[Coco] 6309 Weirdness ??

[Robert Gault]

Darren A. wrote:
> While putting together some new documentation for the 6309 CPU, I came 
> across some odd behavior with the new Inter-Register instructions. I was 
> wondering if anybody else may have come across this?
> 
> All of the new Inter-Register instructions (ADDR, SUBR, ANDR, etc..) are 
> 3 bytes in length, but when the PC register is used as either the source 
> or destination, the value utilized is the address of the instruction 
> plus 4.
> 
> So if X contains 0, and the instruction ADDR PC,X were executed at 
> address $7F00, you end up with $7F04 in X. However, on the MESS 6309 
> CoCo 3 emulator, you end up with $7F03 in X, which is how I assumed the 
> instruction should have worked (and I guess the MESS author(s) did too).
> 
> Even stranger, if you implement an endless loop using the following 
> instructions:
> 
>      LDX   #-4
>      ADDR  X,PC
> 
> ..the loop runs until an IRQ occurs, then when the interrupt routine 
> returns, it resumes execution at the address of the ADDR instruction 
> minus one, which in this case holds the value $FC (LDD extended). If you 
> mask interrupts before executing the loop, it runs forever.
> 
> I am wondering if these are bugs in the 6309 which were never fixed. 
> When Hitachi decided, for whatever reason, not to reveal the new 
> features, perhaps they also abandoned any effort to thoroughly debug them.
> 
> Note: I have performed tests in both native and emulation modes and 
> there is no difference.
> 
> Darren
> 
> _________________________________________________________________
> Make every IM count. Download Messenger and join the i’m Initiative now. 
> It’s free. http://im.live.com/messenger/im/home/?source=TAGHM_MAY07
> 
> 

My tests indicate that you are misinterpreting your results.

When I run the program
	org $7000
start	ldx #$1234
$7003	addr pc,x
$7006	swi
	end
the result is that regX contains $1234+$7003+3=$823A. If the program is 
changed to addr x,pc  there is a problem because the next instruction 
after addr pc,x is not swi. To correctly run the new test a program such 
as the following is needed.
	org $7000
start	ldx #1
$7003	addr pc,x
$7006	swi
$7007	swi
	end

Now when the program is run, it terminates at $7007=$7003+3+1. That is 
exactly what should happen.

Your example LDX #-4  ADDR X,PC  will lead to disaster. Using -3, the 
program should loop indefinitely at the addr x,pc instruction. But with 
-4, the program will jump into the middle of the ldx #-4 instruction. 
The result in most cases will be an illegal instruction resulting in the 
6309 jumping to a special trap which in most cases will not have a 
vector installed.