Fixed-field length arithmetic instructions use a 20-digit accumulator that holds the instruction result within the processor as an operand for a subsequent operation. Every instruction has an implied reference to the accumulator.

The fixed-point (integer) format consists of an implied signed exponent field (+08) of three digits followed by a mantissa field of a single-sign digit followed by eight digits of mantissa.

The fixed-point instructions operate on the 12 most significant digits of the accumulator. This format is similar to the single-precision format in the floating-point, fixed-field length instructions. The same accumulator is used for the fixed-point and floating-point instructions.

When the accumulator is stored in memory, both the sign of the exponent and the sign of the mantissa are stored in standard signed-numeric format (C = positive, D = negative).

If the instruction produces a result greater than seven digits, an overflow occurs and the following characteristics apply:

  1. If the operation specifies a store of the result to memory (IAS, ISS, IMS, IMI), this store is not performed.
  2. The sign and exponent field is set to +08.
  3. The overflow flag is set and the comparison flags are set HIGH.
  4. The final contents of the accumulator are unspecified.

A Trap Fault is a software enabled routine that allows the instruction in error to be examined. The Trap Fault is enabled if the 2-digit key stored at memory address 64, relative to base 0, is FF.

If a trap fault is enabled and a fault occurs, a hardware call procedure is executed with the address of the instruction at fault stored on the stack.

If a trap fault is not enabled and a fault occurs, the next program instruction is executed.

arithmetic/fixed_point.txt · Last modified: 2011/07/21 16:05 by scott
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