On the 040, it seems to do something that actually involves D1. Definitely doesn't touch A1 at all. I didn't test further, but it's possible it just handles the instruction as a normal CAS.
It did cause a system error the first time I stepped through the instruction with MacsBug on my LC 475, but then it was fine after that.
It did cause a system error the first time I stepped through the instruction with MacsBug on my LC 475, but then it was fine after that.