除IEEE754之外，是否还有其他常用的浮点格式？

Question

I am writing a marshaling layer to automatically convert values between different domains. When it comes to floating point values this potentially means converting values from one floating point format to another. However, it seems that almost every modern system is using IEEE754, so I'm wondering whether it's actually worth generalising to allow other formats, or just manage marshaling between different IEEE754 formats.

Does anyone know of any commonly used floating point formats other than IEEE754 that I should consider (perhaps on ARM processors or mainframes)? If so, a reference to the format specification would be extremely helpful.

Answer 1

Virtually all relatively modern (within the last 15 years) general purpose computers use IEEE 754. In the very unlikely event that you find system that you need to support which uses a non-IEEE 754 floating point format, there will probably be a library available to convert to/from IEEE 754.

Some non-ancient systems which did not natively use IEEE 754 were the Cray SV1 (1998-2003) and IBM System 360, 370, and 390 prior to Generation 5 (ended 2002). IBM implemented IEEE 754 emulation around 2001 in a software release for prior S/390 hardware.

Answer 2

As of now, what systems do you actually want this to work on? If you come across one down the line that doesn't use IEEE754 (which as @JohnZwinick says, is vanishingly unlikely) then you should be able to code for that then.

To put it another way, what you are designing here is, in effect, a communications protocol and you obviously seek to make a sensible choice for how you will represent a floating point number (both single precision and double precision, I guess) in the bytes that travel between domains.

I think @SomeProgrammerDude was trying to imply that representing these as text strings (while they are in transit) might offer the most portability, and if so I would agree, but it's obviously not the most efficient way to do it.

So, if you do decide to plump for IEEE754 as your interchange format (as I would) then the worst that can happen is that you might need to find a way to convert these to and from the native format used on some antique architecture that you are almost certainly never going to encounter, and if that does happen then that problem would not be not difficult to solve .

Also, floats and doubles can be big-endian or little-endian, so you need to decide what you're going to use in your byte stream and convert when marshalling if necessary. Little-endian is much more common these days so I'd go with that.

Answer 3

Does anyone know of any commonly used floating point formats other than IEEE754 that I should consider ...?

1. CCSI uses a variation on binary32 for select processors.

it seems that almost every modern system is using IEEE754,

2. Yes, but... various implementations fudge on the particulars with edge values like subnormals , negative zero in visual studio , infinity and not-a-number .

It is this second issue that is more lethal and harder to discern that a given implementation has completely coded IEEE754. See __STDC_IEC_559__

OP has "I am writing a marshaling layer". It is in this coding that likely troubles remain for edge cases. Also IEEE754 does not specify endian so that marshaling issues remains. Recall integer endian may not match FP endian .