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

[Robert Gault]

The last message in the original thread suggested that ZBUG (or EDTASM) 
might have introduced errors in reading non or partially readable bytes 
such as the MMUs. That does not seem to be the case as you can see from 
the following test.

Source code
               org $E00
               rmb  2
        start  ldx #$FFA0
E6 00         ldb  0,x
A6 84         lda   ,x
FD 0E 00      std  $E00
               rts
               end  start

This was assembled and run from Basic, not used under EDTASM control.

Results
?PEEK(&HE00)
248                ie. $F8
?PEEK(&HE01)
56                 ie. $38

Source code #2
               org $E00
               rmb  2
        start  ldx #$FFA0
E6 84         ldb   ,x
A6 84         lda   ,x
FD 0E 00      std  $E00
               rts
               end  start

Results
?PEEK(&HE00)
248                ie. $F8
?PEEK(&HE01)
184                ie. $B8

Source code #3
               org $E00
               rmb  2
        start  ldx #$FFA0
E6 00         ldb  0,x
A6 84         lda   ,x
12            nop
FD 0E 00      std  $E00
               rts
               end  start

Results
?PEEK(&HE00)
56                 ie. $38
?PEEK(&HE01)
56                 ie. $38

  EDTASM does generate correct code or very few if any of our assembled 
programs would run correctly. The above was run from Basic so ZBUG was 
not involved. The results were changed by both the byte preceding the 
Load instruction and that following the Load instruction. In fact, ex. 
#3 had both preceding Loads changed by the first post byte.
  It is a good thing that this only happens when reading non or 
partially readable bytes or one could not write code using the 6809/6309 
cpu.

What this means I'll leave up to the hardware guys. Note that the msn 
gets changed but not the lsn of the info read from $FFA0. Installed 
memory for the tested Coco is 512K so $FFA0 bits 6&7 are not meaningful. 
Above we have seen these bits filled in with 11, 10, and 00. There 
probably is code that sees the bits filled with 01.