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

Page of GA34-0003-6,-7As Updated August 20, 1976By TNL GN34-0340The reserved storage locations are described as follows:Hex Storage Location Contents0-1 The restart instruction. After loading the IPL routine, themachine automatically branches to location 0 to initiateprocessing. This instruction is also used with the checkcontrol switch in the check-restart position as describedin Chapter 5, "Operator Console."2-7 Reserved.8 Address of 5024 IOTB (refer to Chapter 17, "IBM 5024 I/OAttachment Enclosure" for a further description of the 5024IOTB).9—B The addresses of the first instructions in the routines thatservice program check, power failure, and machine checkinterruptions, respectively.IC—DThe IAR backup contents after a power failure or machinecheck interruption, respectively. This is the old instructionaddress (OIA). Control can then be returned to the correctinstruction in the interrupted program after successful completionof the interruption servicing routine. Only one word isrequired to save the interrupted program address for each typeof interruption because only one power failure or machinecheck can occur at any one time (it is not possible to have amachine check or power failure at more than one interruptionlevel at the same time).E—F Reserved.10-13 The addresses of the first word in the level displacement tablesfor interruption levels 0 to 3, respectively. Program-check oldinstruction address is stored at displacement 0 in the displacementtable of the level where the program-check occurred. See Figure 3-4.Locations 0 to 13 are fixed as to size and function of contents. However, additionalstorage is required for the level displacement tables assigned to each of the priorityinterruption levels (0,1,2, and 3). These tables are variable both in number (from 1 to 4)and individual size (from 2 to 17 words). They c,an be located anywhere in main storageexcept the reserved locations discussed above. Figure 3-4 shows the format for a prioritylevel table.The storage space required for the level displacement tables depends on two things:1. The number of different priority levels assigned to I/O devices by the user.2. The number of sublevels assigned to I/O devices on the same priority level.For example, if only two different priority levels are used (such as levels 0 and 1), onlytwo level displacement tables are required in storage. In addition, if sublevels 0 through 5are assigned to devices that are on priority level 0, and sublevels 0 through 11 are assignedto devices that are on priority level 1, then only seven words would be required for thelevel 0 displacement table and 13 words for the level 1 displacement table.Each priority level displacement table must occupy a contiguous block of storage, butneed be only as long as required by the current assignment of sublevels. If more than onepriority level is used, the level tables do not have to be adjacent to each other.System/7 Interruptions 3-7