THE FUNCTIONS OF A COMPUTER
This chapter introduces certain basic computer concepts. It provides background information and definitions which will be useful in later chapters of this manual. Those already familiar with computers may skip this material, at their option.
A typical digital computer consists of:
a) A central processor unit (CPU)
b) A memory
c) Input/output (1/0) ports
The memory serves as a place to store Instructions, the coded pieces of information that direct the activities of the CPU, and Data, the coded pieces of information that are processed by the CPU. A group of logically related instructions stored in memory is referred to as a Program. The CPU "reads" each instruction from memory in a logically determined sequence, and uses it to initiate processing actions. If the program sequence is coherent and logical, processing the program will produce intelligible and useful results.
The memory is also used to store the data to be manipulated, as well as the instructions that direct that manipulation The program must be organized such that the CPU does not read a non-instruction word when it expects to see an instruction. The CPU can rapidly access any data stored in memory; but often the memory is not large enough to store the entire data bank required for a particular application. The problem can be resolved by providing the computer with one or more Input Ports. The CPU can address these ports and input the data contained there. The addition of input ports enables the computer to receive information from external equipment (such as a paper tape reader or floppy disk) at high rates of speed and in large volumes.
A computer also requires one or more Output Ports that permit the CPU to communicate the result of its processing to the outside world. The output may go to a display, for use by a human operator, to a peripheral device that produces "hardcopy," such as a line-printer, to a peripheral storage device, such as a floppy disk unit, or the output may constitute process control signals that direct the operations of another system, such as an automated assembly line. Like input ports, output ports are addressable. The input and output ports together permit the processor to communicate with the outside world.
The CPU unifies the system. It controls the functions performed by the other components. The CPU must be able to fetch instructions from memory, decode their binary contents and execute them. It must also be able to reference memory and 1/0 ports as necessary in the execution of instructions. In addition, the CPU should be able to recognize and respond to certain external control signals, such as INTERRUPT and WAIT requests. The functional units within a CPU that enable it to perform these functions are described below.
A typical central processor unit (CPU) consists of the following interconnected functional units:
Registers are temporary storage units within the CPU. Some registers, such as the program counter and instruction register, have dedicated uses. Other registers, such as the accumulator, are for more general purpose use.
The accumulator usually stores one of the operands to be manipulated by the ALU. A typical instruction might direct the ALU to add the contents of some other register to the contents of the accumulator and store the result in the accumulator itself. In general, the accumulator is both a source (operand) and a destination (result) register.
Often a CPU will include a number of additional general purpose registers that can be used to store operands or intermediate data. The availability of general purpose registers eliminates the need to "shuffle" intermediate results back and forth between memory and the accumulator, thus improving processing speed and efficiency.
Program Counter (Jumps, Subroutines and the Stack):
The instructions that make up a program are stored in the system's memory. The central processor references the contents of memory, in order to determine what action is appropriate. This means that the processor must know which location contains the next instruction.
Each of the locations in memory is numbered, to distinguish it from all other locations in memory. The number which identifies a memory location is called its Address.
The processor maintains a counter which contains the address of the next program instruction. This register is called the Program Counter. The processor updates the program counter by adding "1" to the counter each time it fetches an instruction, so that the program counter is always current (pointing to the next instruction)
The programmer therefore stores his instructions in numerically adjacent addresses, so that the lower addresses contain the first instructions to be executed and the higher addresses contain later instructions. The only time the programmer may violate this sequential rule is when an instruction in one section of memory is a Jump instruction to another section of memory.
A jump instruction contains the address of the instruction which is to follow it. The next instruction may be stored in any memory location, as long as the programmed jump specifies the correct address. During the execution of a jump instruction, the processor replaces the contents of its program counter with the address embodied in the Jump. Thus, the logical continuity of the program is maintained.
A special kind of program jump occurs when the stored program "Calls" a subroutine. In this kind of jump, the processor is required to "remember" the contents of the program counter at the time that the jump occurs. This enables the processor to resume execution of the main program when it is finished with the last instruction of the subroutine.
A Subroutine is a program within a program. Usually it is a general-purpose set of instructions that must be executed repeatedly in the course of a main program. Routines which calculate the square, the sine, or the logarithm of a program variable are good examples of functions often written as subroutines. Other examples might be programs designed for inputting or outputting data to a particular peripheral device.
The processor has a special way of handling subroutines, in order to insure an orderly return to the main program. When the processor receives a Call instruction, it increments the Program Counter and stores the counter's contents in a reserved memory area known as the Stack. The Stack thus saves the address of the instruction to be executed after the subroutine is completed. Then the processor loads the address specified in the Call into its Program Counter. The next instruction fetched will therefore be the first step of the subroutine.
The last instruction in any subroutine is a Return. Such an instruction need specify no address. When the processor fetches a Return instruction, it simply replaces the current contents of the Program Counter with the address on the top of the stack. This causes the processor to resume execution of the calling program at the point immediately following the original Call Instruction.
