Program counter
Encyclopedia
The program counter, commonly called the instruction pointer (IP) in Intel x86 microprocessor
s, and sometimes called the instruction address register, or just part of the instruction sequencer in some computer
s, is a processor register
that indicates where the computer is in its instruction sequence. Depending on the details of the particular computer, the PC or IP holds either the memory address
of the instruction being executed, or the address of the next instruction to be executed.
In most processors, the program counter is incremented automatically after fetching a program
instruction, so that instructions are normally retrieved sequentially from memory, with certain instructions, such as branches
, jumps and subroutine
calls and returns
, interrupting the sequence by placing a new value in the program counter.
Such jump instructions allow a new address to be chosen as the start of the next part of the flow of instructions from the memory. They allow new values to be loaded (written) into the program counter register. A subroutine call is achieved simply by reading the old contents of the program counter, before they are overwritten by a new value, and saving them somewhere in memory or in another register. A subroutine return is then achieved by writing the saved value back in to the program counter again.
(CPU) of a simple computer contains the hardware (control unit
and arithmetic logic unit
) that executes the instructions, as they are fetched from memory. Most instruction cycle
s consist of the CPU sending an address, on the address bus
, to memory, which then responds by sending the contents of that memory location as data, on the data bus. (This is tied up with the idea of the stored-program computer
in which executable instructions are stored alongside ordinary data in memory, and handled identically by it).
The Program Counter (PC) is just one of the many registers
in the hardware of the CPU. It, like each of the other registers, consists of a bank of binary latches (a binary latch is also known as a flip-flop
), with one flip-flop per bit in the integer that is to be stored (32 for a 32-bit CPU, for example). In the case of the PC, the integer represents the address in memory that is to be fetched next.
Once the data (the instruction) has been received on the data bus, the PC is incremented. In some CPUs this is achieved by adding 000..001 to its contents, and latching the result into the register to be its new contents; on most CPUs, though, the PC is implemented as a register that is internally wired so that it counts up to the next value when a specific signal is applied to it externally. Such a register
, in electronics, is referred to as a binary counter
, and hence the origin of the term program counter.
.
The PC imposes a strict sequential ordering on the fetching of instructions from memory (the flow of control
), even where no sequentiality is implied by the algorithm itself (the von Neumann bottleneck). This is why research into possible models for parallel computing considered, at one point, other non von Neumann or dataflow
models that did not use a program counter. For example, functional programming
languages offered much hope at the high level, with combinatory logic
at the assembler level. Even then, most of the researchers emulated this in the microcode
of conventional computers (hence still involving a program counter in the hardware); but, in fact, combinators are so simple, they could, in principle, be implemented directly in the hardware without recourse to microcode or program counters at all.
In the end, though, the results of that research fed back, instead, into ways of improving the execution speed of conventional processors. Ways were found for organising out-of-order execution
so as to extract the sequencing information that is implicit in the data. Also, the pipeline and very long instruction word
(VLIW) organisations allowed the compiler to arrange for multiple calculations to be set off in parallel. At the start of each instruction execution, though, the instruction needs to be fetched from memory, and this is initiated by an instruction fetch cycle that gets the instructions, one at a time, as directed by the program counter.
Even high level programming languages have the program-counter concept engrained deep down in their behavior. You need only to watch how a programmer develops or debugs a computer program to see evidence of this, with the programmer using a finger to point to successive lines in the program to model the steps of its execution. Indeed, a high level programming language is no less than the assembler language of a high level virtual machine -- a computer that would be too complex to be cost-effective to build directly in hardware, so is implemented instead using multiple shells of emulation (with the compiler
or interpreter
providing the higher levels, and the microcode
providing the lower levels).
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
s, and sometimes called the instruction address register, or just part of the instruction sequencer in some computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
s, is a processor register
Processor register
In computer architecture, a processor register is a small amount of storage available as part of a CPU or other digital processor. Such registers are addressed by mechanisms other than main memory and can be accessed more quickly...
that indicates where the computer is in its instruction sequence. Depending on the details of the particular computer, the PC or IP holds either the memory address
Memory address
A digital computer's memory, more specifically main memory, consists of many memory locations, each having a memory address, a number, analogous to a street address, at which computer programs store and retrieve, machine code or data. Most application programs do not directly read and write to...
of the instruction being executed, or the address of the next instruction to be executed.
In most processors, the program counter is incremented automatically after fetching a program
Computer program
A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...
instruction, so that instructions are normally retrieved sequentially from memory, with certain instructions, such as branches
Branch (computer science)
A branch is sequence of code in a computer program which is conditionally executed depending on whether the flow of control is altered or not . The term can be used when referring to programs in high level languages as well as program written in machine code or assembly language...
