[Coco] "Reading" non-readable bytes with PEEK vs ZBUG

[Roger Taylor]

At 06:13 PM 1/24/2008, you wrote:

>Roger, regardless of whether a Coco3 has 128K, 512K, or more memory, 
>when you send $38-$3F to a GIME MMU register you get the same 
>result. If you have only 128 or 512K memory, it does not matter what 
>values you give to the two most significant bits because it won't 
>change anything regardless what you send to these bits on a Store! 
>Those bits are only important with more than 512K RAM when you write 
>to them. They are meaningless on a Read regardless of memory present.
>
>That means it does not matter in your program what you are doing 
>during a LOADM as long as you don't store a value less than $38 or 
>higher than $3F in an MMU register. Depending on the code in your 
>program, it may not matter whether you only use MMU values of 
>$00-$0F because the net result will can still be the same as long as 
>you don't need the full address of the bytes those MMUs control.
>
>If you would post the code you are using during the LOADM (or send 
>it by private e-mail) I could show why the results of the survey 
>don't matter. In terms of LOADM, the survey of what can or can't be 
>read from an MMU register has been pointless except for the 
>interesting results caused by "floating" bits in a Coco3 system. By 
>the way, you get the same "incorrect" values with a Paul Barton 8Meg 
>RAM board installed. Just tested that.


Robert, there's a logic to what I'm doing because of 1) ghosting, 2) 
my program will only use 128k of RAM no matter what CoCo it's on, and 
3) LOADM should see a %01000000 float or actual (doesn't matter to 
me) when it verifies my GIME writes, and 4) I'll reference the same 
block #'s in my program by adding %01000000 to the GIME writes.

The actual MMU block # in the entire 2-meg range that is mapped in on 
any given CoCo the user has shouldn't matter one bit, as long as I 
leave BASIC's area alone, which to the best of my knowledge is 
sitting smack dab at "$38-$3F" on any CoCo 3.  Anything outside of 
that is .... Mine to play with, ghosted or not.

If you see something I don't and want to explain why it's still 
working, feel free.  I'm quite happy.

         org     $FFA1
         fcb     %01000000+$30           switch in block $30 at $2000
         fcb     %01000000+$31           switch in block $31 at $4000
         org     $2000                   load next data into $2000+
         includebin      "scrpart1.dat"  1st 16k of a graphics screen

         org     $FFA1
         fcb     %01000000+$32           switch in block $30
         fcb     %01000000+$33           switch in block $30
         org     $2000                   load next data into $2000+
         includebin      "scrpart2.dat"  2nd 16k of a graphics screen 
(slightly less than 16k)

         org     $FFA1
         fcb     %01000000+$39           restore MMU
         fcb     %01000000+$3A           these blocks were there for any CoCo 3

         org     $1000                   start of our program code

         ... everything else