IBM's "31-bit" architecture

Posted Feb 14, 2014 18:15 UTC (Fri) by khim (subscriber, #9252)
In reply to: IBM's "31-bit" architecture by giraffedata
Parent article: Quotes of the week

Your explanation is essentially correct but leaves one to wonder just how this one bit helped and why it was even needed.

The reality is simple: IBM/360 and it's successors were always 32bit (later 64bit) but early models only used low 24bit for addressing memory and many programs took advantage of that and used few bits here and there as tagged pointers. IBM/370 expanded memory address beyond that, but simple idea to just start using the whole register to address memory was rejected since it broke old programs. Instead most significant bit was used as marker: if it was set then the rest was 31bit address, if it was unset then the rest 7 bits were ignored (and old programs or adaptations of old programs could have used it as tag). It's all described in wikipedia, actually.