Doing the mod of a pointer variable in C

Question

I am using embedded C for motorola 68000 microprocessor. I want to store the pointer address in a uint16 variable and then, want to do the mod of the variable. My main reason for doing this is to check the sectors of EEPROM are correctly assigned.

Eg->

 uint16 a, c;
 uint16 *b;
 b=&a;
 c=b%4;

Something like this. Obviously, I cannot do it this way.

Thanks

unwind · Accepted Answer · 2012-09-06 13:00:37Z

2

Are you limited to a 16-bit address space, i.e. 64 KB of RAM? That's quite small for a 68k system.

Anyway, you need to convert the pointer to an integer type in order to arithmetic on it;

#include <stdint.h>

uint16 a, *b = &a;
intptr_t c = (intptr_t) b;

if( (c % 4) != 0 )
{
  /* bad address! */
}

Note that the above doesn't store the address in a 16-bit variable, since that's not the right way to do this. Instead, it uses intptr_t to hold the pointer as an integer.

answered Sep 6, 2012 at 13:00

unwind

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1 Comment

Vish Over a year ago

Sorry for the late reply, I had tried ths technique before posting the query. It seems my cross compiler was having some issues while compiling it.

Mike Mackintosh · Accepted Answer · 2012-09-06 13:08:03Z

1

b = (uint16*)(((uint16)(&a)) % 4);

edited Sep 6, 2012 at 13:08

Mike Mackintosh

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answered Sep 6, 2012 at 13:00

Rango

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1 Comment

Lundin Over a year ago

b = (uint16*)( ((uint16) &a) % 4 ); is a bit easier to read.

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