* RESET BIT PRIN FOR TOUT MAT START PRINT ALARM SOUND RESTORE KYBD RESET MDT
6.2.5 Read Commands
Read Buffer Command
The Read type commands executed by the IBM 7171 are Read Buffer and Read Modified. Read Buffer causes the entire buffer contents of the addressed terminal to be read into main storage. The operation initiated by Read Modified is deter-mined by display station operator actions. The information read during execution of Read Modified could consist of fields of data modified by keyboard operations, buffer addresses, data of CURSOR SELECT fields, or the code of a Program Function or Program Access key.
An operator action that requires program interaction causes an attention inter-ruption; the program would respond to this attention interruption with a read command.
Programming Note: Unsolicited read commands are not recommended because the information read by these commands may be incomplete. 5
Execution of the Read Buffer command causes all data in the addressed device buffer, from the buffer location at which reading starts through the last buffer location, to be transferred to main storage. This command is provided primarily for diagnostic purposes. The transfer of data begins from the current buffer address if the Read Buffer command is chained. Regardless of where the transfer of data begins, data transfer from the buffer will terminate when the last character location in the buffer has been transferred, or before the last character location has been transferred when the channel byte count reaches 0 (in this case, the buffer address after termination is undefined).
The transferred data stream begins with a three character read heading consisting of the AID character followed by a two character cursor address. The contents of all buffer locations are transferred, including nulls. Start Field (SF) orders are inserted to identify the beginning of each field. The possible Attention Identifica-tion (AID) byte configuraIdentifica-tions are shown in the following figure. An AID byte other than X ' 60' is set when the operator at the selected display station has per-formed an operation that requires program intervention. These operations are either pressing a Program Function or Program Access key, or detecting an atten-tion field with the CURSOR,~ELECT key.
Refer to IBM 3270 Information Display System: 3274 Control Unit Description and Programming Guide.
Chapter 6. IBM 7171 I/O Interface to the Host System
6-11
ACTION AID Reaction to Resultant
(key hit) Generated READ MODIFIED data transferred
(EBCDIC) command to HOST CPU
No AID generated 60 Rd Mod (unsolic- Transfer field addresses and ited read) text for modified fields
No AID generated (PRINTER) E8 RdMod
ENTER key and & (Selector-Light AID code and cursor address,
pen attention) 7D RdMod followed by an SBA order,
PFI key Fl RdMod attribute address
+
1 and textPF2 key F2 RdMod for each modified field. Nulls
PF3 key F3 RdMod are suppressed.
PF4 key F4 RdMod
PF5 key F5 RdMod
PF6 key F6 RdMod
PF7 key F7 RdMod
PF8 key Fl RdMod
PF9 key Fl RdMod
PFIO key 7A RdMod
PFll key 7B RdMod
PF12 key 7C RdMod
PF13 key Cl RdMod
PF14 key C2 RdMod
PF15 key C3 RdMod
PF16 key C4 RdMod
PF17 key C5 RdMod
PF18 key C6 RdMod
PF19 key C7 RdMod
PF20 key C8 RdMod
PF2l key C9 RdMod
PF22 key 4A RdMod
PF23 key 4B RdMod
PF24 key 4C RdMod
Selector-Light Pen Attention space 7E RdMod AID code, cursor address, and
null field addressed only; no data.
PAl key 6C Short Rd
PA2 key 6E Short Rd AID code only
PA3 key 6B Short Rd
CLEAR key 6D Short Rd
TEST REQ or SYSTEM REQ keys FO Test Req Rd A test request message. AID transferred on Read Buffer only.
Figure 6-9. Attention 10 (AID) Configurations
6-12
IBM 7171 Reference Manual and Programming GuideRead Modified Command
Read Modified initiates one of three operations, as determined by operator actions at the display station:
1. a Read Modified, 2. a Short Read, or
3. a Test or System Request Read.
Figure 6-9 on page 6-12 lists the operator actions and the resulting Read Modi-fied command operation initiated by each action.
A major feature of Read Modified command operations is null suppression. The device buffer is cleared to all nulls when the operator turns power on or presses the CLEAR key, or when the erase portion of an Erase/Write command is executed at the selected device. Also, selected portions of a buffer can be cleared to nulls by the Erase All Unprotected command and certain orders. During Read Modified command operations, null codes are not sent.
Read Modified Operation: During a Read Modified command, if an AID other than the CURSOR SELECT key, a PA key, or the CLEAR key is generated, all fields that have been modified by a keyboard or the CURSOR SELECT key are trans-ferred to the program. All nulls are suppressed during data transfer and thus are not included in the read data stream. As a field is modified by the operator, the modified data tag (MDT) bit is set in the attribute byte for that field. Then, when a read modified operation is performed, successive attribute bytes are examined for a set MDT bit. When the bit is found, the data in the associated field is read (with nulls suppressed) before the next attribute byte is examined.
The first three bytes of the read data stream are always the AID code and the two byte cursor address; these bytes are called the "read heading."
Following the read heading is the alphameric data of each modified field. The data for each field is preceded in the data stream by a Set Buffer Address (SBA) order code followed by the two byte buffer address of the first character position in that field (the attribute address
+
1). Thus, the read data stream when data has been modified is as follows:Chapter 6. IBM 7171 I/O Interface to the Host System
6-13
BYTE
-Figure 6-10. Read Modified data stream