"I wanted to have virtual memory, at least as it's coupled with file systems"

Ken Thompson's quote is referring to the concept of virtual memory, which is a form of memory management that enables a computer system to utilize more memory than is physically readily available in the system. Virtual memory works by utilizing a portion of the hard drive as an extension of the RAM, permitting the computer system to save more data than it would otherwise have the ability to. This is specifically helpful when running multiple programs simultaneously, as it allows the computer to access more memory than it would otherwise be able to. By coupling virtual memory with file systems, the computer system can keep data in a more organized manner, making it easier to access and handle. This quote from Ken Thompson is stressing the significance of virtual memory and its ability to enhance the efficiency of a computer system.