2.2 The IBM 7171 ASCII Device Attachment Control Unit - Index of

6.2.5 Read CommandsRead Buffer CommandThe Read type commands executed by the IBM 7171 are Read Buffer and ReadModified. Read Buffer causes the entire buffer contents of the addressed terminalto be read into main storage. The operation initiated by Read Modified is determinedby display station operator actions. The information read during executionof Read Modified could consist of fields of data modified by keyboard operations,buffer addresses, data of CURSOR SELECT fields, or the code of a ProgramFunction or Program Access key.An operator action that requires program interaction causes an attention interruption;the program would respond to this attention interruption with a readcommand.Programming Note: Unsolicited read commands are not recommended because theinformation read by these commands may be incomplete. 5Execution of the Read Buffer command causes all data in the addressed devicebuffer, from the buffer location at which reading starts through the last bufferlocation, to be transferred to main storage. This command is provided primarily fordiagnostic purposes. The transfer of data begins from the current buffer address ifthe Read Buffer command is chained. Regardless of where the transfer of databegins, data transfer from the buffer will terminate when the last character locationin the buffer has been transferred, or before the last character location has beentransferred when the channel byte count reaches 0 (in this case, the buffer addressafter termination is undefined).The transferred data stream begins with a three character read heading consistingof the AID character followed by a two character cursor address. The contents ofall buffer locations are transferred, including nulls. Start Field (SF) orders areinserted to identify the beginning of each field. The possible Attention Identification(AID) byte configurations are shown in the following figure. An AID byteother than X ' 60' is set when the operator at the selected display station has performedan operation that requires program intervention. These operations areeither pressing a Program Function or Program Access key, or detecting an attentionfield with the CURSOR,~ELECT key.Refer to IBM 3270 Information Display System: 3274 Control Unit Description andProgramming Guide.Chapter 6. IBM 7171 I/O Interface to the Host System 6-11