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9 Cards in this Set
- Front
- Back
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Memory Allocation Schemes |
1. Single-User systems 2. Fixed partitions 3. Dynamic partitions 4. Relocatable dynamic partitions |
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Single-User Contiguous Scheme |
Whole job/program loaded into memory. Allocated as much contiguous space in memory it needs. Jobs/programs processed sequentially. Finite memory, if too large, execution moves to next process. Program may be modified to fit by making it smaller. Multiprogramming is unsupported. |
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Fixed Partitions |
Contiguous loading of entire program 1. Supports multiprogramming 2. Protects each job's memory space 3. Memory space allocated to jobs based on info. in a table 4. Allocation method: Loaded into first available partition big enough Small partitions have long turnaround times while large ones waste memory. Internal fragmentation: less-than-complete use of memory space in a fixed partition. Unused memory space in the partition remains idle. Partitions allocated to one job at a time. 5. To work well: Alljobs have similar size and memory size is known ahead of time |
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Dynamic Partitions |
1. Memory allocated in one contiguous block. Jobs given memory as needed when loaded for processing. 2. As new jobs enter after the first few, they are allocated space based on first-come, first-served: loaded into first available partition. 3. Free memory between partitions of allocated memory of allocated memory: external fragmentation. 4. Memory requirements less than size of previous jobs. |
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Free Space Allocation Methods |
Best Fit (best use of memory space) First Fit (faster) OS maintains list of memory locations, tracking which ones are free or busy. Combo yields best system performance. |
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Best-Fit |
Free/Busy lists ordered by size (small to large) Job/Program assigned to smallest memory block it fits. Least wasted space. |
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First-Fit |
Organized by memory locations: low to high-order-memory Job/Program assigned to first memory block it fits. |
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Deallocation |
Release allocated memory space FP: reset status to free w/ codes DP: combines free areas of memory Depending on position of block to be deallocated: 1. UB Isolated from other free blocks 2. UB Between two free blocks 3. UB Adjacent to another free block |
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Relocatable Dynamic Partitions |
Relocates programs. 1. Contiguous 2. Address/reference to address adjusted to account for new location in memory 3. OS flags addresses so they can be adjusted if/when a program is relocated. Empty blocks compacted to make one large enough for some or all waiting jobs. |
