with a clearly defined activity IS called a State. And the inter- val 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.
Instruction Fetch:
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 prepara- tion 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.
Memory Read:
An instruction fetch is merely a special memory read operation that brings the instruction to the CPU's instruc- tion 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 re- ceived is placed in the accumulator or one of the other gen- eral purpose registers (not the instruction register).
Memory Write:
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 s~nds 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 synchroniza- tion provision, which permits the memory to request a Wait state. When the memory receives a read or write enable ~ig nal, it places a request signal on the processor'sR EADY f.ine, 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/Output:
InpLJt and Output operations are similar to memory read and write operations with the exception that a peri- pherall/O device is addressed instead of a memory location. The CPU issues the appropriate input or outppt 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 of bits; each bit is either a one or a zero. Parallel I/O con- sists of transferring all bits in the word at the same time, one bit per line. Serial I/O consists of transferring one bit at a time on a single line. Naturally serial I/O is much slower, but it requires considerably less hardware than does parallel I/O.
Interrupts:
Interrupt provisions are included on many central processors, as a means of improving the processor's effi- ciency. 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 equiva- lent 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 auto- matically 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 untilization of a processor's capacity for high system throughput.
Hold:
Another important feature that improves the through- put 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
