Re: Mask Register Value Mapping

Andrew Waterman

It is the case that mask elements are always one bit wide in this version of the spec. Removing the “.LSB” holdovers will improve clarity.

On Tue, Sep 22, 2020 at 12:25 PM CDS <cohen.steed@...> wrote:

From 0.9 stable spec, 5.3.1, table (no number), vector masking is referred to as having LSB. This suggests, yet does not require, that the mask field for each element is greater than bit-size 1.

From same spec, 4.6.1, each element mask bit is given an explicit location, as a single bit.

And yet, for individual operations, the LSB reference is still intact - such as in section 12.4 (Vector Integer Add-with-Carry / Subtract-with-Borrow Instructions).

This suggests some reformatting may be necessary regardless of the correct interpretation:
- If the single-bit-is-always-correct interpretation is to be asserted clearly, it would make sense to remove all of the ".LSB" references when discussing mask input/output fields.
- Otherwise, section 4.6.1 would need to be overhauled to be consistent with the idea of mask entries being multi-bit-per-element.


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