Question

Consider a computer system which uses segmentation. Explain any differences between:

-Binding of instructions and data at compile time versus the binding of instructions and data at load time

-Binding of instructions and data at load time versus the binding of instructions and data at execution time

Consider cases whenever the program is executed and what happens when the program could be swapped out and swapped back into memory.

Answer 1

Ans to part1:

Addresses may be represented in different ways during these steps.

• Addresses in the source program are generally symbolic (such as $count$).
• A compiler will typically bind these symbolic addresses to relocatable addresses (such as "14 bytes from the beginning of this module").
• The linkage editor or loader will in turn bind the relocatable addresses to absolute addresses (such as 74014).
• Each binding is a mapping from one address space to another.

Compile time binding:

The compiler translates symbolic addresses to absolute addresses. If you know at compile time where the process will reside in memory, then absolute code can be generated (Static).

It is therefore remarkable that compile time binding works only when the load memory location of the process image is known well in advance. It is used to generate the absolute code. In case, the starting location of the process changes, the code must be recompiled.

Load time binding:

The compiler translates symbolic addresses to relative (relocatable) addresses. The loader translates these to absolute addresses. If it is not known at compile time where the process will reside in memory, then the compiler must generate relocatable code (Static).

The process cannot move from one memory location to another during its execution.

Run time /Execution time binding:

If the process can be moved during its execution from one memory segment to another, then binding must be delayed until run time. The absolute addresses are generated by hardware. A special base register stores the starting address of the program. This address is added to each reference generated by the CPU. If the process image is moved, the base register is updated to reflect the new starting address of the process. Most general-purpose OSs use this method (Dynamic).

Note that if the load memory location for a process image is not known at compile time, a relocatable code is generated which can be bound either at load time or run time.

Static-new locations are determined before execution. Dynamic-new locations are determined during execution.

Ans to part 2:

A process can be swapped temporarily out of memory to a backing store,to free up the memory and then brought back into memory for continued execution. The combined sizes of the physical memory and the swap space is the amount of virtual memory available.

Swapping out is necessary for two important reasons.

First, when the system requires more memory than is physically available, the kernel swaps out less used pages and gives memory to the current application (process) that needs the memory immediately.

Second, a significant number of the pages used by an application during its startup phase may only be used for initialization and then never used again. The system can swap out those pages and free the memory for other applications or even for the disk cache. They are swapped in whenever required.

However, swapping does have a downside. Compared to memory, disks are very slow. Memory speeds can be measured in nanoseconds, while disks are measured in milliseconds, so accessing the disk can be tens of thousands times slower than accessing physical memory. The more swapping that occurs, the slower your system will be. Sometimes excessive swapping or thrashing occurs where a page is swapped out and then very soon swapped in and then swapped out again and so on. In such situations the system is struggling to find free memory and keep applications running at the same time. In this case only adding more RAM will help.