A CDE Definition
A software routine that is part of the operating system or network control program which performs the storing/retrieving or transmitting/receiving of data. It is also responsible for detecting a bad transfer of data caused by hardware or network malfunction and correcting it if possible. See also access mode.
Disk Access Methods
Indexed access methods are widely used to keep track of records in a file and files on disk. The index is a table of contents with pointers to the location of each file on the disk or each record within the file. A common approach is the indexed sequential access method (ISAM), which uses an index in sequential order that points to records stored in random order. For fastest retrieval, there direct access methods convert the record's identifying field, such as account number, into a physical storage address. The pure sequential method may also be used if direct access is not required. See ISAM, direct access method and sequential access method.
Communications Access Methods
Local area network (LAN) access methods, such as CSMA/CD in Ethernet, transfer data to and from connected computers on the network. These methods reference layers 1 and 2 of the OSI model. See CSMA/CD.
Mainframe access methods, such as IBM's TCAM and VTAM, transfer data between a host computer and remote terminals. These routines place the data into frames with appropriate control codes and reference layers 3, 4 and 5 of the OSI model. See TCAM and VTAM.
Tape Access Methods
Tapes use the sequential access method (SAM), which keeps records in order by a key field such as account number. Each record must be compared to find the desired one. Modern drives allow fast forwarding to index points where different groups of records are stored. This provides a direct access capability to sections of the tape, although very slow by comparison to disks.
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