C programming/Structures and databus width - Revision history

Heth at 12:36, 8 July 2019

2019-07-08T12:36:22Z

Heth: /* struct_B */

2013-10-23T20:06:58Z

‎struct_B

Heth: /* Source code */

2012-08-26T07:04:05Z

‎Source code

Heth: /* ARM Cortex M3 */

2012-02-22T07:59:05Z

‎ARM Cortex M3

Heth: /* struct_B */

2012-02-22T07:58:37Z

‎struct_B

Heth: /* struct_A */

2012-02-22T07:58:22Z

‎struct_A

Heth: /* Source code */

2012-02-22T07:58:06Z

‎Source code

Heth: /* struct_A */

2012-02-20T11:45:48Z

‎struct_A

Heth: /* struct_B */

2012-02-20T11:45:17Z

‎struct_B

Heth at 06:19, 20 February 2012

2012-02-20T06:19:41Z

@@ Line 91: / Line 91: @@
 *OS: None
 Cortex M3 is a 32 bit microcontroller, but allocates memory in 8 Bytes chunks behaving as the 64 bit Intel CPU described above.
 [[Category:c]][[Category:ARM]]

@@ Line 71: / Line 71: @@
 ===struct_B===
-;The better way: Occupies 24 bytes of RAM using only 23 bytes. (Descending order of variables in size)
+;The better way: Occupies 24 bytes of RAM using 23 bytes. (Descending order of variables in size)
 <source lang=c>
          struct struct_B {

@@ Line 4: / Line 4: @@
 The same source code is used in the different setups below.
 ==Source code==
-<source lang=cli>
+<source lang=c>
 #include <stdio.h>
 int main( void ) {
@@ Line 30: / Line 30: @@
          struct struct_B s_B;
-         printf("Size of s_A: %i\n",(int) sizeof(m1) );
+         printf("Size of s_A: %i\n",(int) sizeof(s_A) );
-         printf("Size of s_B: %i\n",(int) sizeof(m2) );
+         printf("Size of s_B: %i\n",(int) sizeof(s_B) );
          return(0);
+}

@@ Line 91: / Line 91: @@
 *OS: None
 Cortex M3 is a 32 bit microcontroller, but allocates memory in 8 Bytes chunks behaving as the 64 bit Intel CPU described above.
-[[Category:c]][[Categori:ARM]]
+[[Category:c]][[Category:ARM]]

@@ Line 74: / Line 74: @@
 <source lang=c>
          struct struct_B {
-                 long l1; // Occupy from 0x0 to 0x7
+                 long long l1; // Occupy from 0x0 to 0x7
                  int i1;  // Occupy from 0x8 to 0xb
                  int i2;  // Occupy from 0xc to 0xf

@@ Line 61: / Line 61: @@
                  char c2; // 4 bytes used: c2 occupies address 0x8 - address 0x9, 0xa, 0xb are padding
                           // Because next variable is a long and must start 0x0, 0x8 - 0xc to 0xf are padding
-                 long l1; // 8 bytes used: l1 occupies address 0x10 to 0x17
+                 long long l1; // 8 bytes used: l1 occupies address 0x10 to 0x17
                  char c3; // 4 bytes used: c3 occupies address 0x18 - address 0x19, 0x1a and 0x1b are padding
                  int  i2; // 4 bytes used: i1 occupies address 0x1c to 0x1f

