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Memory (:pineapple:Swapping (backing storage (:icecream:Linux, UNIX交换区,…

- - - - :one:<segment-number, offset>
        
        16 bytes for segment-number as well as offset
      - :two:<limit, base>
        
        :one:limit
        
        :explode:the maximum bytes the segment can provide
        
        :warning:if offset > limit, the a trap is caused, and addressing error
        
        :two:base
        
        :explode:the physical location
    - - :snowflake:basic ideas
      - :one:segment-number
        
        :explode:represents the subscript in the segment table
      - :two:
      - :icecream:example
  - - - :one:Segment-table base register(STBR)
        
        :explode:points to the segment table's location in memory
      - :two:Segment-table length register(STLR)
        
        :explode:indicates the number of segments used by a program
        
        :arrow_right:segment number s is legal only if s < STLR
    - - :explode:<base, limit>
      - :one:base
        
        :explode:contains the starting physical address where the segment reside in memory
      - :two:limit
        
        :arrow_right:offset is legal if "offset" < limit
        
        :explode:the length of the segment
- - - - :one:确认十六进制下的页面大小
        
        :explode:4KB => 1000..0 (二进制) => 1000H
      - :two:根据一个页面大小知道右数第4个数字表示第几个页面,所以虚地址2362表示虚页面是2
      - :three:确定指定虚页面的物理页面起始地址,并且加上偏移地址
        
        :explode:254H * 1000H + 362H = 254362H
    - - :one:页号表示虚拟地址的页号
      - :two:Page Frame表示物理内存地址的页号
- - - - :two:Load time
        
        link & load
      - :three:Execution time
      - :one:Compile time
      - :warning:a system using virtual memory does address binding during execution time
    - - :bread:PCB中有个页目录指针,页目录指针指向页目录;页目录有很多指针,这些指针指向多个页表;页表里面也有许多指针,这些指针指向物理页面(里面存储数据,为内存单元)
      - :warning:新线程有独立的PCB,但是指向页目录的指针是和主线程基本一样的=>虚拟地址空间是基本一样
    - - work with limit register
      - \( How\ to\ work \)
        
        CPU => addr1
        
        :one:if addr1 > limit register
        
        TRAP
        
        :two:else
        
        \( physical address = addr1 + limit register \)
        
        :dagger_knife:limit register
        
        :explode:defines the range of logical address
  - - - :star:用户空间是3级,内核空间是0级.级别越低越严格
- - - - :question:Why do we need fixed partitioning given that our system always change its status
      - :question:How to partitioning
      - :moneybag:OS's + unused memory + partition unused + P's
      - What is the name of memory that is allocated but haven't been used by the process and memory that is not allocated yet
    - - \( When \)
        
        :explode:A process is loaded into the main memory
      - \( What \)
        
        把可用内存切出连续的区域分配给该进程
      - :cherry_blossom:algorithm
        
        :one:First fit
        
        地址从小到大
        
        空闲分区表
        
        :question:别人的说法是First fit指在空闲分区表上的地址从低到高来寻找,但是这个空闲分区表是什么东西?是维护在内存里的吗?是OS维护的吗?里面的元素是不是指向空闲frame的指针
        
        :two:Best fit
        
        :three:Worst Fit
        
        :four:Next Fit
        
        :fire:从上次结束的地方开始
        
        :star:If a process cannot fit to any partition, then it will be kept in the waiting queue
  - - - :one:Internal Fragmentation
        
        wasted memory within a single process memory space
        
        :zap:进程不可能总是整除frame大小,其中很小的一部分就会导致碎片
        
        :zap:fixed-size partition & paging & segmentation with paging
      - :two:External Fragmentation
        
        :arrow_right:reduces #processes
        
        :maple_leaf:Compaction
        
        :explode:organize all free memory blocks together in one larger block
        
        :zap:variable-size partition & segmentation, multiple partition