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Chapter 20 The HP Model 9100A computing calculator 255

Fig. 15. Flow chart of a simple digit entry. Some of these flow paths are used by other calculator operations for greater hardware efficiency.

Multiplication

Multiplication is successive addition of the multiplicand as determined by each multiplier digit. Offset in the digit position flip-flops is increased by one after completion of the additions by each multiplier digit. Exponents are added after completion of the product. Then the product is normalized to justify a carry digit which might have occurred.

Division

Division involves repeated subtraction of the divisor from the dividend until an overdraft occurs. At each subtraction without overdraft, the quotient digit is incremented by one at the digit position of iteration. When an overdraft occurs, the dividend is restored by adding the divisor. The division digit position is then incremented and the process continued. Exponents are subtracted after the quotient is formed, and the quotient normalized.

Square root

Square root, in the Model 9100A, is considered a basic operation and is done by pseudo division. The method used is an extension of the integer relationship.

In square root, the divisor digit is incremented at each iteration, and shifted when an overdraft and restore occurs. This is a very fast algorithm for square root and is equal in speed to division.

Circular routines

The circular routines (sin, cos, tan), the inverse circular routines (arcsin, arccos, arctan) and the polar to rectangular and rectangular to polar conversions are all accomplished by iterating through a transformation which rotates the axes. Any angle may be represented as an angle between 0 and 1 radian plus additional information such as the number of times p /2 has been added or subtracted, and its sign. The basic algorithm for the forward circular function operates on an angle whose absolute value is less than 1 radian, but prescaling is necessary to indicate quadrant.

To obtain the scaling constants, the argument is divided by 2p , the integer part discarded and the remaining fraction of the circle multiplied by 2p . Then p /2 is subtracted from the absolute value until the angle is less than 1 radian. The number of times p /2 is subtracted, the original sign of the argument, and the sign upon completion of the last subtraction make up the scaling constants. To preserve the quadrant information the scaling constants are stored in the core memory.

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