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+# Chapter 8 - Main Memory
+
+
+The `base register` holds the smallest legal physcial memory address;
+The `limit register` specifies the size of the range.
+
+E.g.
+
+accessible process memory (inclusive)
+* base: 300040
+* limit: 120900
+
+```plaintext
+          ---------------------------|
+        0 |                          |
+          |  Operating System        |
+          |                          |
+   256000 |--------------------------|
+          |                          |
+          | process                  |
+          |                          |
+   300040 |--------------------------| <--- (300040) base
+          |                          |
+          | process                  |
+          |                          |
+   420940 |--------------------------| <--- (120900) limit
+          |                          |
+          | process                  |
+          |                          |
+   880000 |--------------------------|
+          |                          |
+          |                          |
+          |                          |
+  1024000 |--------------------------|
+```
+
+
+Protection of memory space is accomplished by having the CPU hardware compare
+every address generated in user mode with registers.
+Any attempt by a program executing in user mode to access OS memory or
+other users' memory results in a trap to the operating system, which treats the
+attempt as fatal error.
+
+This scheme prevents a user program from modifying code of data structures of
+either the operating system or other users.
+
+The base and limit registers can only be loaded by the operating system which uses
+a special privileged instruction. Priviliged instructions can only be executed in
+kernel mode by the operating system.
+
+An operating system that provides multiprocessing must store the process state
+and load the process state when context switching.
+
+## Address Binding
+
+Usually, a program resides on a disk as a binary executable file.
+To be executed the program must be loaded into memory and placed within a process.
+
+The processes on the disk that are waiting to be brought into memory for execution form
+the `input queue`. When a process terminates its memory space is marked as available.
+
+Addresses in the source program are generally symbolic. A compiler typically `binds` these
+symbolic addresses to relocatable addresses (such as 14 bytes from the beginning of this module).
+The linkage editor or loader in turn binds the relocatable addresses to absolute addresses (such as
+74014).
+
+Each binding is a mapping from one address space to another.
+
+* Compile time binding: `absolute code` is generated to use hardcoded addresses.
+  * MS-DOS COM format does this.
+  * If location of address changes then the program is broken.
+* Load time binding: The compiler generates `relocatable code` where final binding is delayed until load time.
+  * If the starting address changes, then we need to reload the user code to adjust.
+* Execution time binding: binding can be delayed until run time but special hardware must be available for this scheme to work.