IBM System/7 Functional Characteristics - All about the IBM 1130 ...

I■ 0=11 1111111 MEM11 1111111111 1111111 1111InnINITIAL PROGRAM LOAD (IPL)The host processor determines when to initiate an IPL to the System/7 through the ACCA.The IPL is then initiated by means of a special IPL sequence of characters (see Figure 7-6).This IPL is allowed only when the host attach switch on the System/7 console is in theenable and IPL position, and the ACCA is in standby mode of operation. The System/7program can, however, reject the IPL when the same special character sequence is receivedby the ACCA, as described later in this discussion.An IPL is initiated by a host processor that sends the unique sequence of characters EOA,US, DEL, DS at the beginning of a message. The EOA character places the ACCA in receivemode and the US character is a control character to the ACCA. The DEL characteris decoded as the first data character. If interruption requests have been enabled by a prepareI/O command, this decoded character presents an interruption request to the processor(a character-received request). At this time the controlling program can interpretthe circumstances leading up to the request and, if desired, issue a reset ACCA commandto reject the IPL. If the ACCA is not reset at this time, and the ACCA receives the DScharacter, a system reset occurs in the System/7, the processor is put into the load state,and the ACCA enters IPL mode. IPL data that is received by the ACCA is translated thesame as if the ACCA were operating in binary data mode, described previously.Loading into System/7 storage begins at location 0000 and continues sequentially untilterminated by the host. The System/7 load light is turned on during this time. If no errorsare detected by the host, it terminates the IPL operation with the characters EOB andLRC to indicate a normal ending. If no errors are detected by the ACCA during the message,the ACCA transmits a positive response to the host and exits from IPL mode. Atthis time, the System/7 processor branches to location 0000 to begin program executionon priority level 3, and the ACCA remains in receive mode. If interruption requests areenabled by a prepare I/O command, receipt of an EOT character from the host presentsan interruption request to the processor (ISW bit 13 and the summary status indicatorare set). When the EOT character is decoded, the ACCA enters standby mode of operation.Sequence usedwith station controloption onlyEOT SOAHost ®CD SPSystem/7Notes:0VW60 1 1YAKCan be ON if an error occursIPL sequenceEOA DELUS DSIPL text EOB EOT(binary data)CILRC 0First wordin 00002 3 4 0 5VAYAK,ACCA must be in standby mode'Interruption if prepared3 System reset if host attach switch is in enable and IPL position (ACCA in IPL mode)4 System reset and start program5 ACCA in standby mode6 AddressFigure 7-6. Initial program load operationAV V7-14 GA34-0003