Post by thread:16C84 addressing

This is my first project using a 16C84 and it's special function registers located on page 1 so it is a learning exercise for me. In the include files (MPASM) for the '84 it lists: TRISA EQU H'0085' Further, the data book says that the direct addressing mode uses only 7 bits from the opcode and that operations on page 1 require setting RP0=1. My question is this, why does the file assign the value of 85 to be used when addressing the TRISA register if the msb is not even used? Would not a value of 05 give the same result? Thanks,...K.R...

Kevin Rhoades <spam_OUTPICLISTTakeThisOuTMITVMA.MIT.EDU> wrote: > In the include files (MPASM) for the '84 it lists: > > TRISA EQU H'0085' > > Further, the data book says that the direct addressing mode uses > only 7 bits from the opcode and that operations on page 1 require > setting RP0=1. > > My question is this, why does the file assign the value of 85 to be > used when addressing the TRISA register if the msb is not even used? > Would not a value of 05 give the same result? Kevin: Yes. In fact, if you use the include file and write: MOVWF TRISA MPASM will generate code that's equivalent to "MOVWF 005H". On top of that, it'll give you a warning that the operand was truncated. Given this, it seems silly to use the "085H" definition. Using that definition, however, 1. more accurately depicts what your code's doing, making it more self-documenting, 2. allows simulators and emulators to track the TRISA register, and 3. builds character. There are any number of ways to keep the "truncated" warning messages from appearing... One of the most straightforward is to replace instructions like: MOVWF TRISA with instructions like: MOVWF 080H^TRISA -Andy Andrew Warren - .....fastfwdKILLspam@spam@ix.netcom.com Fast Forward Engineering, Vista, California http://www.geocities.com/SiliconValley/2499

{Quote hidden}> Kevin Rhoades <PICLISTKILLspamMITVMA.MIT.EDU> wrote: > > > In the include files (MPASM) for the '84 it lists: > > > > TRISA EQU H'0085' > > > > My question is this, why does the file assign the value of 85 to be > > used when addressing the TRISA register if the msb is not even used? > > Would not a value of 05 give the same result? > > Kevin: > > Yes. In fact, if you use the include file and write: > > MOVWF TRISA > > MPASM will generate code that's equivalent to "MOVWF 005H". On top > of that, it'll give you a warning that the operand was truncated. > > Given this, it seems silly to use the "085H" definition. Using that > definition, however, > > 1. more accurately depicts what your code's doing, making it > more self-documenting, > 2. allows simulators and emulators to track the TRISA > register, and > 3. builds character. > > -Andy > and... 4. you can write: MOVLW TRISA MOVWF FSR to access the register indirectly. ___Bob