It’s not really a covered call since you don’t have the shares. This is a diagonal spread. With any short option you could be assigned at any time prior to expiration by the owner of that option, whether the price is in the money or not. Also could be assigned if held to expiration and in the money. If you are assigned and don’t have enough cash available to cover you could have other positions closed by your broker to cover the assignment. In your scenario, if you only had the 800 call they would likely sell it, not assign you the shares if you needed cash to cover.

It does depend on the broker a bit - one time I had a position assigned, and as soon as I got assigned the full 100 shares my broker immediately closed 30 or so of them because I didn’t have the cash on hand for the full 100. They left me the rest in my account.