The Register source has this comment:

    // Registers can't be a record type because the values need to be truncated to 8 bits when writing, so setters are needed
    // This is for the web renderer as Fable transpiles uint8 to Number (more than 8 bits) in JS and doesn't apply any truncation
    // Known non-standard behaviour in Fable (https://fable.io/docs/javascript/compatibility.html#numeric-types)

So, I think, it's just conservatively cleaning the data due to Fable's widening via js Number on the web target.

▲ debugnik 4 hours ago | parent [-]

Oof, thanks for pointing that out, I hadn't noticed and I've only ever used F# on .NET.

That's terrible on Fable's part, the least they could do is truncate. I wasn't aware Fable's translation is so naive.

	▲	ibejoeb 3 hours ago \| parent [-]
		I haven't used Fable much, but apparently it maps .NET arrays to js TypedArray. Presumably you could keep the registers in 8-element array and fable will properly produce a Uint8Array. I'd like to benchmark that.