified and idealised representation of the
+available memory of the system.
+
+The main benefits:
+
+- User mode processes do not have to be concerned with the physical memory
+  management
+- There is a buffer between user mode processes and physical memory, meaning
+  that memory cannot be accidentally corrupted by other processes in user space.
+
+When a process writes or reads from a virtual memory address this does not
+directly refer to a hardware memory location. The kernel translates this into a
+physical memory address but this is opaque to the user space process. In fact,
+the physical memory addresses could be distributed accross multiple
+non-contiguous locations such as cache and swap memory, not just DRAM.
+
+Although the physical memory may be distributed and non-contiguous, from the
+viewpoint of user space, the available virtual memory is contiguous. Each user
+space process is presented with the same range of available memory addresses and
+the same total capacity.
+
+Because this is virtual, there is no risk of one process reading or overwriting
+the address of another. The same virtual address for multiple programs maps to
+different physical addresses.
+
+// Next: more memory offered than is physically available.
+![](/img/virtual-memory-diagram.jpg)

zk/Virtual_memory_and_the_MMU_in_Linux.md

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----
-tags:
-  - memory
-  - Linux
----
-
-# Virtual memory and the Memory Management Unit
-
-## What is virtual memory?
-
-Virtual memory is implemented at the level of the operating system and is an
-abstraction on top of 'real', physical memory (i.e the bits stored within the
-[DRAM](./Memory.md#DRAM).
-
-When virtual memory is used, the CPU handles physical memory allocation and
-presents this to the kernel as an idealised representation.
-
-This means that the kernel ( and, by extension, programs and processes) does not
-need to think about accessing the real memory blocks.
-
-This reduces complexity because often memory will be allocated in places that
-are non-contiguous with similar running processes and may even be located in the
-cache or in swap memory on the disk, rather than the actual main memory.
-
-Virtual memory presents a unified abstraction to the kernel over and above these
-specific memory locations.
-
-It would require considerable processing work for the kernel to be tracing these
-disparate memory sources at every instance. By working on an idealised
-(contiguous, unlimited) memory set the kernel can focus on task management and
-CPU sequencing as its primary task.
-
-![](/img/virtual-memory-diagram.jpg)
-
-The memory is idealised in that all locations are represented virtually as being
-contiguous (even when this is not physically the case). Secondly, quantities of
-available memory can be presented as much larger than is actually the case and
-which often exceed the physical memory limits of the device. This is achieved
-through paging, handled by the MMU.
-
-## The Memory Management Unit (MMU)
-
-Without an MMU, when the CPU accesses RAM, the actual RAM locations never
-change. The memory address is always the same physical location within the RAM.
-The MMU is a chip that sits between the CPU and RAM recalculating the actual
-memory address from the virtual memory location requested by the kernel.
-
-## Pages
-
-We use the term **pages** to denote blocks of virtual memory and to distinguish
-them from **addresses** as physical blocks. The MMU possesses a **page table**
-which is registry logging which pages correspond to which physical blocks.
-
-## Shared pages
-
-Virtual memory allows the sharing of files and memory by multiple processes.
-Crucially the shared data doesn't have to be within the address of each process,
-instead there is a reference in the page table that each process has access to
-the shared data.
-
-## Page faults
-
-There are two kinds of error that can occur with relation to paged memory:
-
-- minor page faults
-  - The desired page is in main memory but the MMU doesn't currently know where
-    it is
-- major page faults
-  - The desired page is not in main memory at all. Therefore the kernel must
-    fetch it from disk
-
-Minor page faults are very common and are to be expected; they resolve quickly.
-On the other hand too many major page faults can slow the system down both
-because of the time-costly process of fetching data from disk and because it
-demands more kernel resources to locate the missing page, which puts other
-processes on hold.