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

Module address field:Module addresses:Six-position enclosureThree-position enclosure26 29 310 0 X XXX000 011 000001 100 001010 101 010Two-position enclosure0 or 10 = Main enclosure1 = Expansion enclosure0-5Selects module position000XXXFigure 4-3. System/7 module addressingWith the two-position enclosure, any value in bits 29 to 31 (bits 13 to 15 of the secondinstruction word) other than 000 addresses the single I/O module position. The valuechosen to initially address the single I/O module must also be used for all subsequentaddressing of that module by the same program. However, because of the possibility offuture expansion of such a configuration, it is advisable to specify a value of 010 or 101.By specifying one of these two values, the I/O module can be installed in the same relativeposition in a larger configuration (that is, at the bottom of the enclosure). Thus, theamount of program redesign and customer termination rewiring will be minimized whenchanging to a three- or six-position enclosure.The processor module occupies the first position in the main enclosure; therefore, itsmodule address is always 000000.Each time an I/O command is issued, the processor determines whether (1) the commandwas executed successfully, (2) an error occurred, (3) the device was busy or an interruptionwas pending, or (4) the device was not attached to the system. One of these machineconditions is indicated by the two-bit condition code, whose settings are determined (andmade available to the program) by the results of the I/O operation. The two bits used forcondition codes are the same two bits that serve as carry and overflow indicators forother machine instructions. A summary of the condition codes and their settings as indicatedby the carry and overflow indicators is shown in Figure 4-4.The result indicators are changed only by an immediate read command to reflect thefinal contents of the register specified by the R field of the PIO instruction.ConditioncodeCarry (C)Overflow (0)Command functionsWrite Read PrepareI/OHaltI/OSetinterrupt0 C 00 = 0S S S S S1 C = 0 E E+ E0 = 12C = 10 = 0B 13 . 4- B BA3 C= 1 X X+ X X X**0 = 1vÃv4-56 GA34-0003Key to symbols:S = Satisfactory operationE = Error detectedB = Busy or interruption pendingX = Device not attachedFigure 4-4. Condition codes* = Not used on read status command** = Returned only if set interrupt is directed athost and no host processor is attached.+= With immediate read command, validity oftarget register contents is not guaranteed.Register may have invalid data and/or parity.