In a message dated 11/14/2007 7:45:09 A.M. Central Standard Time, [EMAIL PROTECTED] writes: >My thinking, right or wrong was that if they used the lowest bit to do the signaling, they would have lost addressability to less. The lowest bit was assigned a meaning in the mid-1960s, namely an odd address which causes a specification exception. The reason the bit is on is almost a program error, although it could conceivably be turned on to force a program interrupt for some reason. To use it now as an indication of being in 64-bit addressing mode would thus require using another bit somewhere that the microcode would interrogate to determine which of two ways to interpret the lowest bit. Programs today are still generating odd addresses all the time due to program bugs. The best way to design new function is almost always to use previously unavailable or reserved resources, rather than to assign a new meaning to an already existing resource. It happens sometimes, though, when IBM realizes that no one is using the old function any more. The amount of virtual storage "wasted" by the addressing hole in vanishingly insignificant when compared to the total addressing capability of 2 to the 64th power number of bytes. A rough approximation is that we have only 2 to the 63rd different bytes we can address, using virtual addresses, on z/OS. Bill Fairchild Franklin, TN
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