Important Things 

The PPC has the following registers (Note: On 32 Bit PPCs like the
603e or the 604e r0-r31 are only 32 Bit, and 64 Bit commands are
not available !!!) I listed only the Usermode registers. If you need the Supervisor
mode registers, have a look at Motorola Documentation. But with the given Coding
Systems on PowerUP, you normally don't have to handle the Supervisor mode registers
yourselves.

f0-f31     : 64 Bit Wide FPU-Registers
cr         : Condition Register
fpscr      : Floating Point Condition and Control Register
xer        : 32 Bit Wide Another Condition Register
lr         : 32 Bit Wide Link Register
ctr        : 32 Bit Wide Counter Register
r0-r31     : 64 Bit Wide General Purpose Registers (32 Bit on current PowerUP)
tbl        : Time Base Register
tbu        : Time Base Register

The rename Buffers are used, if the Destination of one command is the source of another
command, for optimization purposes. The xer contains the carry and overflow bits.
The lr contains the branch destination and the return address, when appropriate (for
branch and link instructions). The ctr is decremented and tested automatically as
result of branch and counter operations. The Timebase registers can be read by userlevel
software, but written only by supervisorlevel software.

The condition register is split into 8 4-bit parts, named cr0-cr7. For each crn,
the bits are defined as follows:

0 LT "less than"
1 GT "greater than"
2 EQ "equal"
3 SO "sum overflow"

Normally you use cr0 to store this information, but you can also use other CRs. You
should be aware, that, if you use the below specified . Notation, all Integer Commands
update cr0 according to the result of the operations, and all FPU Commands update the
cr1 with the current setting of the FEX/FX/VX/OX Bits like described in the sections
about the FPU Exceptions and the FPSCR. Also the o Notation (also described below)
modifies (for Integer Commands) the SO Bit of cr0, and (for FPU Commands) the OX
Bit of cr1. Asides from that you are completely free to use whatever crn you want
for your comparisions and other cr-using operations. Usually you simply use cr0,
though...

Read more about this at FPSCR and XER.

Concerning the condition fields cr0-cr7 and concerning XER, it has to be noted, that
some commands have special "modes". If an o is appended to the command name, it reacts
on "sum overflow" and "overflow" by setting the SO and OV Bits in the XER to the correct value.
If a . is appended to the command name, it updates Bits 0-3 of cr0 for Integer-Commands
or Bits 0-3 of cr1 for FPU-Commands (to the corresponding bits in the FPSCR). Like outlined
above, this would be LT/GT/EQ/SO in case of an Integer command, but FEX/FX/VX/OX for
FPU-commands (which have a totally different meaning). Always remember, that the . or
o Notation are not supported by ALL commands. This might sound VERY complicated now to
you: Well, those . and o notations are additional features, in normal code you usually
don't have to use them.

Examples:

add   r6,r7,r8
addo  r6,r7,r8
add.  r6,r7,r8
addo. r6,r7,r8

This is not possible for all commands, and some commands do not support all modes.

Some commands additionally have a 32 and a 64 Bit modes. Also it should be noted, that
the current PPC CPUs used on Amiga PPC Boards (PPC 603e and 604e) are 32 Bit CPUs, they
do *not* have the 64 Bit commands. Also they do not have the 64 Bit versions that are
available for some commands (like ldx for example).

All these things are notified in the command descriptions. Always have a close look in
the description, though, if it only supports the . notation or also the o-notation.
Some commands only support the . notation, but not the o-notation.

Please consider: All commands using 64 Bit Command width (d) only exist on 64 Bit
Implementations. This means: NOT ON POWERUP !!! There might be 64 Bit versions of
PowerUP or other PPC Boards for the Amiga in the future, though.

On PowerUP, the PowerPC *always* runs in Big-Endian mode (like you are used to from
the 68k) !!!

Also always note: The Bit numbering on PPC is just the other way round like on 68k !!!
On PPC Bit 0 is the Bit to the left, while this is the Bit to the right on 68k !!!

Also, there are some important things about register usage:

* r0 has a special meaning. Do not use it, if you do not know EXACTLY what you do.
* r1 is the stack pointer
* r2 is used for Global Variable Stuff
* r3 is the Library Base Pointer (like a6 on 68k !!!)
* r13 is the local stack pointer
* If you change r13-r31, you have to restore the old values
* Never forget to do the Stackframe with prolog/epilog
* Never forget the executable command
* Function-Parameters are transferred on r4-r10 (and possible the stack also)
* When there is talk about macros, the macros in powerpc/ppcmacros.i are meant