In SX Microcontrollers, SX/B Compiler and SX-Key Tool, sammishal wrote:

Mr. Gunther,

 

I am wondering...... The manual says that the PC register is actually a 9 bit register,

BUT in the SXSim (and IDE Debug) the PC register is showing only 2 digit hex which is 8 bits, 
what is happening here when PC should be for example $300 (0011 0000 0000)?

 

 

Now the MostSig 3 bits 001 are stored in the Status register but the next bit 1 is nowhere 

to be acounted for!!!????

 

Also I have a puzzle.......See this code:

======================================

 LIST Q = 37
 DEVICE  SX28L, TURBO, STACKX, OSCHS2
 IRC_CAL IRC_FAST
 FREQ 50_000_000
 RESET   _Main


       org $1FE

_Main
      jmp @Main

 

 

     Org $200
Main
     mov    w,#$01
     call     @subtest
     mov    $08,w
     mov    $08,w
     jmp     $

 
     

    Org $600
SUbTest
    mov     $08,w
    mov     $08,w
    mov     w,#10
    ret

 

END ;End of program code
==========================================================


This code causes the DEbug to go to neverland after the execution of the JMP $ line

and causes an error dialogue on the SXSim. This is obviously due to use of

RET instead of RETP and thus the STATUS register is not restored and therefore

when JMP $ is executed the Jump is made to a bad address (not as intended the current line).

 

What I am puzzled about is this :

 

How does the RET instruction cause the execution to go back to the line right after

the Call @SubTest????????

 

If the RET only pops the PC off the stack and the Page is not restored, HOW???

does the execution go back to the correct page????????

 

It only fails on the JMP instruction due to wrong Page, but WHY does

the execution resume on the correct page even though the SATUS register still shows

the wrong page. 

 

Is there some shadow register that is actually used upon RET instead of the STATUS for the 

Top 3 three bits?

 

 

As a side the RETW instruction also does not restore the Status register. IE RETW behaves

exactly like RET (not RETP) but with the added action of moving a constant into the W register.

 

 

Regards

 

Samuel

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