, jumps and subroutine
Subroutine
In computer science, a subroutine is a portion of code within a larger program that performs a specific task and is relatively independent of the remaining code....
calls and returns
Return statement
In computer programming, a return statement causes execution to leave the current subroutine and resume at the point in the code immediately after where the subroutine was called, known as its return address. The return address is saved, usually on the process's call stack, as part of the operation...
, interrupting the sequence by placing a new value in the program counter.
Such jump instructions allow a new address to be chosen as the start of the next part of the flow of instructions from the memory. They allow new values to be loaded (written) into the program counter register. A subroutine call is achieved simply by reading the old contents of the program counter, before they are overwritten by a new value, and saving them somewhere in memory or in another register. A subroutine return is then achieved by writing the saved value back in to the program counter again.
Working of a simple program counter
The central processing unitCentral processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
(CPU) of a simple computer contains the hardware (control unit
Control unit
A control unit in general is a central part of the machinery that controls its operation, provided that a piece of machinery is complex and organized enough to contain any such unit. One domain in which the term is specifically used is the area of computer design...
and arithmetic logic unit
Arithmetic logic unit
In computing, an arithmetic logic unit is a digital circuit that performs arithmetic and logical operations.The ALU is a fundamental building block of the central processing unit of a computer, and even the simplest microprocessors contain one for purposes such as maintaining timers...
) that executes the instructions, as they are fetched from memory. Most instruction cycle
Instruction cycle
An instruction cycle is the basic operation cycle of a computer. It is the process by which a computer retrieves a program instruction from its memory, determines what actions the instruction requires, and carries out those actions...
s consist of the CPU sending an address, on the address bus
Address bus
An address bus is a computer bus that is used to specify a physical address. When a processor or DMA-enabled device needs to read or write to a memory location, it specifies that memory location on the address bus...
, to memory, which then responds by sending the contents of that memory location as data, on the data bus. (This is tied up with the idea of the stored-program computer
Stored-program computer
A stored-program computer is one which stores program instructions in electronic memory. Often the definition is extended with the requirement that the treatment of programs and data in memory be interchangeable or uniform....
in which executable instructions are stored alongside ordinary data in memory, and handled identically by it).
The Program Counter (PC) is just one of the many registers
Processor register
In computer architecture, a processor register is a small amount of storage available as part of a CPU or other digital processor. Such registers are addressed by mechanisms other than main memory and can be accessed more quickly...
in the hardware of the CPU. It, like each of the other registers, consists of a bank of binary latches (a binary latch is also known as a flip-flop
Flip-flop (electronics)
In electronics, a flip-flop or latch is a circuit that has two stable states and can be used to store state information. The circuit can be made to change state by signals applied to one or more control inputs and will have one or two outputs. It is the basic storage element in sequential logic...
), with one flip-flop per bit in the integer that is to be stored (32 for a 32-bit CPU, for example). In the case of the PC, the integer represents the address in memory that is to be fetched next.
Once the data (the instruction) has been received on the data bus, the PC is incremented. In some CPUs this is achieved by adding 000..001 to its contents, and latching the result into the register to be its new contents; on most CPUs, though, the PC is implemented as a register that is internally wired so that it counts up to the next value when a specific signal is applied to it externally. Such a register
Hardware register
In digital electronics, especially computing, a hardware register stores bits of information, in a way that all the bits can be written to or read out simultaneously.The hardware registers inside a central processing unit are called processor registers....
, in electronics, is referred to as a binary counter
Counter
In digital logic and computing, a counter is a device which stores the number of times a particular event or process has occurred, often in relationship to a clock signal.- Electronic counters :...
, and hence the origin of the term program counter.
The all-pervading nature of the program counter
The presence of the program counter in the CPU has far reaching consequences on our way of thinking when we program computers. Indeed, the program counter (or any equivalent block of hardware that serves the same purpose) is very much central to the von Neumann architectureVon Neumann architecture
The term Von Neumann architecture, aka the Von Neumann model, derives from a computer architecture proposal by the mathematician and early computer scientist John von Neumann and others, dated June 30, 1945, entitled First Draft of a Report on the EDVAC...
.
The PC imposes a strict sequential ordering on the fetching of instructions from memory (the flow of control
Control flow
In computer science, control flow refers to the order in which the individual statements, instructions, or function calls of an imperative or a declarative program are executed or evaluated....
