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

Example:PB instruction(R = 110 specifies indexregister 6 is to be used forcalculating the branch-tolocation)0 5 8 15Op code R Displacement0 0 1 1 1 1 1 0 0 0 0 0 1 1 1 1 1!III 1 1 III 13 E 015Index register 601I 0 0 10 1 011 0 0 1 0 1 1 11 1 1 1 1 1 1 1 1111110 0 F FEffective address generation:Contents of indexregister 6Expanded displacementEffective address0000 0000 1111 1111+0000 0000 0000 11110000 0001 0000 1110Thus, the next instruction to be executed is at storage location 270 (decimal).Branch and Link (PBAL)0 5 8 15Op code0 1 0 1 1I litX X X1 IDisplacement1 1 1 1 1 15 8-F X XThe branch and link instruction (PBAL) branches to another storage location and savesthe contents of the IAR for return to the original sequence. The PBAL instruction isunconditional; the branch is always taken because it is not based upon the result ofa condition or test.When the PBAL instruction is fetched from storage, the IAR contains the address ofthe storage location that immediately follows the PBAL instruction. Execution of the PBALinstruction causes the contents of the IAR to be stored (and thus saved for future usein returning to the original sequence) in the index register specified by the R field.If R=000, the accumulator is used for this purpose.The location of the next instruction to be executed (the location branched to) isdetermined by the effective address as computed from the IAR and displacement fields inthe PBAL instruction. The contents of the displacement field are added algebraically tothe contents of the IAR to form this effective address. Thus, the IAR now contains theaddress of the storage location being branched to. The PBAL instruction does not changethe carry, overflow, or result indicators.4-30 GA34-0003