Support for marking absolute stack access

[fachat]

In "classic" 6502 program sometimes it is assumed that the stack is fixed at $01xx, and is accessed directly, like

     TSX
     LDA $0101,X

to get a parameter value from the stack.

This, however, conflicts with some more modern 6502 variants, most notably the 65816, where the stack can be moved to anywhere in bank zero.

So, to allow 65816-based systems to identify programs that use this approach (and either handle it, or refuse to load it), a new header bit is proposed:

     mode.8 :     dstack   0= (ignored)    1= program accesses stack directly via addresses $01xx