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Pic 24EP and compiler 5

 
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championx



Joined: 28 Feb 2006
Posts: 151

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Pic 24EP and compiler 5
PostPosted: Tue May 12, 2015 12:52 pm     Reply with quote
Hi, I'm trying a pic24EP512GU806 on a board, and I'm writing a small code, but I'm having some strange troubles.

Compiler version 5.042

I'm just printing the task_queue buffer. But, i want to ignore when the characters are 0xFF or 255 in decimal (character: 'ÿ')

This code works fine, the 255 characters are ignored

Code:

#define TASK_QUEUE_SIZE 300
char Task_queue[TASK_QUEUE_SIZE];

void Show_task_queue()
{
   int16 task_index_debug = 0;
   
   fprintf(U2,"Sys Task queue");
   for(task_index_debug = 0; task_index_debug <= TASK_QUEUE_SIZE; task_index_debug++)
   {
      if(task_queue[task_index_debug] != 'ÿ')
         fprintf(U2,"%c",task_queue[task_index_debug]);
   }
   
return;
}



But this one not, it prints all the characters, including the 255.

Code:

#define TASK_QUEUE_SIZE 300
char Task_queue[TASK_QUEUE_SIZE];

void Show_task_queue()
{
   int16 task_index_debug = 0;
   
   fprintf(U2,"Sys Task queue");
   for(task_index_debug = 0; task_index_debug <= TASK_QUEUE_SIZE;   task_index_debug++)
   {
      if(task_queue[task_index_debug] != 0xFF)
         fprintf(U2,"%c",task_queue[task_index_debug]);
   }

return;
}



both codes are exactly the same... only changes:

if(task_queue[task_index_debug] != 'ÿ')

for

if(task_queue[task_index_debug] != 255)
jeremiah



Joined: 20 Jul 2010
Posts: 1334

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PostPosted: Tue May 12, 2015 2:02 pm     Reply with quote
Have you tried (char)(0xFF)?

I know some versions of the PCD compiler have had issues doing comparisons versus defined constants because it assumes a different type promotion for the constant than it does the variable. You end up getting a sign extended version compared to a non sign extended versions

Since the default type of constants on PCD is 16 bit and a char is 8bit, I could see this being a possible cause.

EDIT: Also, as a matter of practice, one should always (always) use {} with their if statements. Not doing so is going to bite you one day.

EDIT2: Just tossed your code into a sample PCD project. This was the LST output:
Code:

....................       if(task_queue[task_index_debug] != 0xFF){
00210:  MOV     #1000,W4
00212:  MOV     112C,W3
00214:  ADD     W3,W4,W0
00216:  MOV.B   [W0],W5L
00218:  SE      W5,W5
0021A:  MOV     #FF,W4
0021C:  CP      W4,W5
0021E:  BRA     Z,220


Notice how the variable gets copied as a byte (MOV.B) and then sign extended (SE) and the is compared to a non sign extended 0xFF (Which happens when 0xFF is considered a 16 bit value).

Casting 0xFF as a char changes it to:

Code:

if(task_queue[task_index_debug] != (char)0xFF){
0020E:  MOV     #1000,W4
00210:  MOV     112C,W3
00212:  ADD     W3,W4,W0
00214:  MOV.B   [W0],W4L
00216:  XOR.B   #FF,W4L
00218:  BRA     Z,21A

which will probably work (I don't have anything to test on).
Ttelmah



Joined: 11 Mar 2010
Posts: 19433

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PostPosted: Wed May 13, 2015 2:05 am     Reply with quote
You are also printing one to many entries. An array with 'TASK_QUEUE_SIZE' entries has entries from 0 to TASK_QUEUE_SIZE-1.
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