those Actually the figure is 29 bits (it was 512M not 512K) but I agree with you 100% in principle. The way I look at it is this: Memory sizes are growing exponentially. So address bus widths can be expected to grow linearly. It has taken roughly 30 years between 64K words or bytes getting tight (early '70s, DEC up the Unibus from 16 bit addressing to 18 bit, but there's probably an example bit addressing looks like it might get tight. (Note this is on small computers. On large computers, I'd estimate 10 years earlier in each case.) I therefore see address buses growing at 16 bits every 30 years. That's just over a bit every 2 years - slower than I expected but not much. Someone (I forget who) said that memory chips double in capacity every 18 months. This would give 16 bits in 24 years. I claim that the assertion that we'll see even 64-bit address spaces being used anything like up by 2003 is unfounded. According to that growth rate above, we will start hitting the limit of 48-bit addressing - 256 TWord - in the '20s, and the limit of 64-bit addressing, 16 exawords (or exabytes, possibly), in the '50s (or '40s at 1 bit per 18 months). Many of us will probably still be alive then (I shall be celebrating my 83rd birthday in March of 2050) - and I for one would like to see what sort of technology will be used to store 16 exabytes in a space smaller than a mountain! Philip.