[Libre-soc-isa] [Bug 213] SimpleV Standard writeup needed

bugzilla-daemon at libre-soc.org bugzilla-daemon at libre-soc.org
Fri Oct 16 17:06:14 BST 2020


--- Comment #49 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
(In reply to Jacob Lifshay from comment #5)
> the chosen prefixes should mesh well with the 64-bit instructions that were
> added in v3.1 of the spec.

unfortunately the use of one extra major opcode by the 64 bit prefixes is
mutually exclusively incompatible with using 8 major opcodes for SV P32, P48,
P64, VBLOCK and planned Compressed.

(this is why i designed the data pointer concept which effectively implicitly
merges a (micro-op) LDST into any immediate operation and gives arbitrary
overloads of the immediate length to anything the programmer desires.)

to get 8 major opcodes free and clear is essential to providing enough bit
bandwidth for SV Prefixing in the different types.  those 8 major opcodes are
for example the only reason why we have 11 bits available as prefixes in SVP32
and P48... and consequently would not need to throw away several weeks design
work that you did on SVP, javob.

(6 bits per major opcode leaves only 10 bits for SV P32 and P48.  that is not
enough.  however by using *two* major opcodes, in pairs, you get 11 bits.  we
need P48, P64, P32, VBLOCK and Compressed, that's 4x2 major opcodes and v3.1
instructions took one of them).

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