IBM 5150 maximum memory?

Does anyone recall what the maximum memory was for an original IBM 5150 PC at launch time? The way I recall it, IBM only offered 64KB expansion cards back in the day (256KB ones came later) and the 5150 would only take four of them (wasn't the fifth expansion slot wired differently or something)? That still gives a maximum of 320KB of memory though (4 x 64KB, plus 64Kb on the motherboard) - yet I was remembering the maximum total memory as being 256KB. Maybe just bit-rot on my part. Or did the motherboard memory somehow get disabled if memory expansion boards were in use? Or was there some kind of maximum limit dictated by the 5150's BIOS? I'm sure other things could be done later on via third-party boards of course (or via the 256KB expansion boards), but in the context of the thread that I found myself involved in, the question was what the maximum memory config in a 5150 at launch time was... cheers Jules