Overflow Flag

The Overflow Flag is contained within the processor COM and OVF flags two-digit field. This field is defined as:

Information Digit Bit
Reserved 01) 3
Reserved 2
Reserved 1
Reserved 0
Reserved 1 3
Overflow Flag 2
Comparison Low (COML) flag 1
Comparison High (COMH) flag 0

The Overflow Flag indicates that the result field length of an arithmetic, Move Alphanumeric, or Move Numeric operation is not sufficient to store the result.

The Overflow Flag is not cleared at the beginning of an arithmetic operation, but is preserved. Therefore, it indicates overflow that has occurred at any time before or during a series of arithmetic operations or other interspersed non-arithmetic operations.

The Overflow Flag is cleared by the Conditional Branch on Overflow (OP = 28), Edit (OP = 49) and Search (OP = 39) instructions.

The Overflow Flag is stored in memory and reset by the Branch Communicate (OP = 30), Enter (OP = 31), Virtual Enter (OP = 35), and Hyper Call (OP = 62) instructions. It is also stored in memory and reset by the Interrupt and Hardware Call procedures.

The Overflow Flag is unconditionally restored from memory by the Return (OP = 63) and Virtual Branch Reinstate (OP = 93) instructions and conditionally restored from memory by the Exit (OP = 32) instruction.

1) Digit 0 is used as a flag during the Exit and Return instructions to specify whether to restore the setting of the Overflow and Comparision flags
processor_state/overflow_flag.txt · Last modified: 2011/07/23 14:23 by scott
 
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