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RFC 401

Updates RFC 387

Network Working Group                                 Jim Hansen
Request for Comment #401                              Center for Advanced
NIC #11923                                              Computation
Category:  D.6                                        University of Illinois
Updates:  RFC #387                                    October 23, 1972
Obsoletes: None

               Conversion of NGP-0 Coordinates to Device
                          Specific Coordinates

Conversion of NGP-0 coordinates to floating point PDP-10 coordinates
was discussed in RFC #387.  In general, however, it is undesirable to
convert NGP coordinates to floating point coordinates because real
devices require integer addressing.  To this end, a means is described
to convert NGP coordi- nates to integer coordinates in the range zero
to M, where M is the maximum address of the device screen on a machine
using 2's complement arithmetic.  It would not, however, be difficult
to modify this algorithm to operate on machines using one's complement
or sign-magnitude arithmetic.

First consider the NGP coordinate format:

                   |  |   n       |
                    s ^  FRACTION

Where the sign occupies the most significant bit of the coordinate
followed by bits of numerical information (initial implementation of
NGP requires N=15).  Negative numbers are represented by 2's
complement.  Conversion to device coordinates is accomplished by:

                    D = S * f + S

Where D =>integer device coordinate
      S =>scaling factor (typically M/2)
      f =>NGP fractional coordinate

Let us rewrite this as:

                            n     n
                    D = S*(2 *f)/2 +S

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Now factor S into two terms:

                    S= Q * 2

Where Q is an odd integer and I is an integer.

When:                        I   n     n
                    D = Q * 2 *(2 *f)/2  +S

                             I-n   n
                      = Q * 2   *(2 *f)  +S
The factor (2 *f) is represented in 2's complement form simply by
extending the sign bit of f into the upper portion of the computer
word, If Q = 1 (as it would be with many devices), it can be ignored.
If Q >< 1, we may console ourselves that an integer multiply is faster
on most machines than a floating point multiply.  In fact, on a
PDP-10, this multiply can usually be performed with no access to
memory since Q is usually small.

We are now left with the 2    factor.  This can be accomplished with an
arithmetic shift left by (I-n) or an arithmetic shift right by (n-I)
as is appropriate.  The offset factor, S, may now be added using an
integer add.

The procedure for converting NGP coordinates to integer device
coordinates is then:

               1.   move coordinate to a register and extend sign
               2.   integer multiply by Q (if necessary)
               3.   arithmetic shift left by (I-n)
               4.   integer add S

This procedure would generally be much faster than:

               1.   move coordinate to register and extend sign
               2.   float fractional coordinate
               3.   floating point multiply
               4.   floating point add
               5.   conversion to fixed point

       [ This RFC was put into machine readable form for entry ]
       [ into the online RFC archives by BBN Corp. under the   ]
       [ direction of Alex McKenzie.                      1/97 ]

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