Contiguous Allocation Policy and Address Translation
Duration: 9 min
This video lesson is available to enrolled students.
AI Summary
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This educational video lecture provides a detailed explanation of the contiguous allocation policy used in operating systems for memory management. The instructor begins by defining the policy, emphasizing that a process must be loaded entirely into main memory and stored in a contiguous block of addresses. He illustrates the data flow between the CPU, main memory, and secondary memory using hand-drawn diagrams. The lecture then explores the implications of this policy, specifically how processes are allocated within memory partitions and the constraints imposed by non-contiguous free space. Finally, the instructor introduces the Memory Management Unit (MMU) and the relocation register, explaining the mechanism of translating logical addresses to physical addresses by adding a base address to the logical address generated by the CPU.
Chapters
0:00 – 2:00 00:00-02:00
The video opens with a slide titled 'Contiguous allocation policy'. The instructor explains that when a process is executed, it must be loaded into main memory. He lists two key implications visible on the slide: 'It must be loaded to main memory completely for execution' and 'Must be stored in main memory in contiguous fashion'. To visualize this, he draws a diagram showing a box labeled 'CPU' pointing to 'M.M' (Main Memory), which in turn interacts with 'S.M' (Secondary Memory). He explains that the CPU generates logical addresses which are then translated into physical addresses in main memory.
2:00 – 5:00 02:00-05:00
The instructor elaborates on the 'contiguous fashion' requirement. He draws a representation of main memory divided into partitions. He presents a scenario where a process of size 50 needs to be loaded. He draws available memory blocks of size 30 and 40. He explains that even though the total free space (30 + 40 = 70) is greater than the process size (50), the process cannot be loaded because the free space is not contiguous. This demonstrates the fragmentation issue inherent in contiguous allocation, where a process requires a single continuous block of memory.
5:00 – 8:52 05:00-08:52
The lecture transitions to the hardware mechanism used to manage these addresses. A slide appears stating, 'Here we use a memory management unit(OS) whose duty is to take logical address and translate it into physical address.' The instructor draws a diagram of the MMU containing a 'relocation register' and an adder. He explains that the relocation register holds the base address of the process in main memory. He provides a numerical example: if the CPU generates a logical address of 42 and the base address in the relocation register is 100, the adder sums them to produce a physical address of 142. He also briefly mentions the advantage (simple strategy) and disadvantage (security risk if logical address exceeds limit) of this method.
The lecture systematically builds the concept of contiguous allocation, starting from the fundamental requirement that processes must reside in continuous memory blocks. It then demonstrates the practical limitations of this approach through memory partition examples, highlighting the inability to utilize fragmented free space. The lesson concludes by introducing the MMU and relocation register as the essential hardware components that enable this policy by mapping logical addresses to physical addresses, ensuring processes run in their allocated memory regions.