Memory management - Revision history

Mziwisky: typo (in Memory Allocators: "per-thread", not "pre-thread")

2011-10-06T19:50:02Z

typo (in Memory Allocators: "per-thread", not "pre-thread")

Mziwisky: Grammar.

2011-09-09T18:02:11Z

Grammar.

Michael: /* Memory Protection */ Fleshed out memory protection

2009-07-29T22:37:47Z

Memory Protection: Fleshed out memory protection

Michael: /* Memory Allocators */ Defined the memory allocators

2009-07-29T22:00:53Z

Memory Allocators: Defined the memory allocators

Michael: Added stubs for memory management page

2009-07-29T20:32:45Z

Added stubs for memory management page

New page

Memory management is an important aspect of any operating system, as such [[Embedded Xinu]] makes use of some aspects of the underlying hardware to build up a simple to understand memory management system.

== Memory Allocators ==

Embedded Xinu uses two main memory allocators that work in tandem to provide dynamic memory to both kernel and user software.

=== Kernel Allocator ===

The kernel allocator trusts the programmer to keep track of accounting information on the stack and provides no mapping or protection.

=== User Allocator ===

The user allocator stores accounting information adjacent to the allocated memory and provides distinct address spaces for each running thread.

==== Region Allocator ====

The region allocator runs below the user allocator and provides more page aligned memory regions upon request.

== Memory Protection ==

Since Embedded Xinu has limited resources to work with it does not provide a virtual memory system. It does take advantage of separate address spaces for each user thread running in the system, which provides simple memory protection for low overhead costs.

=== Translation Lookaside Buffer ===

To facilitate memory protect Embedded Xinu uses the translation lookaside buffer (TLB) build into the MIPS processors of the [[WRT54GL]] series of routers.

@@ Line 3: / Line 3: @@
 == Memory Allocators ==
-Embedded Xinu maintains two memory allocators that work in tandem to provide dynamic memory to both kernel and user software.  The first allocator is the kernel allocator which allocates small chunks of memory from the global memory heap as needed by the kernel.  The second allocator is a user allocator, that allocates memory from a pre-thread memory heap as needed by user processes.
+Embedded Xinu maintains two memory allocators that work in tandem to provide dynamic memory to both kernel and user software.  The first allocator is the kernel allocator which allocates small chunks of memory from the global memory heap as needed by the kernel.  The second allocator is a user allocator, that allocates memory from a per-thread memory heap as needed by user processes.
 === Kernel Allocator ===

← Older revision		Revision as of 18:02, 9 September 2011
Line 1:		Line 1:
−	Memory management is an important aspect of any operating system, ~~as such~~ [[Embedded Xinu]] makes use of some aspects of the underlying hardware to build up a simple to understand memory management system.	+	Memory management is an important aspect of any operating system. As such, [[Embedded Xinu]] makes use of some aspects of the underlying hardware to build up a simple-to-understand memory management system.

	== Memory Allocators ==		== Memory Allocators ==

@@ Line 37: / Line 37: @@
 == Memory Protection ==
-Since Embedded Xinu has limited resources to work with it does not provide a virtual memory system.  It does take advantage of separate address spaces for each user thread running in the system, which provides simple memory protection for low overhead costs.
+Since Embedded Xinu has limited resources to work with it does not provide a virtual memory system.  It does take advantage of separate address spaces for each user thread running in the system, which provides simple memory protection for low overhead costs.  As such, when allocating pages to the thread via the user allocator those pages will be mapped to the protection domain of the currently running thread.  These protection domains are inserted into a single global page table, that hold all the page table entries and the address space identifier of the protected page.
 === Translation Lookaside Buffer ===
-To facilitate memory protect Embedded Xinu uses the translation lookaside buffer (TLB) build into the MIPS processors of the [[WRT54GL]] series of routers.
+To facilitate memory protection, Embedded Xinu uses the translation lookaside buffer (TLB) built into the MIPS processors of the [[WRT54GL]] series of routers.  When a piece of software attempts to access memory in the [[Memory#User Segment|user segment]], a TLB load or store exception will occur.  When this occurs the processor jumps to a specific exception handler which allows the kernel to look up the page table entry, check if the faulting thread is in the same memory space as the entry, and load the entry into the TLB.  If there is no mapping or the thread is not in the same address space, a memory protection violation occurs and the thread is killed.

@@ Line 3: / Line 3: @@
 == Memory Allocators ==
-Embedded Xinu uses two main memory allocators that work in tandem to provide dynamic memory to both kernel and user software.
+Embedded Xinu maintains two memory allocators that work in tandem to provide dynamic memory to both kernel and user software.  The first allocator is the kernel allocator which allocates small chunks of memory from the global memory heap as needed by the kernel.  The second allocator is a user allocator, that allocates memory from a pre-thread memory heap as needed by user processes.
 === Kernel Allocator ===
-The kernel allocator trusts the programmer to keep track of accounting information on the stack and provides no mapping or protection.
+The most basic memory allocator in the system is the kernel allocator which uses the <code>memget</code> and <code>memfree</code> functions.  This operates on the global kernel heap that uses the <code>memlist</code> global variable.  In this allocator the kernel developer is trusted to keep track of the accounting information for memory blocks.  This makes a rather straightforward API.
 === User Allocator ===
-The user allocator stores accounting information adjacent to the allocated memory and provides distinct address spaces for each running thread.
+Unlike the kernel allocator, the user allocator does not trust the programmer to remember the amount of memory requested and instead stores the accounting information immediately before the allocated memory.  To the programmer the API for user memory is simply:
 ==== Region Allocator ====
-The region allocator runs below the user allocator and provides more page aligned memory regions upon request.
+The region allocator works beneath the user allocator and is initialized during the boot process.  During system boot Embedded Xinu uses <code>UHEAP_SIZE</code> as defined in <code>xinu.conf</code> to allocate memory for the user heap.  This memory is allocated via the kernel <code>memget()</code> function and is then passed to the <code>memRegionInit()</code> function.  Once the region allocator is initialized, the only user level interface to the region allocator is hidden behind the <code>malloc</code> and <code>free</code> routines.
 == Memory Protection ==