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Jerry I
Joined: 14 Sep 2003
Posts: 96
Location: Toronto, Ontario, Canada
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| Make8() function using a variable as the offset !!! SOLVED |
 Posted: Tue Jun 20, 2017 3:00 pm |
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Hi
I am using Version PCM 5.52
Are you not allowed to use a variable for the offset?
From Manual
make8( )
Syntax: i8 = MAKE8(var, offset)
Parameters: var is a 16 or 32 bit integer.
offset is a byte offset of 0,1,2 or 3.
Returns: An 8 bit integer
Function: Extracts the byte at offset from var. Same as: i8 = (((var >> (offset*8)) &0xff)
except it is done with a single byte move.
Availability: All devices
Requires: Nothing
Examples: int32 x;
int y;
y = make8(x,3); // Gets MSB of x
Using their example as shown i8 = (((var >> (offset*8)) & 0xff
| Code: |
int32 var;
int8 i8[4], i;
var = 123456789;
for (i = 0; i < 4; i++)
{
i8[i] = (((var >> (i*8)) & 0xff;
}
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This works.
| Code: |
for (i = 0; i < 4; i++)
{
i8[i] = make8(var, i);
}
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This is an ERROR: Invalid parameters for built in function.
The above example does nothing, it will just show the error!
I thought make8() function would have saved me some code space?
Any Thoughts?
Thanks
Jerry
Last edited by Jerry I on Wed Jun 21, 2017 10:25 pm; edited 1 time in total |
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gaugeguy
Joined: 05 Apr 2011
Posts: 303
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| Posted: Tue Jun 20, 2017 3:27 pm |
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unroll it and it is faster and takes fewer instructions.
| Code: |
i8[0] = make8(var,0);
i8[1] = make8(var,1);
i8[2] = make8(var,2);
i8[3] = make8(var,3);
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Ttelmah
Joined: 11 Mar 2010
Posts: 19552
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| Posted: Wed Jun 21, 2017 12:54 am |
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It should make it plain that offset is a constant. However if you think about it a single byte move implicitly must involve a constant, since otherwise there will need to be calculations to find the byte. As Gaugeguy says 'unroll it'.
The same applied to lots of things where variables are allowed. So (for instance):
Where 'n' is a constant, codes as a single instruction. However using a variable, one single call codes to perhaps 20 instructions.... 
This is an area where 'tidy code', may not result in efficient code. |
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gaugeguy
Joined: 05 Apr 2011
Posts: 303
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| Posted: Wed Jun 21, 2017 9:34 am |
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| In embedded systems when coding in C the prettiest or most "standard" routines are not always the most efficient. This is where understanding the assembler code can make for a more efficient routine in C. CCS does a lot of special functions like make8 that allows C code to be written as efficiently as assembler. |
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asmboy
Joined: 20 Nov 2007
Posts: 2128
Location: albany ny
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| Posted: Wed Jun 21, 2017 11:23 am |
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| If this was facebook- the above would merit a very smiley faced 'like' |
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Jerry I
Joined: 14 Sep 2003
Posts: 96
Location: Toronto, Ontario, Canada
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| Make8() function using a variable as the offset !!! SOLVED |
| Posted: Wed Jun 21, 2017 10:24 pm |
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Hi All,
Thanks for the replies.
My problem is that I am transmitting a 32 bit integer via the i2c bus within the #int_ssp interrupt.
I transmit 1 byte at a time and the index = i2c state & 7 would have been the offset for the make8() function to send 4 bytes of 32 bit integer.
I will try something like this, possibly save me some code.
| Code: |
//// NEW IDEA ////
if (state >= 0x80)
index = state & 7
switch (index)
{
case:0
i2c_write(make8(32BitNumber, 0));
break;
case:1
i2c_write(make8(32BitNumber, 1));
break;
case:2
i2c_write(make8(32BitNumber, 2));
break;
case:3
i2c_write(make8(32BitNumber, 3));
break;
}
/// This is the original code ////
if (state >= 0x80)
index = state & 7
i2c_write((int8)32BitNumber) >> 8 * index;
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I just did a check.
The original code took about 35 lines of code, but these 35 lines execute 4 times.
The above code idea using the switch/case statement overall took more code about 8 extra lines. But each case within the switch was about 11 line which made the execution time much faster.
  
Thanks again for all the great help.
-Jerry |
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temtronic
Joined: 01 Jul 2010
Posts: 9246
Location: Greensville,Ontario
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| Posted: Thu Jun 22, 2017 7:13 pm |
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I appreciate the time and effort 'Mr. T' took to cut and compile code to show HOW and WHY it's important to understand how a PIC works. While most don't know 'dreaded' machine code, aka Assembler aka the 'Instruction Set for PICs' this one example shows how a programmer can truly benefit from reading a few chapters and experimenting.
Jay |
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