10. Endian Mapping
181
PowerSpan II User Manual
80A1010_MA001_09
Integrated Device Technology
www.idt.com
In PowerPC little-endian mode, the processor munges the address and places the scalar on the external
byte lanes starting at this modified address. The scalar is still in big-endian order. This operation is only
defined for starting addresses that are a multiple of the size of the scalar. In PowerPC literature, this is
referred to as being naturally aligned.
The munging performed by the processor is illustrated in
.
When the processor bus is operating in PowerPC little-endian mode, END bit must be set to PowerPC
little-endian mode. In this case, the PB Slave munges the processor bus address, and maps byte lanes to
register addresses to preserve the significance of the scalar.
Ti
p
Munging the address makes the address appear to the processor bus that individual aligned
scalars are stored as little-endian values when they are actually stored in big-endian order.
They are stored at different byte addresses with a double word.
Table 49: Processor Bus
Address Munging
Transfer Size
Address modification
4 bytes
XOR with 0b100
2 bytes
XOR with 0b110
1 byte
XOR with 0b111