Making Lemonade (Floating-point format changes)

I've made another long-overdue change in the Concertina II
architecture on the page about 17-bit instructions.

Since I describe the individual instructions there, with their opcodes
and what they do, I've illustrated the floating-point formats of the
architecture on that page.

The good people in charge of the IEEE 754 standard had seen fit to
define a standard 128-bit floating-point format which included a
hidden first bit.

This annoyed me greatly, because I was going to take the 8087's
temporary real format, and extend the mantissa for my 128-bit format.

I've decided that it's necessary to fully accept the 128-bit standard
and support it in a consistent manner.

Therefore, I have taken the following actions:

I have dropped the option of supporting 80-bit temporary reals
entirely, as they are now incompatible as an internal format.

I have instead defined a 256-bit format for floats which does not have
a hidden first bit, which looks like the old temporary reals, except
that the exponent field is one bit wider.

And in addition, just as the IBM 704 used two single-precision floats
to make a double-precision float, and the IBM System/360 Model 85
started using two double-precision floats to make an extended
precision float... I've defined how the 256-bit internal format floats
can be doubled up to make a 512-bit float.

I'm not really sure such floating-pont precision is useful, but I do
remember some people telling me that higher float precision is indeed
something to be desired. Well, the IEEE 754 standard has forced my
hand.

John Savard