The Atari 600XL was sort of the ‘little brother’ to the Atari 800XL. It was physically only a little smaller, just in depth, and had a whopping 16KB of RAM, as opposed to 64KB that the 800XL had. The only other difference was the lack of a monitor port, which didn’t really matter because most everyone only ever used a TV with these computers.
The 600XL had BASIC built in so you could stat to program as soon as you turned it on. Once on, you would be prompted with the message ‘READY’ with a small cube under the ‘R’ that was your cursor.
I found this one on a buy/sell board and paid $20 for it.