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Register Arithmetic

This section deals with the instructions that perform register-to-register arithmetic.

This diagram shows the standard floating-point types provided in this architecture:

They include the types specified in the IEEE 754 standard, and a 128-bit Extended type without a hidden first bit.

As well, in addition to the standard 32-bit single precision and 64-bit double precision types, one additional type has been added.

A 48-bit type has been added to provide a precision that closely matches that which had been found adequate for a large class of scientific problems. The size of the exponent field was chosen so as to include the exponent range provided by typical electronic scientific calculators - both pocket calculators, and calculators like the Wang 500, the Monroe 1655, and the Hewlett-Packard 9100A which preceded them. As well, this format provides about eleven decimal digits of precision.

The nine-bit opcode for the standard register-to-register arithmetic instructions is sufficient to provide a wide range of operations.

The original floating-point format of the System/360 is handled, so as to permit simulation programs to run more efficiently. However, the floating-point registers will normally be set up to contain numbers in the IEEE 754 Extended format, with conversion taking place when they are loaded with Single, Long Single, Medium, or Double floating-point numbers. So any other format of number must instead be held in the integer registers.

This is the primary reason why there are Double Long memory reference instructions, to handle the form of extended precision introduced on the System/360 Model 85.

As well, the integer registers are also used for packed decimal numbers, and the Decimal Floating-Point standard included in the newest revision of the IEEE 754 standard.

Another numeric format that may be handled by this system, but from the integer registers, is the new standard for Decimal Floating Point.

Decimal digits are represented using the 10-bit coding that belongs to IBM's Densely Packed Decimal scheme:

BCD digits         Densely Packed
                   Decimal encoding

0abc 0pqr 0uvw     abcpqr0uvw
0abc 0pqr 100W     abcpqr100W
0abc 100R 0uvw     abcuvR101w
100C 0pqr 0uvw     uvCpqr110w
0abc 100R 100W     abc10R111W
100C 0pqr 100W     pqC01r111W
100C 100R 0uvw     uvC00R111w
100C 100R 100W     00C11R111W

The overall format of a Decimal Floating-Point number is illustrated below:

A DFP number is composed of four fields, with lengths as given in the table below:

Overall     Sign   CF     BXCF    CCF

    32         1    5        6     20
    64         1    5        8     50
   128         1    5       12    110

The first part of the number is the sign, which is one bit long.

The second part of the number is the Combination Field (CF), which is always five bits long.

The combination field contributes one decimal digit, and two binary bits, to the significand and the exponent of the number respectively.

Its format is as is shown in this table:

bbaaa Number begins with 0 to 7
11bbA Number begins with 8 or 9
11110 Infinity
11111 NaN

First digit of number:
aaa:       A:
  0 000     8 0
  1 001     9 1
  2 010
  3 011
  4 100
  5 101
  6 110
  7 111

The exponent is binary, and its first two bits may only be 00, 01, or 10, never 11.

The third field is the rest of the exponent, called the Binary eXponent Continuation Field (BXCF). The exponent bias is as given in the table below:

Length of       Exponent
number          Bias

 32               101
 64               398
128             6,176

Thus, a 32-bit number has an eight bit exponent, two bits from the combination field plus six bits from the BXCF, that varies from 000000 to 101111, and it is an excess-101 exponent.

The fourth and final field is the Coefficient Continuation Field (CCF), and consists of 2, 5, or 11 groups of three decimal digits represented by ten binary bits in Densely Packed Decimal encoding. These are the digits which follow the first digit of the number that was given by the Combination Field.

Note that the coding of the Combination Field allows the first digit to be zero. Therefore, unnormalized numbers can be expressed in this format. This is intentional. The description of the format includes rules for determining the "ideal exponent" of an arithmetic operation. Essentially, those rules are based on the principle of treating all inputs to the arithmetic operation as exact, and producing a result which has no additional padding of zeroes after the least significant digit.

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