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3.2 / COMPUTER FUNCTION 73

Instruction Operand Operand
fetch fetch store

Multiple Multiple
operands results

Instruction Instruction Operand Operand
Data
address operation address address
calculation decoding calculation operation calculation

Instruction complete, Return for string
fetch next instruction or vector data

Figure 3.6 Instruction Cycle State Diagram

address of the previous instruction. For example, if each instruction is 16 bits
long and memory is organized into 16-bit words, then add 1 to the previous ad-
dress. If, instead, memory is organized as individually addressable 8-bit bytes,
then add 2 to the previous address.
• Instruction fetch (if): Read instruction from its memory location into the
processor.
• Instruction operation decoding (iod): Analyze instruction to determine type
of operation to be performed and operand(s) to be used.
• Operand address calculation (oac): If the operation involves reference to an
operand in memory or available via I/O, then determine the address of the
operand.
• Operand fetch (of): Fetch the operand from memory or read it in from I/O.
• Data operation (do): Perform the operation indicated in the instruction.
• Operand store (os): Write the result into memory or out to I/O.
States in the upper part of Figure 3.6 involve an exchange between the processor
and either memory or an I/O module. States in the lower part of the diagram involve
only internal processor operations.The oac state appears twice, because an instruction
may involve a read, a write, or both. However, the action performed during that state
is fundamentally the same in both cases, and so only a single state identifier is needed.
Also note that the diagram allows for multiple operands and multiple results,
because some instructions on some machines require this. For example, the PDP-11
instruction ADD A,B results in the following sequence of states: iac, if, iod, oac, of,
oac, of, do, oac, os.
Finally,on some machines,a single instruction can specify an operation to be per-
formed on a vector (one-dimensional array) of numbers or a string (one-dimensional
array) of characters. As Figure 3.6 indicates, this would involve repetitive operand
fetch and/or store operations.