Matthew Hannigan wrote:
On Fri, Sep 30, 2005 at 10:09:40AM +1000, O Plameras wrote:
.... and as C is closely bound
to hardware architecture you must have said something about these data
Actually, C is not necessarily that closely bound to hardware architecture.
The the following C program illustrates the relationship of C and
hardware architecture on
16-bit and 32-bit (Two different architectures). We cannot use 32-bit
and 64-bit (AMD) to
illustrate because the int size are the same in both. This is an
aberration for AMD CPUs, I
think.
Many documentations says that a 64-bit should really have int size=8
instead of 4
(see http://www.osdata.com/topic/language/asm/datarep.htm
-DEC VAX *16* *bit* [2 *byte*] *word*; 32 bit [4 *byte*] longword; 64
bit [8 *byte*]quadword;
132 bit [16 *byte*] octaword; data may be unaligned at a speed penalty *...*
)
There are other documentations similar to the above assertions on the net.
But check the C-codes.
#include <stdio.h>
struct verify {
char initials[2];
int birthdate;
};
int main(void)
{
struct verify holes;
printf ("%d\n", sizeof(holes.initials[0]));
printf ("%d\n", sizeof(holes.initials));
printf ("%d\n", sizeof(holes.birthdate));
printf ("%d\n", sizeof(holes));
return 0;
}
Given that the word-byte
In 16-bit computer = 2 bytes word the output is,
1
2
2
4
in 32-bit computer = 4 bytes word the output is,
1
2
4
8
Two things are different due to computer architecture.
1. The same struct have different memory sizes allocated.
2. The struct has no hole in 16-bit and has 2-byte hole in 32-bit
because C-compiler always
align int data types beginning at the next word byte.
The following quote is from wikipedia
(http://en.wikipedia.org/wiki/C_variable_types_and_declarations#Size)
"There is some confusion in novice C programmers as to how
big these types are. The standard is specifically vague
in this area:
* A short int must not be larger than an int.
* An int must not be larger than a long int.
* A short int must be at least 16 bits long.
* A long int must be at least 32 bits long.
The standard does not require that any of these sizes are
necessarily different. It is perfectly valid, for example,
that all 3 types be 64 bits long."
--
Matt
--
O Plameras
http://www.acay.com.au/~oscarp/tutor
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