Michael Rigby-Jones wrote:
> 
>> -----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
 > x40 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,...

I think it is.
To use the BSR, the machine inst for ADDWF "2400" would have been
"2500". The difference is the "a" bit. a = "0" => access bank.
And the "MOVWF   _SSPBUF" also uses the access bank, so it weird...

Maybe the asm listing isn't the final PIC code ?
Notice that the target adress of both MOVWF and MOVFF is h'000',
SSPBUF is at h'FC9' (in one of the PIC18 I looked at).

I'd try to write a few lines in asm in MPALB and run it
through MPSIM and see what happens.

Jan-Erik.


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