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

CTL CHK (Control Check) LightThis light turns on under any one of the following conditions:1. The processor has requested or taken more than one type of storage cyclesimultaneously.2. More than one interruption level is active simultaneously.3. No interruption level is active, but the processor is requesting or takingstorage cycles.A machine check interruption occurs if the check control switch is in the processposition.CHK (I/O Check) LightA machine error condition that prevents further communication with the I/O modules turnson this light and causes a machine check interruption. (Refer to "SEQ (Sequence) Light"following.)SEQ (Sequence) LightThis light operates in conjunction with the I/O check light. The sequence light is turnedon if the machine error occurred during an interruption; the light is not turned on ifthe machine error occurred during an execute I/O instruction.Program Check (PCK) Indicator LightsINV OP (Invalid Operation) LightThis light turns on when an instruction contains an invalid value in the operation code,modifier, or function field. This results in a program check interruption. The MSP/7Macro Library/Relocatable, in saving registers across calls to system routines, uses theprogram check interruption. This causes the INV OP light to flicker. See MSPI 7 Messages,Codes and Operating Procedures, Order Number GC34-0023, for proper use of STOP ONERROR in this environment. A system message is generated when an operation codeviolation occurs.INV ADR (Invalid Address) LightWhen the storage address register attempts to access a main-storage address that exceedsthe limits of the storage installed in the system, this light turns on and a program checkinterruption occurs.INV CT (Invalid Count) LightThis light turns on when an invalid count field is detected in a shift instruction, causinga program check interruption.Cycle Indicator LightsIF1 REQ (Instruction Fetch Request) LightThis light turns on to indicate that an instruction is about to be taken from storage(that is, the machine will next be in the instruction fetch cycle).El REQ and E2 REQ (Execution Request) LightsThese lights turn on to indicate that the machine will next be in the first or secondexecution cycle, respectively.5-4 GA34-0003