RTEMS 4.6.0 On-Line Library

Memory Model Byte Addressable versus Word Addressable

RTEMS TI C3x/C4x Applications Supplement

3.2: Byte Addressable versus Word Addressable

Processor in the Texas Instruments C3x/C4x family are word addressable. This is in sharp contrast to CISC and RISC processors that are typically byte addressable. In a word addressable architecture, each address points not to an 8-bit byte or octet but to 32 bits.

On first glance, byte versus word addressability does not sound like a problem but in fact, this issue can result in subtle problems in high-level language software that is ported to a word addressable processor family. The following is a list of the commonly encountered problems:

String Optimizations: Although each character in a string occupies a single address just as it does on a byte addressable CPU, each character occupies 32 rather than 8 bits. The most significant 24 bytes are of each address are ignored. This in and of itself does not cause problems but it violates the assumption that two adjacent characters in a string have no intervening bits. This assumption is often implicit in string and memory comparison routines that are optimized to compare 4 adjacent characters with a word oriented operation. This optimization is invalid on word addressable processors.
Sizeof: The C operation sizeof returns very different results on the C3x/C4x than on traditional RISC/CISC processors. The sizeof(char), sizeof(short), and sizeof(int) are all 1 since each occupies a single addressable unit that is thirty-two bits wide. On most thirty-two bit processors, sizeof(char is one, sizeof(short) is two, and sizeof(int) is four. Just as software makes assumptions about the sizes of the primitive data types has problems when ported to a sixty-four bit architecture, these same assumptions cause problems on the C3x/C4x.
Alignment: Since each addressable unit is thirty-two bit wide, there are no alignment restrictions. The native integer type need only be aligned on a "one unit" boundary not a "four unit" boundary as on numerous other processors.

RTEMS TI C3x/C4x Applications Supplement