COE 628 Lecture Notes - Lecture 8: Memory Management Unit, Logical Address, Limit Up

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March 27th, 2017
Chapter 7 in textbook!
Simple MMU – memory management unit
CPU  logical address  MMU  Physical address  memory
Base limit
- Simple MMU – every time there is a process switch, and there is a context switch, MMU needs
to be reprogrammed for the running task. The physical address = logical address + base.
Logic address –(translate)  added to relocation address
- If there is an error, then there is an interrupt
An address is followed my segment number and an offset
- Data, stack, text, heap
Descriptor consists of base, limit, permissions, present, write, user mode operation
- Every single descriptor has its own base limit
- Up to 64k descriptors, roughly 4 bytes long, less than 300 kB to hold the entire table. Held on
chip
- Easy to share between different process, useful for dynamically linked libraries (ie. Printf)
Recall 2 modes of operation (kernel vs user)
Segmentation is good because:
- Easy to share
- Small table
Segmentation is bad because
- Segments can be huge, this is a problem because the format of the code
- Ex. Initialization of variables is long, but the calculations are short as they occur in a loop
- If this is the case then after initialization, we are wasting memory because the segmented
memory architecture. We use more memory than needed once the program is running.
Segmentation is the first type of memory management
Logical address vs Physical address
- Physical address is accessing the physical memory
- Paging – is with virtual address and virtual memory instead of logical memory.
- Page – consists of a certain number of bytes
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