[MI400] MI Newbie with XLATE instruction problem

[Winchester Terry <terry.winchester@xxxxxxxxxxxxxxx>]

As the subject implies, I'm an MI newbie and having a 
problem using the XLATE instruction. My goal is pretty 
simple - create a program that accepts 3 parameters

Parm1= String   10 chars
Parm2= OldChar   1 char
Parm3= NewChar   1 char

and translate each character in "string" from OldChar
to NewChar.  At least I *thought* this was simple :(
My ultimate goal is to handle a string of 2000-3000
characters callable from any HLL.

Here is my code, but it does not seem to translate
the characters properly.  The SHOWMSG routine is
straight from Leif's MI examples.  

DCL SPCPTR .P1 PARM;                         
DCL DD PARM-STRING   CHAR(10) BAS(.P1);      
                                             
DCL SPCPTR .P2 PARM;                         
DCL DD PARM-OLDCH    CHAR(1)  BAS(.P2);      
                                             
DCL SPCPTR .P3 PARM;                         
DCL DD PARM-NEWCH    CHAR(1)  BAS(.P3);      
                                             
DCL OL PARMS(.P1, .P2, .P3) EXT PARM MIN(3); 
                                             
DCL SPCPTR .NEWSTR INIT(NEWSTR);             
DCL DD      NEWSTR CHAR(10);                 
                                                            
ENTRY * (PARMS) EXT;                                        
    CPYBLA      NEWSTR, PARM-STRING;                        
    CPYBREP     PARM-STRING, " ";                           
    XLATE       PARM-STRING, NEWSTR, PARM-OLDCH, PARM-NEWCH;
    CPYBLA      PARM-STRING, NEWSTR;                        
                                                            
    CAT         MSG-TEXT, PARM-STRING, "|";                 
    CALLI SHOW-MESSAGE, *, .SHOW-MESSAGE;                   
    RTX          *;                                         
%INCLUDE SHOWMSG                                            
 
When I call this program from a command line:

CALL PGM(MYPGM) PARM('1234567890' '4' 'A')

I see the results in my message queue but
nothing has changed:

>From  . . . :   TWINCHES       08/31/04   09:47:43  
1234567890|                                         

Of course, I was expecting to see: 123A567890|

Originally, the XLATE code looked like this:

XLATE       PARM-STRING, PARM-STRING, PARM-OLDCH, PARM-NEWCH;

but since *that* didn't work I assumed that separate string 
variables were needed, hence my new version above that uses 
NEWSTR as a temporary variable to hold the inbound string 
parameter.

I'm missing something simple here...but just can't see it...
Any help would be appreciated :)

Terry