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

The branch and link instruction (PBAL) permits branching only to storage locationsthat are +127 or —128 positions away from the address contained in the IAR when thePBAL instruction is fetched. This limit exists because the displacement field (bits 8to 15) of the instruction can contain only numbers in the range of +127 through —128.Example: Assume that the PBAL instruction is at storage location 88.0 5 8 15Op codeDisplacementPBAL instruction 0 1 0 1 1 1 1 1 0 0 1 0 1 0 0 0 11 1 1 I 1 1 1 1 f5 F 2 1IAR0 15 0PBAL instruction (Decimal 89)is fetched 0 0 0 0 10 0 0 0 1 0 1 0 1 1 1 0 0 111111 I 111111111 I 1 10 0 5 9Index register 715IAR storedby PBALinstructionBranch-to addressgeneration:PBAL instruction IAR 0000 0000 0101 1001execution Disp +0000 0000 0010 00010000 0000 0111 101000 0 0 0 0 01 1 1 I 10(Decimal 89)150 0 0 1 0 1 1 0 0 11 1111111110 5 9IAR0 15 0 15After PBALinstruction execution(Decimal 122) (Decimal 89)0 0 0 0 10 0 0 0 0 1 1 1 11 0 1 00 0 0 0 0 0 0 0 0 1 0 1 1 0 0 11111111111111111111111111111110 0 7 A 0 0 5 9The IAR contains the effective address of the next instruction to be executed; that is,the instruction branched to, which is at location 122. The branch to location 122 istaken and execution begins. The PBAL instruction does not change the carry, overflow,or result indicators.When it is desired to return to the original sequence (at location 89), the contents ofindex register 7 must be moved to the IAR. A branch (PB) instruction, with 0displacement and R=111 (specifying XR7), can be used to accomplish this.Processor Instructions 4-31