Re: Making Lemonade (Floating-point format changes)

On 12/05/2024 05:44, John Savard wrote:

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-point 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. 

 
YES, I'd use something similar:
I never cared nor supported any odd 10 byte formats and I give a fart
to all these weird IEEE standards.
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