Destination can be W or F

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micropad

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Hi
happy New yea All.
As per the attached picture, I pointed out in red cycle
that is Destination. so Destination can be W or F but in table sows only F
please guide me

pic 16f877a
 

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micropad said:
Hi
happy New yea All.
As per the attached picture, I pointed out in red cycle
that is Destination. so Destination can be W or F but in table sows only F
please guide me
Click to expand...

What's to guide?

Fairly obviously some instructions can't possibly have the option of either - for example MOVLW which can only have a destination of w.
 
Dear Nigel,
Thanks for reply

but ADDWF can be destination W or F as per my picture it is only F ( Result we can put either W or F)

please guide
 
Have a look at my attachment.

Does that help explain it?

JimB
 

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micropad...when executing a 2 operand instruction (i.e. most instructions that end with "wf" are 2 operand instructions), the second operand is the destination operand where the result of the instruction will get stored. There are only two choices for this...W or F.

If you use F, the result gets stored in the file location that the instruction was performed on.

If you use W, the result gets stored in the W register while the contents of the file that the instruction was performed on remains unchanged.
 
If you are using an assembler or a compiler you don't have to worry about the encoding. That is all taken care of for you.

In the ADDWF instruction, it starts 000111

The next bit is the destination, "d". It is either 0 representing a destination of the W register, or 1 representing a destination of the addressed register, F.

The last 7 bits, "fffffff", are the addressed register, F.

In assembler, you just write

ADDWF 0x20, F

and the assembler will convert that to

0001111010000
 
micropad said:
but ADDWF can be destination W or F as per my picture it is only F
Click to expand...

You can put a W in place of the "f". The "f" is used as a variable here. Type W or WREG in any place the "f" is shown and it will go to W.
 
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Basically the destination operand can only be a 0 (F) or a 1 (W). But 0 and 1 are equated to F and W respectively in the device header file.
 
JimB said:
Have a look at my attachment.

Does that help explain it?

JimB
Click to expand...

Dear JimB

Thanks for reply

please see picture

I need to know why datasheet not included W in that Location

Please guide
 

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That is because d can be either W or F. You are confusing two different things together.

The f in your original picture (i.e. ADDWF f, d) is for the register (or file) that is going to be operated on and can be a value between 0 and 0x7F. The d is the destination, which can be 0 or 1. If it is 0 the result is stored back in W, if it is 1 the result is stored back to the register you oprate on (i.e. the 0 - 0x7F).

In the datasheet 'f' is used to describe two different things.

In the processor header w and f are defined to 0 and 1 respectively to make assembly writing easier. It's easier to see what "addwf porta, w" does as opposed to "addwf porta, 0".
 

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Yes..."f" and "d" are variables that the data sheet uses for the purpose of illustrating the syntax of the instruction only. You would replace "f" with the file address that you wish for the instruction to be performed on, then replace the "d" with either W or F, depending on where you want it to store the result of the instruction.
 
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Thanks hantto,
you pointed out it's very well. I got the point
Thanks again and again
 
It looks like instruction ADDWF does not include the W register as a third operand in table 15-2 because the W register can be chosen as a destination by the second 'd' operand. If you are asking why the W register is not included as a source operand, I would guess that is because it is implied as a source operand in the instruction name itself, and it's address is already known. A more detailed description is shown in section 15.2.
 
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