[Libre-soc-isa] [Bug 562] New: make elwidth's encoding more intuitive

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=562

            Bug ID: 562
           Summary: make elwidth's encoding more intuitive
           Product: Libre-SOC's first SoC
           Version: unspecified
          Hardware: PC
                OS: Other
            Status: CONFIRMED
          Severity: enhancement
          Priority: ---
         Component: Specification
          Assignee: lkcl at lkcl.net
          Reporter: oliva at libre-soc.org
                CC: libre-soc-isa at lists.libre-soc.org
   NLnet milestone: ---

currently it's:

00      DEFAULT         default behaviour for operation
01      ELWIDTH=b       Byte: 8-bit integer
10      ELWIDTH=h       Halfword: 16-bit integer
11      ELWIDTH=w       Word: 32-bit integer 

which seems out of order to me.  make make the two bits encode i and have the
bit length be 8 << i, or 8 << (3-i) if the default is to be zero.

for floating-point encodings, if we wish 64-bit to be the default, but also
support 128 bits instead of either f16/bf16, we could still do this, having the
shift amount be a signed 2-bit number ;-)

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