Instruction Sets: Characteristics and Functions
The essential elements of a computer instruction are the opcode, which specifies the operation to be performed; the source and destination operand references, which specify the input and output locations and for the operation; and a next instruction reference, which is usually implicit. (understood)
Opcode specify operation in one of the following general categories: arithmetic and logic operations; movement of data between two registers, register and memory or two memory locations; I/O; and control.
Operand references specify a register or memory location of operand data. The type of data may be addresses, numbers, characters and logical data.
A common architectural feature in processor is the use of stack, which may or may not be visible to the programmer. Stacks are used to manage procedure calls and returns and may be provided as an alternative form of addressing memory. The basic stacks operation are PUSH, POP, and operations on the top one or two stack locations. Stacks typically are implemented to grow from higher addresses to lower addresses.
The machine instructions set provides the functional requirements for the CPU: Implementing the CPU is a task that in large part involves implementing the machine instructions set.
MACHINE INSTRUCTIONS CHARACTERISTICS
The operation of the CPU is determined by the instructions it executes referred to as Machine Instructions or computer instructions
The collection of different instructions that the CPU can execute is referred to as the CPU’s instruction set. * Machine instructions operate on data. The important general categories of data are : * Addresses (locations) * Numbers (integer/fixed point and floating point/decimal * Characters (text/ character strings) * Logical data (1s-0s)

Elements of Machine Instructions
Operation Code- Specifies the operation to be performed (e.g., ADD, I/O). The operation is specified by a binary code, known as the operation code or opcode.
Opcodes are represented by abbreviations, called mnemonics, that indicate the operation. Common examples include:
ADD Add
SUB Subtract
MPY Multiply
DIV Divide
LOAD Load Data from memory
STOR Store data to memory

Elements of Machine Instructions
Source operand reference- the operation may involve one or more source operands, that is, operands that are inputs for the operation.
Result operand reference- the operation may produce a result.
Next Instruction reference- this tells the CPU where to fetch the next instructions after the execution of this instruction is complete.

Instruction Cycle State Diagram
Source and result operands can be in one of three areas:
Main or Virtual Memory- as with next instruction references, the main or virtual memory address must be supplied.
CPU Register- with rare exceptions, a CPU contains one or more registers that may be referenced by machine instructions. If only one register exists, reference to it may be implicit. If more than one register exists, then each register is assigned a unique number and the instruction must contain the number of the desired register.
I/O Device- the instruction must specify the I/O module and device for the operation. If memory-mapped is used, this is just another main or virtual memory address.
Categorize Instructions Types:
Data Processing- Arithmetic and Logic instructions
Data Storage- Memory Instructions
Data Movement- I/O Instructions
Control- Test and Branch instructions

Stacks * Stack is a last-in-first-out set of locations. * Is an ordered set of elements, only one of which can be accessed at a time. * The point of access is called top of the stack * Items may only be added to or deleted from the top of the stack. * A stack is also known as a pushdown list or a last-in-first-out (LIFO) list. * A push operation appends one new item to the top of the stack. * A pop operation removes the top item from the stack. * Zero-address instructions are applicable to a special memory organization, called stack. * Three addresses are needed for proper operation, and these are often stored in CPU registers: * Stack Pointer- contains the address of the top of the stack * Stack Base- contains the address of the bottom location in the reserved block * Stack Limit- contains the address of the other end of the reserved block.
Activity
Figure 1. Illustrate and explain the Basic Stack Operation.

Figure 2. Illustrate and explain the Typical Stack Organizations

3. Explain the Instruction Cycle State Diagram