64, not 640.
I had one of these in mid 1982 to late https://en.wikipedia.org/wiki/IBM_Personal_Computer
But only the first 256 KB was on the motherboard, you needed a memory expansion card for the extra 384 KB, and I don't think they really existed in 1982.
Actually, you could somewhat break the 640KB limit in the early days (not that most people could afford that much RAM) days. Until EGA came along in 1984, there was free address space upto 704KB that you could use as conventional memory without much issue (or 736KB if you limited yourself to CGA).
And later we had the mess of EMS, which would work on the original 5150 with the right expansion card (and the right software to take advantage of it, like Lotus 123)
Some clone makers quickly found out ways take advantage of that space adding more RAM there. Then bit later it came area where you could even map over 1MB placed memory blocks there with a config.sys loaded driver.