Byte Alignment

Realtime systems consist of multiple processors communicating with each other via messages. For message communication to work correctly, the message formats should be defined unambiguously. In many systems this is achieved simply by defining C/C++ structures to implement the message format. Using C/C++ structures is a simple approach, but it has its own pitfalls. The problem is that different processors/compilers might define the same structure differently, thus causing incompatibility in the interface definition.

There are two reasons for these incompatibilities:

• Byte Alignment Restrictions
• Byte Ordering

Byte Alignment Restrictions

Most 16-bit and 32-bit processors do not allow words and long words to be stored at any offset. For example, the Motorola 68000 does not allow a 16 bit word to be stored at an odd address. Attempting to write a 16 bit number at an odd address results in an exception. 

Why Restrict Byte Alignment?

32 bit microprocessors typically organize memory as shown below. Memory is accessed by performing 32 bit bus cycles. 32 bit bus cycles can however be performed at addresses that are divisible by 4. (32 bit microprocessors do not use the address lines A1 and A0 for addressing memory).

The reasons for not permitting misaligned long word reads and writes are not difficult to see. For example, an aligned long word X would be written as X0, X1, X2 and X3. Thus the microprocessor can read the complete long word in a single bus cycle. If the same microprocessor now attempts to access a long word at address 0x000D, it will have to read bytes Y0, Y1, Y2 and Y3. Notice that this read cannot be performed in a single 32 bit bus cycle. The microprocessor will have to issue two different reads at address  0x100C and 0x1010 to read the complete long word. Thus it takes twice the time to read a misaligned long word.

  Byte 0 Byte 1 Byte 2 Byte 3 0x1000         0x1004 X0 X1 X2 X3 0x1008         0x100C   Y0 Y1 Y2 0x1010 Y3      

Compiler Byte Padding

Compilers have to follow the byte alignment restrictions defined by the target microprocessors. This means that compilers have to add pad bytes into user defined structures so that the structure does not violate any restrictions imposed by the target microprocessor.

The compiler padding is illustrated in the following example. Here a char is assumed to be one byte, a short is two bytes and a long is four bytes.

User Defined Structure

struct Message{  short opcode;  char subfield;  long message_length;  char version;  short destination_processor;};

 

Actual Structure Definition Used By the Compiler

struct Message{  short opcode;  char subfield;  char pad1;            // Pad to start the long word at a 4 byte boundary  long message_length;  char version;  char pad2;            // Pad to start a short at a 2 byte boundary  short destination_processor;  char pad3[4];         // Pad to align the complete structure to a 16 byte boundary};

In the above example, the compiler has added pad bytes to enforce byte alignment rules of the target processor. If the above message structure was used in a different compiler/microprocessor combination, the pads inserted by that compiler might be different. Thus two applications using the same structure definition header file might be incompatible with each other.

Thus it is a good practice to insert pad bytes explicitly in all C-structures that are shared in a interface between machines differing in either the compiler and/or microprocessor. 

General Byte Alignment Rules

The following byte padding rules will generally work with most 32 bit processor. You should consult your compiler and microprocessor manuals to see if you can relax any of these rules.

• Single byte numbers can be aligned at any address
• Two byte numbers should be aligned to a two byte boundary
• Four byte numbers should be aligned to a four byte boundary
• Structures between 1 and 4 bytes of data should be padded so that the total structure is 4 bytes.
• Structures between 5 and 8 bytes of data should be padded so that the total structure is 8 bytes.
• Structures between 9 and 16 bytes of data should be padded so that the total structure is 16 bytes.
• Structures greater than 16 bytes should be padded to 16 byte boundary.

Structure Alignment for Efficiency

Sometimes array indexing efficiency can also determine the pad bytes in the structure. Note that compilers index into arrays by calculating the address of the indexed entry by the multiplying the index with the size of the structure. This number is then added to the array base address to obtain the final address. Since this operation involves a multiply, indexing into arrays can be expensive. The array indexing can be considerably speeded up by just making sure that the structure size is a power of 2. The compiler can then replace the multiply with a simple shift operation.