At 22:00 -0500 on 10/20/2008, Paul Gilmartin wrote about Re:
Addressing Scheme with 64 vs 63 bits:
On Mon, 20 Oct 2008 20:31:08 -0500, W. Kevin Kelley wrote:
On Mon, 20 Oct 2008 15:00:48 -0500, Paul Gilmartin
<[EMAIL PROTECTED]> wrote:
I thought 31-bit was for compatibility with existing exploitation
of the sign bit; ...
Nope, it was the use of a X'80' in the high-order byte of a fullword to
terminate a variable-length parameter list (of fullwords).
"Nope"? Where do you perceive us to be in disagreement?
Of course, with the transition from 370 to XA, the bit could
no longer be set with MVI; OI was required. This almost
certainly provoked an interminable thread on ASSEMBLER-LIST
concerning the performance impact. I'd hardly be surprised,
though, if OI on XA ran faster than MVI on 370.
-- gil
There was also the secondary use of the high bit to signal AM24 vs
AM31 in addresses used for branching to/from subroutines. This
required replacing BALR with BASR and BR with BSM to do the AM Mode
Switch.
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