Memory allocation/ Paging/ Segmentation/ Compaction/ Fit

• these methods are for dividing the main memory to run multi programming
  
• two techniques for organizing, allocating memory
  

✅ Memory

1. Continuous memory
   
2. Incontinuous memory
   

✅ Paging

used for incontinuous memory allocation
divide into small processes of same size

• 운영 체제에서 외부 단편화 문제 해결 위해 사용(external fragmentation)
• 👍🏻 can use needed data as fast as possible

• In physical memory, Process A and Process B are not all together
• processes are not places continuously
• 👍🏻 no wasted space
• 👍🏻 Prevent external fragmentation

• 👍🏻 used more frequently than segmentation(no holes, no external fragmentation)

• 👎🏻 less secure, difficult sharing than segmentation
• divide code+data+stack into same size
• areas can be mixed while dividing
• areas can be mixed while sharing

✅ Segmentation

also used for incontinuous memory allocation
divide into small processes of various size

• devide memory into logical unit called segment
• segments are various size
• allocate memory, change physical address into logical address
• 👍🏻 no internal segmentation,

• 👎🏻 but external segmentation can occur

• 👍🏻 better than paging for security, sharing
• segmentation adds read/write/execute authorization to table logically
• to configure bit is simple, sage

• divided flexibly, areas do not mix while sharing

• 👎🏻 various segment size
• more holes occur, more external segmentation

☑️ Internal Segmentation

When process < memory

• process is saved, but there is left memory
• 20MB is too small to save any other process,
• left

☑️ External Segmentation

Also process < memory, but not continuous

• memory to save process is more than enough
• but not continuous
• process cannot be allocated on memory
• 💊 compaction

💊 Compaction

to solve external segmentation issue
make two seperated memory continuous

✅ Fit 🟰 Memory allocation Algorithm

allocate left memory to process

✔️ First Fit

• allocate first memory found to be available

✔️ Best Fit

• allocate the most fitting memory
• smallest left space
• internal segmentation ⬇️

✔️ Worst Fit

• allocate the wost fitting memory

✔️ Next Fit

• begin search from the last allocated place

• A: first fit
• B: best fit
• C: worst fit