Referencing C variables in In-line assembly - Electronic Circuits Projects Diagrams Free

zevious · 6th February 2009, 07:27 AM

Hi,

I'm wrote a small 18F in-line assembly snippet in MPLAB referencing a local C variable. From the results I'm not referencing the C variable correctly.

Any ideas?

unsigned char count = 0;
unsigned char current_count = 10;

count = current_count;

_asm

// Loop until count is 0
start:
DECFSZ count, 1, 0
BRA start
GOTO done
done:

_endasm

Pommie · 6th February 2009, 08:32 AM

To use a variable without managing the banking you need to have it in access ram. Try moving your variable so it is global and put it in the access area by doing,

Code:

#pragma udata access somename
near unsigned char count;
#pragma udata

Mike.

zevious · 6th February 2009, 03:03 PM

Thanks Mike,

How would I manage the banking to avoid allocating it in access ram?

Pommie · 6th February 2009, 03:11 PM

Well, if you run out of access ram you need to load the bsr with the bank number of your variable. What is wrong with using the access ram, the compiler doesn't seem to!!

Mike.

zevious · 6th February 2009, 09:20 PM

Thanks again Mike,

I believe I can use access RAM (near), and it do sound like it would end up being faster and require less cycles. But lets say I wanted to set the BSR with bank information to access the count variable in my simple loop example above. How would I accomplish that?

I am sorry for these newbie questions but I'm just starting up with MPASM syntax (as you can tell)

Pommie · 7th February 2009, 02:54 AM

You would simply do movlb count.

Mike.

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6th February 2009, 07:27 AM	#1
zevious	Referencing C variables in In-line assembly Hi, I'm wrote a small 18F in-line assembly snippet in MPLAB referencing a local C variable. From the results I'm not referencing the C variable correctly. Any ideas? unsigned char count = 0; unsigned char current_count = 10; count = current_count; _asm // Loop until count is 0 start: DECFSZ count, 1, 0 BRA start GOTO done done: _endasm

6th February 2009, 08:32 AM	#2
Pommie	To use a variable without managing the banking you need to have it in access ram. Try moving your variable so it is global and put it in the access area by doing, Code: #pragma udata access somename near unsigned char count; #pragma udata Mike.

6th February 2009, 03:03 PM	#3
zevious	Thanks Mike, How would I manage the banking to avoid allocating it in access ram?

6th February 2009, 03:11 PM	#4
Pommie	Well, if you run out of access ram you need to load the bsr with the bank number of your variable. What is wrong with using the access ram, the compiler doesn't seem to!! Mike.

6th February 2009, 09:20 PM	#5
zevious	Thanks again Mike, I believe I can use access RAM (near), and it do sound like it would end up being faster and require less cycles. But lets say I wanted to set the BSR with bank information to access the count variable in my simple loop example above. How would I accomplish that? I am sorry for these newbie questions but I'm just starting up with MPASM syntax (as you can tell)

7th February 2009, 02:54 AM	#6
Pommie	You would simply do movlb count. Mike.