PowerPC - Design Features - Endian Modes

Endian Modes

Most PowerPC chips switch endianness via a bit in the MSR (Machine State Register), with a second bit provided to allow the OS to run with a different endianness. Accesses to the "inverted page table" (a hash table that functions as a TLB with off-chip storage) are always done in big-endian mode. The processor starts in big-endian mode.

In little-endian mode, the three lowest-order bits of the effective address are exclusive-ORed with a three bit value selected by the length of the operand. This is enough to appear fully little-endian to normal software. An operating system will see a warped view of the world when it accesses external chips such as video and network hardware. Fixing this warped view requires that the motherboard perform an unconditional 64-bit byte swap on all data entering or leaving the processor. Endianness thus becomes a property of the motherboard. An OS that operates in little-endian mode on a big-endian motherboard must both swap bytes and undo the exclusive-OR when accessing little-endian chips.

AltiVec operations, despite being 128-bit, are treated as if they were 64-bit. This allows for compatibility with little-endian motherboards that were designed prior to AltiVec.

An interesting side effect of this implementation is that a program can store a 64-bit value (the longest operand format) to memory while in one endian mode, switch modes, and read back the same 64-bit value without seeing a change of byte order. This will not be the case if the motherboard is switched at the same time.

Mercury Computer Systems and Matrox ran the PowerPC in little-endian mode. This was done so that PowerPC devices serving as co-processors on PCI boards could share data structures with host computers based on x86. Both PCI and x86 are little-endian. Solaris and Windows NT for PowerPC also ran the processor in little-endian mode.

Some of IBM's embedded PowerPC chips use a per-page endianness bit. None of the previous applies to them.