Autosave: 2024-07-16 17:00:03

zk/Memory_Management_Unit.md

@@ -4,6 +4,41 @@ tags: [memory, Linux]
 created: Monday, July 08, 2024
 ---
 
-# Memory_Management_Unit
+# Memory Management Unit (MMU)
 
-## Related notes
+The MMU is the bridge between the physical memory devices of the machine and the
+virtual memory space. It is a chip that sits between the CPU and the RAM and
+determines the physical location of the memory requested by the kernel as
+virtual memory.
+
+![Virtual memory diagram](/img/virtual-memory-diagram.jpg)
+
+## 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.

zk/VirtualMemory.md

@@ -45,17 +45,33 @@ _How Computers Really Work_ (2021) p.206
 
 ## Virtual memory and the kernel
 
-The kernel itself utilises virtual memory. The kernel virtual memory has a
-different range of virtual addresses to work with than user space virtual
-memory.
+The kernel itself utilises virtual memory.
+
+During the [boot_process](Boot_process.md), it runs in physical memory mode
+however it rapidly creates a 1:1 mapping of physical to virtual memory for
+itself. This direct mapping allows the kernel to easily access any physical
+memory location as needed, whilst retaining the benefits of virtual memory.
+
+With its own virtual memory established, it can then build additional virtual
+memory structures for itself and user processes.
+
+While the kernel uses virtual memory it can also access physical memory at any
+time wherease user space processes can only ever access virtual memory.
+
+The kernel virtual memory has a different range of virtual addresses to work
+with than user space virtual memory.
 
 Unlike user space virtual memory, the kernel has access to everything running in
 kernel address space whereas processes in user address space are partitioned
 from each other with separate address spaces that cannot interact.
 
-// Next: the kernel also uses virtual memory however isn't also responsible for
-the appportioning of virtual memory. Confused.
+In addition to being able to access its own virtual memory (and physical memory,
+as required) the kernel can also access any user processes' virtual address.
 
-// See Claude convo
+This said, the virtual memory space of the kernel and the virtual memory space
+of the user processes are distinct. It is not the case that the kernel is
+superset of all available virtual memory. It can access user space virtual
+memory because it sets up the tables and locations, not because it is a subset
+of its own virtual memory.
 
 ![Virtual memory diagram](/img/virtual-memory-diagram.jpg)