@@ Line 58: / Line 58: @@
                  char c1; // 4 bytes used: c1 occupies address 0x0 - address 0x1, 0x2 and 0x3 are padding
                           // (Cant be used because next variable is a 32 bit and must start at 0x0, 0x4, 0x8...)
-                 int    i1; // 4 bytes used: i1 occupies address 0x4 to 0x7
+                 int  i1; // 4 bytes used: i1 occupies address 0x4 to 0x7
                  char c2; // 4 bytes used: c2 occupies address 0x8 - address 0x9, 0xa, 0xb are padding
-                             // Because next variable is a long and must start 0x0, 0x8 - 0xc to 0xf are padding
+                         // Because next variable is a long and must start 0x0, 0x8 - 0xc to 0xf are padding
                  long l1; // 8 bytes used: l1 occupies address 0x10 to 0x17
                  char c3; // 4 bytes used: c3 occupies address 0x18 - address 0x19, 0x1a and 0x1b are padding
-                 int   i2; // 4 bytes used: i1 occupies address 0x1c to 0x1f
+                 int  i2; // 4 bytes used: i1 occupies address 0x1c to 0x1f
-                 int   i3; // 4 bytes used: i1 occupies address 0x20 to 0x23
+                 int  i3; // 4 bytes used: i1 occupies address 0x20 to 0x23
-                           // Because this is the end of the struct which must end at 0x0, 0x8 address 0x24 t o0x27 are padding
+                         // Because this is the end of the struct which must end at 0x0, 0x8 address 0x24 t o0x27 are padding
          };
 </source>
 The struct_A initiatet in s_A  occupies from 0x0 to 0x27 = 0x28 byte locations 0x28 = 40 decimal
 ===struct_B===
 ;The better way: Occupies 24 bytes of RAM using only 23 bytes. (Descending order of variables in size)

@@ Line 76: / Line 76: @@
                  int i1;  // Occupy from 0x8 to 0xb
                  int i2;  // Occupy from 0xc to 0xf
-                 int i3; // Occupy from 0x10 to 0x13
+                 int i3;  // Occupy from 0x10 to 0x13
                  char c1; // Occupy 0x14
                  char c2; // Occupy 0x15
                  char c3; //Occupy 0x16
-                      // Padding: 0x17
+                         // Padding: 0x17
   };
 </source>
 ==Embedded controllers==
 ===ARM Cortex M3===

@@ Line 8: / Line 8: @@
 int main( void ) {
-         struct ma1 {
+         struct struct_A {
                  char c1;
                  int i1;
@@ Line 17: / Line 17: @@
                  int i3;
          };
-         struct ma2 {
+         struct struct_B {
                  long l1;
                  int i1;
@@ Line 27: / Line 27: @@
          };
-         struct ma1 m1;
+         struct struct_A s_A;
-         struct ma2 m2;
+         struct struct_B s_B;
-         printf("Size of m1: %i\n",(int) sizeof(m1) );
+         printf("Size of s_A: %i\n",(int) sizeof(m1) );
-         printf("Size of m2: %i\n",(int) sizeof(m2) );
+         printf("Size of s_B: %i\n",(int) sizeof(m2) );
          return(0);
+}
@@ Line 43: / Line 43: @@
 The two structs - containing the same amount of data - occupies different sizes of RAM chunks.
 <source lang=cli>
-heth@mars2:/tmp$ ./ma3
+heth@mars2:/tmp$ ./show_struct
-Size of m1: 40
+Size of s_A: 40
-Size of m2: 24
+Size of s_B: 24
 </source>
 ==Explanation==
@@ Line 51: / Line 51: @@
 ;Best practice for defining structures:Start with the variables that occupy the most RAM and work down to the variables that occupy least RAM.
 :Use the CPU's native buswidth for variables as far as possible. Using smaller variables may cost performance because it might be necessary to unmask and mask the data when reading from RAM and reading and masking before writing to RAM.
-===Struct m1===
+===struct_A===
 ;The bad way: Occupies 40 bytes of RAM using only 23 bytes
-When defining the structure as ma1:
+When defining the structure as struct_A:
 <source lang=c>
-         struct ma1 {
+         struct struct_A {
                  char c1; // 4 bytes used: c1 occupies address 0x0 - address 0x1, 0x2 and 0x3 are padding
                           // (Cant be used because next variable is a 32 bit and must start at 0x0, 0x4, 0x8...)
@@ Line 68: / Line 68: @@
          };
 </source>
-The ma1 struct occupies from 0x0 to 0x27 = 0x28 byte locations 0x28 = 40 decimal
+The struct_A initiatet in s_A  occupies from 0x0 to 0x27 = 0x28 byte locations 0x28 = 40 decimal
-===Struct m2===
+===struct_B===
 ;The better way: Occupies 24 bytes of RAM using only 23 bytes. (Descending order of variables in size)
 <source lang=c>
-         struct ma2 {
+         struct struct_B {
                  long l1; // Occupy from 0x0 to 0x7
                  int i1;  // Occupy from 0x8 to 0xb