Subroutines are often Nested; that is, one subroutine will sometimes call a second subroutine. The second may call a third, and so on. This is perfectly acceptable, as long as the processor has enough capacity to store the necessary
return addresses, and the logical provision for doing so. In other words, the maximum depth of nesting is determined by the depth of the stack itself. If the stack has space for storing three return addresses, then three levels of subroutines may be accommodated.
Processors have different ways of maintaining stacks. Some have facilities for the storage of return addresses built into the processor itself. Other processors use a reserved area of external memory as the stack and simply maintain a Pointer register which contains the address of the most recent stack entry. The external stack allows virtually unlimited subroutine nesting. In addition, if the processor provides instructions that cause the contents of the accumulator and other general purpose registers to be "pushed" onto the stack or "popped" off the stack via the address stored in the stack pointer, multilevel interrupt processing (described later in this chapter) is possible. The status of the processor (i.e., the contents of all the registers) can be saved in the stack when an interrupt is accepted and then restored after the interrupt has been serviced. This ability to save the processor's status at any given time is possible even if an interrupt service routine, itself, is interrupted.
Every computer has a Word Length that is characteristic of that machine. A computer's word length is usually determined by the size of its internal storage elements and interconnecting paths (referred to as Busses); for example, a computer whose registers and busses can store and transfer 8 bits of information has a characteristic word length of 8 bits and is referred to as an 8bit parallel processor. An eight-bit parallel processor generally finds it most efficient to deal with eight-bit binary fields, and the memory associated with such a processor is therefore organized to store eight bits in each addressable memory location. Data and instructions are stored in memory as eight-bit binary numbers, or as numbers that are integral multiples of eight bits: 16 bits, 24 bits, and so on. This characteristic eight-bit field is often referred to as a Byte.
Each operation that the processor can perform is identified by a unique byte of data known as an Instruction Code or Operation Code. An eight-bit word used as an instruction code can distinguish between 256 alternative actions, more than adequate for most processors.
The processor fetches an instruction in two distinct operations. First, the processor transmits the address in its Program Counter to the memory Then the memory returns the addressed byte to the processor. The CPU stores this instruction byte in a register known as the Instruction Register, and uses it to direct activities during the remainder of the instruction execution.
The mechanism by which the processor translates an instruction code into specific processing actions requires more elaboration than we can here afford. The concept, however, should be intuitively clear to any logic designer.
The eight bits stored in the instruction register can be decoded and used to selectively activate one of a number of output lines, in this case up to 256 lines. Each line represents a set of activities associated with execution of a particular instruction code. The enabled line can be combined with selected timing pulses, to develop electrical signals that can then be used to initiate specific actions. This translation of code into action is performed by the Instruction Decoder and by the associated control circuitry.
An eight-bit instruction code is often sufficient to specify a particular processing action. There are times, however, when execution of the instruction requires more information than eight bits can convey
One example of this is when the instruction references a memory location. The basic instruction code identifies the operation to be performed, but cannot specify the object address as well In a case like this, a two or three-byte instruction must be used. Successive instruction bytes are stored in sequentially adjacent memory locations, and the processor performs two or three fetches in succession to obtain the full instruction. The first byte retrieved from memory is placed in the processor's instruction register, and subsequent bytes are placed in temporary storage; the processor then proceeds with the execution phase. Such an instruction is referred to as Variable Length.
A CPU may use a register or register pair to hold the address of a memory location that is to be accessed for data If the address register is Programmable, (i e., if there are instructions that allow the programmer to alter the contents of the register) the program can "build" an address in the address register prior to executing a Memory Reference instruction (i.e., an instruction that reads data from memory, writes data to memory or operates on data stored in memory).
Arithmetic/Logic Unit (ALU):
All processors contain an arithmetic/logic unit, which is often referred to simply as the ALU The ALU, as its name implies, is that portion of the CPU hardware which performs the arithmetic and logical operations on the binary data .
The ALU must contain an Adder which is capable of combining the contents of two registers in accordance with the logic of binary arithmetic. This provision permits the processor to perform arithmetic manipulations on the data it obtains from memory and from its other inputs.
Using only the basic adder a capable programmer can write routines which will subtract, multiply and divide, giving the machine complete arithmetic capabilities. In practice, however, most ALUs provide other built-in functions, including hardware subtraction, Boolean logic operations, and shift capabilities
The ALU contains Flag Bits which specify certain conditions that arise in the course of arithmetic and logical manipulations. Flags typically include Carry, Zero, Sign, and Parity. It is possible to program jumps which are conditionally dependent on the status of one or more flags. Thus, for example, the program may be designed to jump to a special routine if the carry bit is set following an addition instruction
The control circuitry is the primary functional unit within a CPU. Using clock inputs, the control circuitry maintains the proper sequence of events required for any processing task After an instruction is fetched and decoded, the control circuitry issues the appropriate signals (to units both internal and external to the CPU) for initiating the proper processing action. Often the control circuitry will be capable of responding to external signals, such as an interrupt or wait request An Interrupt request will cause the control circuitry to temporarily interrupt main program execution, jump to a special routine to service the interrupting device, then automatically return to the main program. A Wait request is often issued by a memory or 1/0 element that operates slower than the CPU. The control circuitry will idle the CPU until the memory or 1/0 port is ready with the data.
