Function exchanged_value_for_store 

pub(crate) fn exchanged_value_for_store(
    cfg: &SystemConfiguration,
    source: &Unsigned36Bit,
    dest: &Unsigned36Bit,
) -> Unsigned36Bit

Perform an exchange operation suitable for a store operation; that is, emulate the operation of the exchange unit diring e.g. STE.

I believe that in this direction there is no sign extension. This is based on my reading of Chapter 13 of the Technical Manual. Section 13-4.1 (covering loads) says “The sign extension process which follows step 4 is described in 13-4.3.”. But section 13-4.2 (covering stores) contains no similar statement. I suspect this means that sign extension does not happen for stores.