[PicForth] Possible code generation bug involving 'if' and program memory bank switching

[Alex Holden]

Hi, I think I've found another code generation bug involving program 
bank switching. This time it seems to cause the PIC to jump to lala land 
when an 'if' in program memory bank 1 calls a word in program memory 
bank 0 and then loops back to do the test again.

Here's a minimal example:
7f9 org
: sometest 1 ;
: dosomething ;
main : test begin sometest if dosomething then again ;

My program (which is much bigger but has a very similar construct in it) 
goes mad when 'sometest' returns true. If I change the code to the 
equivalent of 'begin sometest drop dosomething again' it works fine 
(except of course it unconditionally calls dosomething every time around 
the loop).

Here's the generated code:

0x0000  018A    clrf    0x0A
0x0001  2FFE    goto    0x7FE   ; (init-picforth)
         ; name: sometest
         ; max return-stack depth: 0
0x07F9  3001    movlw   0x01
0x07FA  0384    decf    0x04,f
0x07FB  0080    movwf   0x00
0x07FC  0008    return
         ; name: dosomething
         ; max return-stack depth: 0
0x07FD  0008    return
         ; name: (init-picforth)
         ; max return-stack depth: 0
0x07FE  3032    movlw   0x32
0x07FF  0084    movwf   0x04
         ; name: test
         ; max return-stack depth: 1
0x0800  018A    clrf    0x0A
0x0801  27F9    call    0x7F9   ; sometest
0x0802  0800    movf    0x00,w
0x0803  0A84    incf    0x04,f
0x0804  3800    iorlw   0x00
0x0805  158A    bsf     0x0A,3
0x0806  1903    btfsc   0x03,2
0x0807  280A    goto    0x00A   ; test + 0x00A (0x80A)
0x0808  018A    clrf    0x0A
0x0809  27FD    call    0x7FD   ; dosomething
0x080A  2800    goto    0x000   ; test (rs depth: 1) (0x800)

Where I think it's going wrong is the sequence at 0x808, 0x809, 0x80A 
which is executed when 'dosomething' returns true. It sets PCLATH to 
bank 0, calls a word in bank 0, then does a jump to an address in bank 1 
without setting PCLATH to bank 1 first.

-- 
------------ Alex Holden - http://www.linuxhacker.org ------------
If it doesn't work, you're not hitting it with a big enough hammer