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别名：指令指针、指令地址寄存器、程序计数器；

操作：顺序操作（计数器加一）、分支操作（计数器修改）；

The

program counter

(

PC

), commonly called the

instruction pointer

(

IP

) in

Intel


x86

and

Itanium


microprocessors

, and sometimes called the

instruction address register

(

IAR

),


^[1]


the

instruction counter

,


^[2]


or just part of the instruction sequencer,


^[3]


is a

processor register

that indicates where a

computer

is in its

program

sequence.


^{[note 1]}

In most processors, the PC is incremented after fetching an


instruction


, and holds the


memory address


of (”


points


to”)

the next instruction

that would be executed.

Processors usually fetch instructions sequentially from memory,

but

control transfer

instructions change the sequence

by placing a new value in the PC.

These include


branches


(sometimes called jumps),


subroutine


calls, and


returns


.

A transfer that is conditional on the truth of some assertion lets the computer follow a different sequence under different conditions.

A branch provides that the next instruction is fetched from elsewhere in memory. A subroutine call not only branches but saves the preceding contents of the PC somewhere. A return retrieves the saved contents of the PC and places it back in the PC, resuming sequential execution with the instruction following the subroutine call.

https://en.wikipedia.org/wiki/Program_counter

Branch (computer science)

Mechanically, a branch instruction can change the


program counter


(PC) of a


CPU


.

The program counter stores the memory address of the next instruction to be executed. Therefore, a branch can cause the CPU to begin fetching its instructions from a different sequence of memory cells.

When a branch is

taken

,

the CPU’s program counter is set to the


argument


of the jump instruction.

So, the next instruction becomes the instruction at that address in

memory

. Therefore, the flow of control changes.

When a branch is

not taken

, the CPU’s program counter is unchanged. Therefore, the next instruction executed is the instruction after the branch instruction. Therefore, the flow of control is unchanged.

https://en.wikipedia.org/wiki/Branch_(computer_science)