There are certain operations that are basic to almost any computer A sound understanding of these basic operations is a necessary prerequisite to examining the specific operations of a particular computer.
The activities of the central processor are cyclical. The processor fetches an instruction, performs the operations
required, fetches the next instruction, and so on. This orderly sequence of events requires precise timing, and the CPU therefore requires a free running oscillator clock which furnishes the reference for all processor actions The combined fetch and execution of a single instruction is referred to as an Instruction Cycle. The portion of a cycle identified with a clearly defined activity IS called a State. And the inter vat between pulses of the timing oscillator is referred to as a Clock Period. As a general rule, one or more clock periods are necessary for the completion of a state, and there are several states in a cycle.
The first state(s) of any instruction cycle will be dedicated to fetching the next instruction. The CPU issues a read signal and the contents of the program counter are sent to memory, which responds by returning the next instruc tion word. The first byte of the instruction is placed in the instruction register. If the instruction consists of more than one byte, additional states are required to fetch each byte of the instruction. When the entire instruction is present in the CPU, the program counter is incremented (in preparation for the next instruction fetch) and the instruction is decoded. The operation specified in the instruction will be executed in the remaining states of the instruction cycle. The instruction may call for a memory read or write, an input or output and/or an internal CPU operation, such as a register to register transfer or an add registers operation.
An instruction fetch is merely a special memory read operation that brings the instruction to the CPU's instruction register. The instruction fetched may then call for data to be read from memory into the CPU. The CPU again issues a read signal and sends the proper memory address; memory responds by returning the requested word. The data received is placed in the accumulator or one of the other general purpose registers (not the instruction register).
A memory write operation is similar to a read except for the direction of data flow. The CPU issues a write signal, sends the proper memory address, then sends the data word to be written into the addressed memory location.
Wait (memory synchronization):
As previously stated, the activities of the processor are timed by a master clock oscillator. The clock period determines the timing of all processing activity.
The speed of the processing cycle, however, is limited by the memory's Access Time. Once the processor has sent a read address to memory, it cannot proceed until the memory has had time to respond. Most memories are capable of responding much faster than the processing cycle requires. A few, however, cannot supply the addressed byte within the minimum time established by the processor's clock.
Therefore a processor should contain a synchronization provision, which permits the memory to request a Wait state. When the memory receives a read or write enable signal, it places a request signal on the processor's READY line, causing the CPU to idle temporarily. After the memory has had time to respond, it frees the processor's READY line, and the instruction cycle proceeds
Input and Output operations are similar to memory read and write operations with the exception that a peripheral 1/0 device is addressed instead of a memory location. The CPU issues the appropriate input or output control signal, sends the proper device address and either receives the data being input or sends the data to be output.
Data can be input/output in either parallel or serial form. All data within a digital computer is represented in binary coded form. A binary data word consists of a group 5
of bits; each bit is either a one or a zero. Parallel 1/0 consists of transferring all bits in the word at the same time, one bit per line. Serial 1/0 consists of transferring one bit at a time on a single line. Naturally serial 1/0 is much slower, but it requires considerably less hardware than does parallel 1/0.
Interrupt provisions are included on many central processors, as a means of improving the processor's efficiency. Consider the case of a computer that is processing a large volume of data, portions of which are to be output to a printer. The CPU can output a byte of data within a single machine cycle but it may take the printer the equivalent of many machine cycles to actually print the character specified by the data byte. The CPU could then remain idle waiting until the printer can accept the next data byte. If an interrupt capability is implemented on the computer, the CPU can output a data byte then return to data processing. When the printer is ready to accept the next data byte, it can request an interrupt. When the CPU acknowledges the interrupt, it suspends main program execution and automatically branches to a routine that will output the next data byte. After the byte is output, the CPU continues with main program execution. Note that this is, in principle, quite similar to a subroutine call, except that the jump is initiated externally rather than by the program.
More complex interrupt structures are possible, in which several interrupting devices share the same processor but have different priority levels. Interruptive processing is an important feature that enables maximum utilization of a processor's capacity for high system throughput.
Another important feature that improves the throughput of a processor is the Hold. The hold provision enables Direct Memory Access (DMA) operations.
In ordinary input and output operations, the processor itself supervises the entire data transfer. Information to be placed in memory is transferred from the input device to the processor, and then from the processor to the designated memory location. In similar fashion, information that goes from memory to output devices goes by way of the processor.
Some peripheral devices, however, are capable of transferring information to and from memory much faster than the processor itself can accomplish the transfer. If any appreciable quantity of data must be transferred to or from such a device, then system throughput will be increased by having the device accomplish the transfer directly. The processor must temporarily suspend its operation during such a transfer, to prevent conflicts that would arise if processor and peripheral device attempted to access memory simultaneously. It is for this reason that a hold provision is included on some processors.
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