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

EFFECTIVE ADDRESS GENERATIONThe displacement field is eight bits long. Its function is determined by the instructionto be performed, as indicated in the operation-code field. The displacement field maycontain:1. Data to be used for generating an effective address (EA) for the instruction beingperformed. (See "Effective Address Generation," which follows.)2. Data to be manipulated in some manner by the instruction.3. Data that modifies the operation code so that a particular instruction isexecuted.The address field of a long-format instruction is 16 bits long. Its contents are used ingenerating an effective address to locate the operand in four of the eight long-formatinstructions (load index long, branch conditional, branch and link long, branch andunmask long). The remaining four long-format instructions (execute I/O and the threestorage protect instructions) use the address field to specify additional modifiers and/orthe address of the I/O module and/or device.Storage addresses are expressed internally in 16-bit binary numbers, ranging from 0 (16 bitsset to 0) to a maximum value determined by the storage size of the System/7 processormodule. Any attempt to address a location that is beyond the limits of the storage installedin the system causes a program check class interruption due to the invalid storage address.Most instructions refer to a storage address to locate the data that is to be used. Thisstorage address is called the effective address (EA).Address ArithmeticThe System/7 operates on a base-register-displacement principle for storage addressing.In a short-format instruction, the effective address is derived by adding algebraicallythe contents of the displacement field to the contents of the register indicated by theR field. The R field in the instruction specifies that the contents of the instructionaddress register (R=000) or one of the index registers (R=001 to 111) are added to thedisplacement value to compute the effective address. An example using the IAR (R=000) isshown below:O 5 8 15Add instruction(located at storageaddress 721 decimal)Op code1 0 0 0 0I III0 0 01 1Displacement0 0 1 0 1 0 1 1 011111118 0 2 6(displacement value of +38)0 15IARO 0 0 0 0 0 1 0 1 1 0 1 0 0 1 01111111111 11111O 2 D 2(base register address of 722)0 15O 0 0 0 0 0 1 0 1 1 1 1 1 0 0 0I 1 I 1 I I 1 I 1 I I I 1 1 1O 2 F 8(effective address of 760 decimal)4-6 GA34-0003