Paul himself told that story in his autobiography, "The Idea Man" [1], in colorful details. The first half of the book is full of anecdotes of those early days, the relationship with Bill Gates, and the beginnings of Microsoft. It's a great read.

Really sad he went so soon; RIP.

[1] https://www.amazon.com/Idea-Man-Memoir-Cofounder-Microsoft/d...

There is a detailed account in Allen's book "Idea Man":

https://www.amazon.com/Idea-Man-Memoir-Cofounder-Microsoft/d...

Paul Allen (Microsoft co-founder) writes about the Z80 SoftCard and the early Microsoft history in his book "Idea Man": http://www.amazon.com/Idea-Man-Memoir-Cofounder-Microsoft/dp...