), even where no sequentiality is implied by the algorithm itself (the von Neumann bottleneck). This is why research into possible models for parallel computing considered, at one point, other non von Neumann or dataflow
Dataflow
Dataflow is a term used in computing, and may have various shades of meaning. It is closely related to message passing.-Software architecture:...
models that did not use a program counter. For example, functional programming
Functional programming
In computer science, functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids state and mutable data. It emphasizes the application of functions, in contrast to the imperative programming style, which emphasizes changes in state...
languages offered much hope at the high level, with combinatory logic
Combinatory logic
Combinatory logic is a notation introduced by Moses Schönfinkel and Haskell Curry to eliminate the need for variables in mathematical logic. It has more recently been used in computer science as a theoretical model of computation and also as a basis for the design of functional programming...
at the assembler level. Even then, most of the researchers emulated this in the microcode
Microcode
Microcode is a layer of hardware-level instructions and/or data structures involved in the implementation of higher level machine code instructions in many computers and other processors; it resides in special high-speed memory and translates machine instructions into sequences of detailed...
of conventional computers (hence still involving a program counter in the hardware); but, in fact, combinators are so simple, they could, in principle, be implemented directly in the hardware without recourse to microcode or program counters at all.
In the end, though, the results of that research fed back, instead, into ways of improving the execution speed of conventional processors. Ways were found for organising out-of-order execution
Out-of-order execution
In computer engineering, out-of-order execution is a paradigm used in most high-performance microprocessors to make use of instruction cycles that would otherwise be wasted by a certain type of costly delay...
so as to extract the sequencing information that is implicit in the data. Also, the pipeline and very long instruction word
Very long instruction word
Very long instruction word or VLIW refers to a CPU architecture designed to take advantage of instruction level parallelism . A processor that executes every instruction one after the other may use processor resources inefficiently, potentially leading to poor performance...
(VLIW) organisations allowed the compiler to arrange for multiple calculations to be set off in parallel. At the start of each instruction execution, though, the instruction needs to be fetched from memory, and this is initiated by an instruction fetch cycle that gets the instructions, one at a time, as directed by the program counter.
Even high level programming languages have the program-counter concept engrained deep down in their behavior. You need only to watch how a programmer develops or debugs a computer program to see evidence of this, with the programmer using a finger to point to successive lines in the program to model the steps of its execution. Indeed, a high level programming language is no less than the assembler language of a high level virtual machine -- a computer that would be too complex to be cost-effective to build directly in hardware, so is implemented instead using multiple shells of emulation (with the compiler
Compiler
A compiler is a computer program that transforms source code written in a programming language into another computer language...
or interpreter
Interpreter (computing)
In computer science, an interpreter normally means a computer program that executes, i.e. performs, instructions written in a programming language...
providing the higher levels, and the microcode
Microcode
Microcode is a layer of hardware-level instructions and/or data structures involved in the implementation of higher level machine code instructions in many computers and other processors; it resides in special high-speed memory and translates machine instructions into sequences of detailed...
providing the lower levels).
See also
- Branch prediction
- Instruction cache
- Instruction cycleInstruction cycleAn instruction cycle is the basic operation cycle of a computer. It is the process by which a computer retrieves a program instruction from its memory, determines what actions the instruction requires, and carries out those actions...
- Instruction unitInstruction unitThe instruction unit in a central processing unit is responsible for organising for program instructions to be fetched from memory, and executed, in an appropriate order...
- Instruction pipelineInstruction pipelineAn instruction pipeline is a technique used in the design of computers and other digital electronic devices to increase their instruction throughput ....
- Instruction registerInstruction registerIn computing, an instruction register is the part of a CPU's control unit that stores the instruction currently being executed or decoded. In simple processors each instruction to be executed is loaded into the instruction register which holds it while it is decoded, prepared and ultimately...
- Instruction schedulingInstruction schedulingIn computer science, instruction scheduling is a compiler optimization used to improve instruction-level parallelism, which improves performance on machines with instruction pipelines...
- Out-of-order executionOut-of-order executionIn computer engineering, out-of-order execution is a paradigm used in most high-performance microprocessors to make use of instruction cycles that would otherwise be wasted by a certain type of costly delay...
- Program status wordProgram status wordThe Program status word is an IBM System/360 architecture and successors control register which performs the function of a Status register in other architectures, and more....
External links
- http://www.google.com/codesearch/p?hl=en#1plSxYlzhvw/trunk/projects/work/design/program_counter.vhdl
- http://www.google.com/codesearch/p?hl=en#ES4QsxKMkGQ/templates/program_counter.vhdl