Condition Register (CR), Link Register and Count Register(CTR)

Branching registers

Branches in PowerPC make use of three special-purpose registers: the condition register, the count register, and the link register.

The condition register

The condition register consists conceptually of seven fields. A field is a segment of four bits used to store status information about the results of an instruction. Two of the fields are somewhat special-purpose, and will be covered shortly, and the remaining fields are available for general use. The fields are named cr0 through cr7.

The first field, cr0 is used for the results of fixed-point computation instructions, which use non-immediate operands (with a few exceptions). The result of the computation is compared with zero, and the appropriate bits are set (negative, zero, or positive). To indicate in a computational instruction that you want it to set cr0, you simply add a period (.) to the end of the instruction. For example, add 4, 5, 6 adds register 5 to register 6 and stores the result in register 4, without setting any status bits in cr0. However, add. 4, 5, 6 does the same thing, but sets the bits in cr0 based on the computed value. cr0 is also the default field for use on compare instructions.
The second field (called cr1) is used by floating-point instructions using the period after the instruction name. Floating-point computation is outside the scope of this article.
Each field has four bits. The usage of those bits varies with the instruction being used. Here are their possible uses (floating-point uses are listed but not described):

Condition register field bits

Bit	Mnemonic	Fixed-point comparison	Fixed-point computation	Floating-point comparison	Floating-point computation
0	lt	Less than	Negative	Less than	Exception summary
1	gt	Greater than	positive	Greater than	Enabled exception summary
2	eq	Equal	Zero	Equal	Invalid operation exception summary
3	so	Summary overflow	Summary overflow	Unordered	Overflow exception

Later you see how to access these fields both implicitly and directly.
The condition register can be loaded to or from a general-purpose register using mtcr, mtcrf, and mfcr. mtcr moves a specified general-purpose register to the condition register. mfcr moves the condition register to a general-purpose register. mtcrf loads the condition register from a general-purpose register, but only the fields specified by an 8-bit mask, which is the first operand.
Here are some examples:

Listing 1. Condition register transfer examples

#Copy register 4 to the condition register
mtcr 4

#Copy the condition register to register 28
mfcr 28

#Copy fields 0, 1, 2, and 7 from register 18 to the condition register
mtcrf 0b11100001, 18

The count and link registers

The link register (called LR) is a special-purpose register that holds return addresses from branch instructions. All branch instructions can be told to set the link register, which, if the branch is taken, sets the link register to the address of the instruction immediately following the current instruction. Branch instructions set the link register by appending the letter l to the end of the instruction. For instance, b is an unconditional branch instruction, and bl is an unconditional branch instruction that sets the link register.
The count register (called CTR) is a special-purpose register designed to hold loop counters. Special branch instructions can decrement the count register and/or conditionally branch depending on whether CTR has reached zero.
Both the link and count registers can be used as a branch destination. bctr branches to the address specified in the count register, and blr branches to the address specified in the link register (like the return statement in C language level). 

The link and count registers can also be loaded and copied from general-purpose registers. For the link register, mtlr moves a given register value to the link register, and mflr moves a value from the link register to a general-purpose register. mtctr and mfctr do the same for the count register.