I was not aware that the System/360 had 16 bit addressing, I thought it was always 24 bit.
There have been some other 32 bit processors with 24 bit addressing as well (such as the 68000). Having the address size different than the word size of the machine has usually resulted in problems. What often happens is that the latter versions of the processor use a larger address size like 32 bits in the 68000, and 31 bits in 370/XA. The result is incompatibilities with older software, which lead to things like above the line and below the line memory in OS/390. I suspect that IBM went to 64 bit addressing on the zSeries to avoid this, although it may have been because Sun and HP went to 64 bit addressing on their 64 bit processors. It is also worth noting that some AS/400 customers ran out of virtual memory on the 48 bit systems. This is partly due to the persistent virtual address scheme used in OS/400, but one still has to conceder this when deciding how many address bits are needed by todays applications. The AS/400 group insisted that the Power based systems only used the 64 bit chips for this reason. -----Original Message----- From: Phil Payne [mailto:[EMAIL PROTECTED]] Sent: Friday, February 14, 2003 4:46 AM To: [EMAIL PROTECTED] Subject: Re: IBM stops Linux Itanium effort > Careful. The question isnt "who needs 64bits" its "who cant manage with > 32". The answer to the two is quite different. And doesn't have to be 64 bits. System/360 has used 16 bits, 24 bits, 31 bits - why not 48? The AS/400 managed quite well. It was in fact my point that you don't need 64 bits - the size is forced upon us by the applications we want to port to it - the first of all of them having been written for DEC Alpha. System/390 only went all the way to 64 bits, IMO, because USS was seen as a 'landing strip' for such applications. Linux/390 got lucky. (So did IBM. Someone told me that Linux/390 has been very useful in testing out 64-bit virtual execution.) -- Phil Payne http://www.isham-research.com +44 7785 302 803 +49 173 6242039
