>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>On Behalf Of Peter Todd
>Sent: 16 October 2007 12:00
>To: piclist@mit.edu
>Subject: [PIC] movff ignores banksel, correct?
>
>
>Using this macro:
>
>#define i2c_write(c) \
>  PIR1bits.SSPIF = 0; \
>  SSPBUF = (c); \
>  while (!PIR1bits.SSPIF); // cleared at 9th (ACK) clock
>
>
>i2c_write(i);
>
>!=
>
>i2c_write(0x40 + i);
>
>
>Looking at the outputted asm code the former produces:
>
>00346 ;       .line   71; mcp23017.c  i2c_write(i);
>0030 9600      00347         BCF     _PIR1bits, 3
>0032 C000 F000 00348         MOVFF   r0x00, _SSPBUF
>0036           00349 _00108_DS_:
>0036 A600      00350         BTFSS   _PIR1bits, 3
>0038 EF00 F000 00351         GOTO    _00108_DS_
>
>and the latter:
>
>00346 ;       .line   71; mcp23017.c  i2c_write(0x40 + ((i)));
>0030 9600      00347         BCF     _PIR1bits, 3
>0032 0E40      00348         MOVLW   0x40
>0034 2400      00349         ADDWF   r0x00, W
>0036 6E00      00350         MOVWF   _SSPBUF
>0038           00351 _00108_DS_:
>0038 A600      00352         BTFSS   _PIR1bits, 3
>003A EF00 F000 00353         GOTO    _00108_DS_
>
>
>Basically identical. However the latter outputs 0x00 on the 
>i2c bus regardless of what value i is. The former works fine, 
>although I rather need that 0x40 bit. I'm guessing that sdcc 
>is missing a banksel instruction. It's the only thing I can 
>think of that would make the two versions act so differently.
>
>Does this make any sense to you guys?

I think your conclusion is correct.

MOVFF takes a 12 bit address, so it can operate on any address in the data memory with no banking.  The second bit of code loads 0x40 into W and tries to add it to your 'i' value by using ADDWF.  Unfortunately ADDWF takes only an 8 bit address, and since the access bank is not being used here, if the BSR register is not correctly set at this point the result will be invalid.

It may be worth trying this with the optimiser disabled to see if the bank selction is being removed by the optimiser or never inserted in the first place.


Regards

Mike

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