Re: Why VAX Was the Ultimate CISC and Not RISC

On Tue, 11 Mar 2025 22:48:17 +0000, Lawrence D'Oliveiro wrote:

On Tue, 11 Mar 2025 22:15:30 -0000 (UTC), Waldek Hebisch wrote:
>

Trouble is that such "common" operations have rather low frequency
compared to simple stuff.  They are really library functions.

>
Inline library functions. And they did contribute to keeping the code
compact, as Bell said.
>
One thing, though, I don’t think the POLYx instruction was all that
useful. It is typical, when computing functions approximated by
polynomials, for the polynomial to actually be infinite. And so you have
a
loop that computes each term in turn, accumulates it to the result,
works
out an estimate of the remaining error, and stops only when this falls
below some threshold.
>
This cannot be expressed by some fixed-length table of coefficients, as
the POLYx instruction expects.

People quit doing it that way because Chebyshev (and later Remez)
coefficients can be sufficiently accurate whereas Taylor (or Maclaurin)
cannot. By the time of the Cody and Waite Book the continue the poly
until convergence was already old in the tooth.