Post by thread:bittest on W

I tried the following to see if the reslt of a substraction was negative... movf test, w sublw 0x0x btfss w, 7 ; see if the MSB in W is set do this do that but it did not work...... what works is after the substraction movwf temp btfss temp, 7 di this do that Can anyone tell how come? Cannot bits in W be tested? Henk Tobbe VK2GWK

Agnes en Henk Tobbe wrote: > Can anyone tell how come? Cannot bits in W be tested? Nope. W is not a register as all the others. W can be written too and read from, but cannot be manipulated as with other registers. W is the working register for the ALU and shouldn't be thought of as an actual register. You have to do things like you did in your second example. Quentin

On Thu, 18 Nov 1999 06:56:49 +1100 Agnes en Henk Tobbe <spam_OUTtobbeTakeThisOuTBIGPOND.COM> writes: >I tried the following to see if the reslt of a substraction was >negative... >movf test, w >sublw 0x0x >btfss w, 7 ; see if the MSB in W is set >do this >do that > >but it did not work...... >what works is >after the substraction > >movwf temp >btfss temp, 7 >di this >do that > >Can anyone tell how come? Cannot bits in W be tested? > On most PICs, the W register is not in the "file register" map, so "F" instructions such as btfss do not work. The 17c and 18c processors have wreg, which IS in the file register map, so you can use these instructions there. Note that "w" is 0 and "f" is 1 in the processor include files so you can use them as the destination bit in instructions that take a destination bit. Finally, with the SUB instructions on a PIC, you can check for a negative result by checking the carry bit of the status register. The bit is set if the result of the subtraction was positive and clear if the result was negative. Harold Harold Hallikainen .....haroldKILLspam@spam@hallikainen.com Hallikainen & Friends, Inc. See the FCC Rules at http://hallikainen.com/FccRules and comments filed in LPFM proceeding at http://hallikainen.com/lpfm ___________________________________________________________________ Get the Internet just the way you want it. Free software, free e-mail, and free Internet access for a month! Try Juno Web: dl.http://www.juno.com/dynoget/tagj.

> <tobbeKILLspamBIGPOND.COM> writes: > >I tried the following to see if the reslt of a substraction was > >negative... > >movf test, w > >sublw 0x0x > >btfss w, 7 ; see if the MSB in W is set > >do this > >do that Try using the carry flag instead. See this page for some examples. http://www.picnpoke.com/projects/compares.txt -- Best regards Tony http://www.picnpoke.com Email .....salesKILLspam.....picnpoke.com

Hi Agnes en Henk Tobbe. > I tried the following to see if the reslt of a substraction was negative... > movf test, w > sublw 0x0x > btfss w, 7 ; see if the MSB in W is set > do this > do that It will not work. W register in 16Cxx is something special ;-) You can't apply btfss/btfsc operations to it. You need to put W value to some temp location first and finally check temp with btfss/btfsc commands. BTW. For what you've copied test in W register. Apply btfsc/btfss directly to test register. ;-) WBR Dmitry. HINTS: 1. ANDLW 0x80 ; will test bit .7 in W, other bits are cleared ;-) ANDLW 0x40 ; test bit .6 ;... result of operation in _Z flag (=0 means bit=1 otherwise bit=0 result to _Z=1) ANDLW is destructive command. You cannot restore W if you didn't save it before. 2. ADDLW 0x88 ; will test bits .7 and .3 results of operation in _C and _DC flags. To restore initial W just ADDLW -0x88 ;-) 3. You can also apply XORLW value command to check identity W to some value. It is useful command (sometimes) 'cause you can do comparison for alot of values and restore W after that to initial value. XORLW A_CONST SKPNZ GOTO A_CONST_MATCH XORLW B_CONST^A_CONST ;compare with B_CONST SKPNZ GOTO B_CONST_MATCH XORLW C_CONST^B_CONST ;compare with C_CONST ;.... XORLW LAST_CONST ;last used const value -> restore initial W ;.....