PDP-11 floating point format query

[Richard Hargrove]

(Sorry for the distribution to the questionable newsgroups but I'm trying
 to get this to as many old PDP-11 hands as I can.)

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.

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

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...?)

I obviously don't have access to a PDP-11 (or PDP-11 documentation) or I 
would have checked this out myself. Just knowing the binary representations 
of 3.0 and -3.0 should allow me to answer the questions I have.

Please send responses to me since this is probably of little general interest.

Thanks for listening,

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