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What does the number 8.6, -2.5 mean in some instruction example?

Robert W
Mastermind 7100 points

  • 0 in reply to Robert W
    Mastermind 7100 points

    Hi,

    I find several C674x instructions have 8.6, -2.5 in its examples. What do these numbers mean?

    Thanks,

  • 0 in reply to Robert W
    TI__Mastermind 20790 points

    Robert,

    Refer the ADD example:
    0000 325Ah ->12,890 (decimal)
    FFFF FF12h -> -238 (decimal)

    Similar you can understand about the CMPEQDP.
    Refer the Compare for Equality, Double-Precision Floating-Point Values (CMPEQDP) in TMS320C674x DSP
    CPU and Instruction Set Reference Guide (sprufe8).

  • 0 in reply to Robert W
    Mastermind 7100 points

    Hi,

    I cannot understand yet. On page 187, sptrfe8b.pdf, there is another example for 8.6. Its hex it 4109,999A. How many integer bits? How many fraction bits? Thanks,

  • 0 in reply to Robert W
    TI__Mastermind 20790 points

    Robert,

    double-precision number is described by:

    (-1)^sign * 2^(exponent - bias) * 1.mantissa

    The formula means that for non-NAN, non-INF, non-zero and non-denormal numbers.

     you take the bits in the mantissa and add an implicit 1 bit at the top. This makes the mantissa 53 bits in the range 1.0 ... 1.111111...11 (binary).

    To get the actual value, you multiply the mantissa by the 2 to the power of the exponent minus the bias (1023) and either negate the result or not depending on the sign bit. The number 1.0 would have an unbiased exponent of zero (i.e. 1.0 = 1.0 * 2^0) and its biased exponent would be 1023 (the bias is just added to the exponent). So, 1.0 would be sign = 1, exponent = 1023, mantissa = 0 (remember the hidden mantissa bit).

    Putting it all together in hexadecimal the value would be 0x3FF000000000 == 1.0