Proposal for minor mods to evio

[Elliott Wolin <wolin@jlab.org>]

To the JLab CODA group,

I propose a minor mod to evio for Hall D.  Currently segments use a one-word
header: 16-bit length, 8-bit data_type, but only an 8-bit tag.  Thus only 256
tags are allowed.  This is not enough.

I propose a new type of fragment, a "tagsegement", with a 12-bit tag and a 4-bit
data type.  This allows 4096 tags but only 16 types.  Currently all the atomic
types (int, float, etc.) use 4 bits (0 to 0x8, ignoring VAX stuff), and only
repeating structures, banks, and segments need add'l bits (H-D won't use
packets).  

There are 6 undefined or unneeded 4-bit types.  I propose 0xe for tagsegments,
and 0x9 and 0xa as repeats of bank and segment, so that a tagsegment can hold
banks and segments. If we don't want to redefine the VAX types, then 0xb-0xd are
available instead (n.b. in this case there'd only be one available 4-bit type
left for future needs).

The only code affected by this (I think) is the byte-swapping routine in
evio...an additional case statement must be added to handle tagsegments.  Note
that handling of tagsegments is identical to that of segments except for
unpacking the top 16 bits of the header word into tag and data_type, so the
change to evio should be simple.  Hall D probably won't use cefdmp or xdefdmp.

An alternative is to pick up a few bits from the segment length, but I prefer
shortening the data_type.

Comments, suggestions, ...

				Sincerely,
					Elliott
 

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