PPC405A Data Organization

Data organization refers to the ordering of the data during transfers. There are two general types of ordering:

• BIG ENDIAN – the most significant portion of an operand is stored at the lower address

• LITTLE ENDIAN – the most significant portion of an operand is stored at the higher address.

The PPC405A supports both of these, but is left in its default Big Endian mode after a reset. To use Little Endian data, you would need to configure the hard PPC405 core accordingly (refer to the relevant PowerPC documentation available from Xilinx).

Words, Half-Words and Bytes

The PPC405A operates on the following data sizes:

• 32-bit words

• 16-bit half-words

• 8-bit bytes.

There are dedicated load and store instructions for these three data types.

Figure 1 shows how these different sizes of data are organized relative to each other over an 8-byte memory range in the PPC405A.

Figure 1. Organization of data types for the PPC405A.