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- 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
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- 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
- i
- g
- n
-
- 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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-
-
- [Page 1]
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- Now factor S into two terms:
-
- I
- 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
- n
- 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.
-
- I-n
- 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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- [Page 2]
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