Path: utzoo!utgpu!watmath!clyde!att!ihlpa!normt
From: normt@ihlpa.ATT.COM (Norman R Tiedemann)
Newsgroups: comp.sys.dec
Subject: Re: PDP-11 floating point format query
Keywords: PDP-11 floating point format
Message-ID: <10693@ihlpa.ATT.COM>
Date: 30 Nov 88 18:55:55 GMT
References: <6215@killer.DALLAS.TX.US>
Organization: AT&T Bell Laboratories - Naperville, Illinois
Lines: 60

In article <6215@killer.DALLAS.TX.US>, richardh@killer.DALLAS.TX.US (Richard Hargrove) writes:
> Can anyone answer some questions I have about data objects in PDP-11 floating 
> point format?
> 
> What I know is this:
> 	+-+--------+-----------------------+
> 	|s|eeeeeeee|mmmmmmmmmmmmmmmmmmmmmmm|
> 	+-+--------+-----------------------+
> where
> 	s == sign bit
> 	e == 8 bit exponent
> 	m == 23 bit mantissa (with hidden 1 bit for 24 bits of precision)
> 
> I'm also aware of a 64-bit format similar to above with 55 mantissa bits.
> I'm assuming that the exponent is in excess 128 format, producing an exponent
> range of -128..127. Is this assumption correct? This is critical since, for 
> example, an excess 127 format would modify the exponent range to be -127..128.

Yes, you are correct, the exponent is in excess 128 format so zero's in this
field imply an exponent of -128.

> I'm assuming the sign/mantissa combination are implemented in signed magnitude
> format. Is this assumption correct?

	Yes.

> The question that has me stumped is where is the binary point in relation to 
> the hidden bit (does a mantissa of xxxxx... express the number 1.xxxxx... 
> or .1xxxxx...?)

	.1xxxxxx is the correct representation for the hidden bit.

Smart people will note here, that with the hidden one format, there is no
true representation for 0.000... That is the case. Zero is a special
case and is represented by zero's in all 32 bits. (This is actually the
representation for .100000x10^-128, but DEC takes this as a special case
of zero.
> Just knowing the binary representations of 3.0 and -3.0 should allow me 

+3.0 =>   0 10000010 10000000000000000000000
-3.0 =>   1 10000010 10000000000000000000000

Boy this made me think up old memories of the PDP class I used to teach
at the U of Wisconsin.


> richard hargrove
> ...!{ames | decwrl}!killer!richardh
> --------------------------------------------

	Norm Tiedemann		AT&T Bell Labs IH 2G-331
	att!ihlpa!normt		2000 Naperville Rd.
	normt@ihlpa.att.com	Naperville, IL	60566


-- 

	Norm Tiedemann		AT&T Bell Labs IH 2G-331
	att!ihlpa!normt		2000 Naperville Rd.
	normt@ihlpa.att.com	Naperville, IL	60566