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

Miscellaneous Machine RegistersStorage Data Register (SDR)The storage data register holds 16 data bits and two parity bits. Every word of data transferredto or from main storage by the processor passes through the SDR. The contents ofthe storage data register can be displayed by the console data lights.Operation Register (Op)The operation register is a 16-bit register that holds the first 16 bits of. the instructionaccessed from storage. In most cases these 16 bits are •the complete instruction. TheSystem/7 processor interprets these bits and performs the operation required. The contentsof the operation register can be displayed by the data lights on the operator console.PROGRAM INDICATORSThe System/7 has six indicators that show the status of the result field after an instructionis executed. These indicators are called carry, overflow, result-zero, result-even, resultpositive,and result-negative. At the completion of most instructions, these programindicators are set accordingly. Some of the branching instructions can test these indicatorsfor conditional branching situations.Once an instruction turns the indicators either on or off, they remain in this state(and can be tested) until the execution of another instruction that changes the indicatorsaccording to the result of the operation. (Refer to the detailed description of theindividual instructions in Chapter 4, "Processor Instructions," to determine the effectof each instruction on the setting of these six program indicators.)Each of the four priority interruption levels has a set of program indicators. Thecarry and overflow indicator settings are described separately in each instructiondescription. The four result indicators are grouped together and referred to as:the resultindicators, since they always indicate the results of an operation in a register or inthe accumulator.Carry IndicatorThe carry indicator is used for three functions:1. To indicate that an operation has produced a result that exceeds the physicalcapacity of the system.2. To hold one of the two condition code bits during an I/O instruction.3. To hold the summary status bit during interruption servicing.The following three conditions turn on the carry indicator when the physical capacityof the accumulator is exceeded:1. During a shift-left operation (excluding shift-left circular) when the last bitshifted out of bit position 0 (the sign bit) was a 1-bit.2. During an add operation, when a carry out of bit position 0 (the sign bit) wasa 1-bit.3. During a subtract operation, when a borrow beyond bit 0 occurs.The carry indicator is automatically reset prior to each add, subtract, and noncircularshift-left operation.At the conclusion of an I/O instruction the carry and overflow indicators are set toreflect a two-bit condition code returned by the I/O device. Values of the condition codeare covered in more detail in Chapter 4 under "Input/Output Instruction."The use of the carry indicator to hold the summary status bit during interruptionsis covered more fully in Chapter 3, "System/7 Interruptions."2-12 GA34-0003