[Coco] 6309 Weirdness ??

[Robert Gault]

Darren A. wrote:
> OK, things are even stranger than I first thought.  Below is a BASIC 
> program that can be used to demonstrate my new theory:
> 
> 10 CLEAR 200,&H6FFF:A=&H7000
> 20 READ B$:IF B$="XX" THEN 40
> 30 POKE A,VAL("&H"+B$) :A=A+1:GOTO 20
> 40 EXEC &H7000
> 50 PRINT "X = ";HEX$(PEEK(&H600));HEX$(PEEK(&H601))
> 100 DATA 8E,FF,FD: REM LDX #-3
> 110 DATA 10,30,15: REM ADDR X,PC
> 120 DATA 12: REM NOP
> 130 DATA BF,06,00: REM STX $600
> 140 DATA 39: REM RTS
> 150 DATA XX
> 
> The program pokes the following code into memory, executes it, and 
> displays the value of the X register that gets saved at address $600:
> 
> 7000  LDX   #-3
> 7003  ADDR  X,PC
> 7006  NOP
> 7007  STX   $0600
> 700A  RTS
> 
> When I run the program I see:  X = FFF2
> 
> I think the actual instructions executed are:
> 7000  LDX   #-3
> 7003  ADDR  X,PC     ; PC = $7003+3+1-3 = $7004
> 7004  LEAX  -11,X    ; X = -3-11 = $FFF2
> 7006  NOP
> 7007  STX   $0600
> 7009  RTS
> 
> Note that the instruction following ADDR is a single byte (NOP). Now 
> elimintate that NOP by deleting line 120 and run the program again.
> 
> This time I see:  X = FFFD
> 
> I think the actual instructions executed are:
> 7000  LDX   #-3
> 7003  ADDR  X,PC     ; PC = $7003+3+3-3 = $7006
> 7006  STX   $0600
> 7009  RTS
> 
> Note that on this run, the size of the instruction following ADDR was 3 
> bytes long (STX $600).
> Now let's insert a 2-byte instruction after ADDR by entering the following:
> 
> 120 DATA 86,00 : REM LDA #0
> 
> If you run the program with this change, the CPU crashes (actually 
> invokes the illegal instruction trap).
> I think the actual instructions executed are:
> 
> 7000  LDX   #-3
> 7003  ADDR  X,PC     ; PC = $7003+3+2-3 = $7005
> 7005  FCB  $15     ; Illegal Instruction
> 
> So, given this information, my new theory (and this seems really weird) 
> is that the PC value used in the ADDR instruction is having the length 
> of the following instruction added to it. I know this is getting crazy, 
> but I just can't stop!
> 
> Darren
> 

Interesting!
I can confirm some of this on my Coco3. But there is a gotcha!

  org $7000
start ldx #-1
  addr x,pc
  nop
  stx $400
  rts
Word at $400=$ffff

  org $7000
start ldx #-3
  addr x,pc
  stx $400
  rts
Word at $400=$fffd.

Now if the above were true then the following should work.
  org $7000
start ldx #-2
  addr x,pc
  bra next
next stx $400
  rts
Crash!!!!! Same explanation and this code should work. Since it doesn't, 
something else must be happening.

The following doesn't work either.
  org $7000
start orcc #$50
  ldx #-2
  addr x,pc
  bra next
next stx $400
  andcc #$af
  rts
Crash!!!!!