Must fields within a block of a Mill instruction all be the same width?
The lecture on encoding indicates that all ops encoded in a single block are all of equal width. However, if the op-count for some block is N (!= 0), we know that the block contains operations for slots 0 through N-1 (of the slots for that stream (either exu or flow-side.) So long as the bit-widths for operations in the block are fixed for a given Mill model, the total bit width of this block (and thus the location of the next block) should be trivially calculable from that count and the widths, even if the fields within the block are not of uniform width. And similarly for the existence and location of the alignment hole, used (if present) to encode a compact no-op for the other bundle stream.
Even in the uniform-width case, the decoders must calculate/look up the starting point of block N+1, based on the starting position and op counts of this and previously parsed blocks — even if that calc is a simple multiply by a constant. So it seems to me that handling non-uniform widths within a block wouldn’t significantly complicate block decoding over the uniform-width case. Is my reasoning wrong, and/or are there other reasons why the fields in a block must have uniform widths?
Note: since I posed this question to Ivan (off list, so as not to speculate in a forum post), I now see details in the Wiki (for example http://millcomputing.com/wiki/Cores/Gold that describe different bit widths for both exu and flow slot operation encodings. But, as is often said in the videos, some of the existing docs/videos are simplifications, so I’ll be curious what the Millcomputing folks say on this issue.
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This reply was modified 9 years, 4 months ago by
LarryP. Reason: clarity