Introduction to Memory

1. Background

• Program must be brought (from disk) to memory and placed within process for it to be run
• Main memory and registers are only storage CPU can access directly
• Memory unit only sees a stream of addresses + read requests
• or address +data and write request
• Register access in one CPU clock (or less)
• Main memory can take many cycles (stalls)
• Cache sit between main memory and CPU registers
• Protection of memory required to ensure correct operation 

2. Address binding

• Addresses represented in different ways at different stages of a program's life
• source code addresses usually symbolic
• compile code addresses bind to relocated addresses
• i.e., 14 bytes from beginning of this module
• Linker or loader will bind relocatable addresses to absolute addresses
• Each binding maps one address space to another
• Address binding of instructions and data to memory addresses can happen at three different stages
• compile time: if memory location known a priori, absolute code can be generated; must recompile code if starting location changes
• load time: must generate relocatable code if memory location is not known at compile time
• execution time: binding delayed until run time if the process can be moved during its execution from one memory segment to another
• need hardware support for address maps (e..g, base and limit registers)

3. Multistep Processing of a user program

4. Logical v.s. Physical Address Space

• The concept of a logical address space that is bound to a separate physical address space is central to proper memory management
• logical address: generated by the CPU; also referred to as virtual address
• physical address: address seen by the memory unit
• Logical and physical addresses are the same in compile-time and load-time address-binding schemes; logical (virtual) and physical addresses differ in execution-time address-binding scheme.
• Logical address space is the set of all logical addresses generated by a program
• Physical address space is the set of all physical addresses generated by a program

5. Dynamic loading

• routine is not loaded until it is called
• better memory-space utilization; unused routine is never loaded
• all routines kept on disk is relocatable load format
• useful when large amounts of code are needed to handle infrequently occurring cases
• no special support from the operation system is required
• implemented through program design
• OS can help by providing libraries to implement dynamic loading

6. Dynamic Linking

• Static linking
• system libraries and program code combined by the loader into the binary program image
• Dynamic liking
• linking postponed until execution time
• small piece of code, stub, used to locate the appropriate memory-resident library rountine
• Stub replaces itself with the address of the routine, and executes the routine
• Operating system checks if routine is in processes' memory address
• if not in address space, add to address space
• Dynamic linking is particularly useful for libraries
• system also known as shared libraries
• Consider applicability to patching system libraries
• versioning may be needed

7. Base and Limit Registers

• A pair of base and limit registers define the logical address space

8. Hardware address protection with base and limit registers

9. Memory Management Unit (MMU)

• Hardware device that at run time maps virtual to physical address
• The user program deals with logical address; it never sees the real physical addresses
• execution-time binding occurs when reference is made to location in memory
• logical address bound to physical address

10. Dynamic relocation using a relocation register