| 1. | Any pair of registers can be used as a 16-bit index register.
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| 2. | Simple arithmetic ( add, subtract ) may be performed on all index registers.
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| 3. | The first three bits after the sign specified an index register.
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| 4. | Some 7090 features, including index registers, character instructions and floating point, were extra-cost options.
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| 5. | Instructions contained a 12 bit operand, either fixed or offset from an index register.
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| 6. | Memory locations 1 through 63 were used as index registers.
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| 7. | Registers 1 through 15 can be used as index registers.
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| 8. | Some also subtract the " decrement " field from the contents of the index registers.
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| 9. | The IMP-16 provided four 16-bit accumulators, two of which could be used as index registers.
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| 10. | A change in refractive index registers as clear ice.
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