It's only the single bit that causes ambiguity:
80000000 or 00000000800000000
0000000081234567 can be 01234567 (31bit mode)
0000000081234567 can be 81234568 (64bit mode)
SO, they just said that any address with the following bit is invalid:
0000000080000000
Tony Thigpen
Paul Gilmartin wrote on 11/04/2014 07:22 PM:
On 2014-11-04, at 17:07, Tony Thigpen wrote:
The first bit of the address in the PSW indicates that the address is a 31 bit
address. So, there was ambiguity for any address with the first bit set on.
Was the bit on because it was 31bit code setting the flag, or was it a real
address in 64bit mode?
It was easier to just block out the range as unusable.
Wouldn't that be similarly true for ranges such as 6GiB <= A < 8GiB,
10 GiB <= A < 12 GiB, ...? Are all such ranges blocked out?
-- gil
