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# Join 2 words to make a dword

Posted on 2004-08-26
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I am reading data back from a microcontroller using modbus.  I need to join two words together to make a dword.

I have both word values stored in variables in C, as below:

var1 = 15
var2 = 25

I need to join them so var3 has the value 1525

I also need to be able to split them back up into 2 words.

Are there standard C functions for performing these operations? cast 2 words to a dword?
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Question by:richardoc
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LVL 46

Expert Comment

ID: 11903939
Pls explain? I assume 15 is 0x15, that fits a byte. So does 0x25. Joint, they for a word 0x1525, if you use
var3= (var1<<8)|var2

Other operations are comparable (but always bitwise).
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Author Comment

ID: 11904013
Basically I have 2 integers, and I need to join them to make a long . I used 15 and 25 just as examples, but the end value must be <value1><value2>

Does this make sense?
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LVL 9

Expert Comment

ID: 11904079

int num1=15,num2=25;

long longnum=(num1*100)+num2;
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LVL 9

Expert Comment

ID: 11904105
>I also need to be able to split them back up into 2 words.

int num1,num2;

num1=longnum%100;

num2=longnum/100;
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LVL 9

Expert Comment

ID: 11904129
Oh My God,WHat am i typing?

I need some sleep.
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LVL 46

Accepted Solution

Sjef Bosman earned 125 total points
ID: 11904200
Standard way is using bitwise operators. To join the two vars (portable):

unsigned long var3;
unsigned int var1, var2;

var3= ((unsigned long)var1<<16) | var2

To split them once again:
var1= var3<<16;
var2= var3&0xFFFF;
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LVL 23

Expert Comment

ID: 11904371
small bug [notice right shift]:

To split them once again:
var1= var3>>16;
var2= var3&0xFFFF;

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LVL 46

Expert Comment

ID: 11904490
I might need some sleep as well... :(
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LVL 22

Expert Comment

ID: 11905689

Most computers nowdays are binary base, so when you put two bytes together, you end up with 256 times the high byte plus the low byte.

From your example, it looks like you expect to get 100 times the high part plus the low part.

Please clarify, do you want to combine the digits decimally, or binary-wise?

Regards,

grg99

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LVL 46

Expert Comment

ID: 11906259
Your question is also rather schizo. You seem to know well about words and dword (assembler stuff) but seems not to be familiar with more or less elementary bit-fiddling operators or assembler. If my solution above is what you were after. Or am I mistaken?? Or is this a trick question?
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Author Comment

ID: 11913235
I'm a high level programmer working with assembly programmers, so I know some terms but don't neccessarily understand them :)  Bad I know, but hey, you gotta start somehow.  sjef_bosman's answer did the job nicely. Thanks all.
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