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

Overflow IndicatorThe overflow indicator is set on if the result of an arithmetic operation exceeds thelogical capacity of the accumulator. This happens when an add or subtract operationproduced a result greater than 2 15 -1 or less than —2 15 . This range is from +32,767 to—32,768 in decimal notation, or from 7FFF to 8000 in hexadecimal notation.The overflow indicator may be reset only when it is tested by either the skip or branchconditional instruction with the overflow-save flag equal to 0. These two instructions aredescribed in more detail in Chapter 4, "Processor Instructions." The overflow indicatoris also used as part of the I/O condition code as described previously under "CarryIndicator," and in Chapter 4 under "Input/Output Instruction."Result-Zero IndicatorThe result-zero indicator is set on when the result field of an operation contains all 0's.Result-Even IndicatorThe result-even indicator is set on when the low-order bit (bit 15) of a result field is setto 0, indicating that the operation has produced an even number.Result-Positive IndicatorThe result-positive indicator is set on when the high-order bit (bit 0, the sign bit) of aresult field is set to 0, indicating a positive number. If the numeric value of the entireresult field is 0, this indicator is set off because the result-positive definition does notinclude a 0 value.Result-Negative IndicatorThe result-negative indicator is set on when the high-order bit (bit 0, the sign bit) of aresult field is set to 1, indicating a negative number. A negative 0 is not represented inthe System/7.STORAGE PROTECT (5010 PROCESSOR MODULE MODEL E ONLY)Storage protect on the 5010 Model E provides protection against inadvertentstoring in a selected location. No read protection is provided; this protection is onlyagainst storing.Storage is divided into blocks of 512 words. Each block has a storage key 3 bits longset by the alter storage key instruction. (See "Storage Protect Instructions" in Chapter4).Each of the 4 priority interruption levels is provided with a 3-bit protect key. A protectkey can be assigned in 2 ways. The first method is by an alter protect key instruction.The second method is automatic and occurs during the first instruction read cyclefollowing an interruption. The protect key register for the new level is set to the samevalue as the storage key associated with the block containing the next instruction to beexecuted. This provides automatic assigning of the correct key following an interruption.Validity of the key picked by this mechanism is ensured by the location of the startinstruction address in the vector table in that area of storage controlled by the program.In this case, the address arrived at by the automatic sublevel branching technique is ina storage block with the correct key assigned to this particular interruption routine.This method of control of the storage protect mechanism permits using a storage load thatdoes not recognize the existence of storage protection provided that a system reset hasdisabled it before execution of the storage load commences. The system does not supporta mixed storage load in which some programs use the storage protect mechanism and othersdo not. In this case, an attempt by any program to access a protected area without a keymatch causes a program check interruption.IBM 5010 Processor Module 2-13