5.Internal file structure How to locate an offset within a file? Logical file (record)(vary in length) -Physical block(fixed size) Solution:Packing-packing a number of logical records into physical blocks. Pack unpack:convert between logical records and physical blocks Internal fragmentation will occur Logical records Physical blocks 4口”4814在4色，主)QC
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Internal file structure ▶ How to locate an offset within a file? ▶ Logical file (record) (vary in length) → Physical block (fixed size) ▶ Solution: Packing – packing a number of logical records into physical blocks. ▶ Pack & unpack: convert between logical records and physical blocks ▶ Internal fragmentation will occur Logical records Physical blocks
Outline Access Methods(访问方式) 4口”484在4色，主月QC
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Outline Access Methods (访问方式)
Access Methods(访问方式) Files store information.When it is used,this information must be accessed and read into computer memory On a logical perspective of users,access a file of records 1.Sequential Access(顺序访问方式) 2.Direct Access(直接访问方式) 3.Indexed Access(索引访问方式) 4口”484在4色，主月QC
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Access Methods (访问方式) ▶ Files store information. When it is used, this information must be accessed and read into computer memory ▶ On a logical perspective of users, access a file of records 1. Sequential Access (顺序访问方式) 2. Direct Access (直接访问方式) 3. Indexed Access (索引访问方式)
1.Sequential Access(l顺序访问方式) Sequential Access(顺序访问方式)：the simplest access method. Information in the file is processed in order,one record after the other. This is a most common access mode. For example:editors,compilers A tape model of file File operations the effect on file pointer read/write next reset rewind/forward n current position beginning end rewinc- ead or write→ 4口”4614在4，主9QC
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Sequential Access (顺序访问方式) ▶ Sequential Access (顺序访问方式): the simplest access method. Information in the file is processed in order, one record after the other. ▶ This is a most common access mode. For example: editors, compilers ▶ A tape model of file ▶ File operations & the effect on file pointer ▶ read/write next ▶ reset ▶ rewind/forward n beginning current position end rewind read or write