Accessing Special Purpose Registers 

Principially there are two sort of commands to access the special purpose
registers of the PPC. mtxxx ("move to") moves data FROM a general purpose
register TO a special purpose register, mfxxx ("move from") moves data
FROM a special purpose register TO a general purpose register.

The Syntax is:

mtXXX Rs
mfXXX Rd

where d,s=0..31

xxx can mean (don't bother, if most of these don't mean anything to you,
most of these registers should NOT be modified by a program that is NOT
the OS itself). You definitely should NOT modify the BAT registers yourselves,
for example. Use the Memory-Allocation functions of powerpc.library to
access the BAT registers...

xer:    The XER register
lr:     The Link register
ctr :   The Count register (One of the most useful commands here)
dsisr:  The DSISR register
dar:    The DAR register
dec:    The Decrementer register
sdr1:   The SDR1 register
srr0:   Save and restore Register 0
srr1:   Save and restore Register 1
asr:    Address Space Register
ear:    External Access Register
tbl:    Time Base Lower
tbu:    Time Base Upper
ibatu:  IBAT Register, Upper
ibatl:  IBAT Register, Lower
dbatu:  DBAT Register, Upper
dbatl:  DBAT Register, Lower

mtspr n,Rs
mfspr rD,n

Modify/Save SPRG0-SPRG3, n is the number of the SPRG to modify/to save.

mfpvr rD

Get the value of the processor version register (which tells you what
PPC is inside your system). It is NOT possible to write TO the pvr.

It is also possible to use the "general form" mtspr/mfspr instead of
these extended commands, but as the "general form" looks very abstract
(mfspr Rd,8 means mflr Rd, for example), it is recommended to use the
extended commands.

There are some not-extended commands also (asides from the general form):

mtcrf CRM,Rs

CRM is a mask specifying for each crn, if it will be modified by this
command or not. If CRM(i) = 1, CR field i (CR bits 4*i through 4*i+3)
will be modified. The command replaces the not-masked-out bits by the
low-order 32 Bits of Rs.

mcrxr crf

The contents of XER Bits 0-3 are copied into the Condition Field specified
(for example cr2).

mfcr Rd

The contants of the Condition Register are placed into the low-order 32 Bits
of Rd. The Contents of the high-order 32 Bits of Rd are cleared in 64 Bit
Implementations.

None of the Special Purpose Register Commands support the . or o Notation (which
is described in Important things.