http://www.csclub.uwaterloo.ca/media/1989%20Bill%20Gates%20Talk%20on%20Microsoft.html
"1989 Bill Gates Talk on Microsoft"

At 22:04

In that original design I took the upper 384k and
decided a certain amount should be for video memory
and a certain amount for the ROM and IO,
and that left 640k for general-purpose memory.

So that leads to today's situation where people talk about the 640k barrier,
the limit of how much memory you can put into these machines.
I have to say that in 1981 making those decisions,
I felt like I was providing enough freedom for ten years.

Erm. <audience laugh>.

That is, a move from 64k to 640k felt like something that would last a great deal of time.
Er. Well it didn't.
Took about only 6 years before people started to see that as a real problem.