I was reluctant to open up the issue that Alan Fargusson has addressed, but since he has done so . . .
Historically, IBM mainframe applications were executed in AMODE(24) address spaces containing at most 2**24 = 16,777,216 bytes; and there are in fact many, far too many, such applications srill in heavy use, all plagued by what in the jargon are called virtual-storage constraint-relief (VSCR) problems.
They were succeeded by AMODE(31) applications, which make address spaces of at most 2**31 = 2,147,483,648 bytes available. This a 127-fold increase (2*31/2**24 = 2**7 = 128), but for many applications it too is proving to be inadequate.
AMODE(64) now makes address spaces of at most 2**64 bytes = 18,446,744,073,709,551,616 bytes available. This is a qualitatively different kind of increase, a 1.86-billion-fold one that for the foreseeable future is likely to provde adequate storage for almost any application.
Moreover, compiled C code can already make very effective use of this space and the new z/Architecture instructions made available with it.
Thus, while the economic case for converting existing 31-bit applications to 64-bit ones may or may not be persuasive (Often it is not), the case for developing new ones in anything bit a 64-bit environment is a dubious one.
John Gilmore SystemCraft LLC
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