A Step-by-Step Guide to 8085 Assembly Programming
Writing an assembly language program requires a systematic approach to ensure correctness and efficiency. Here are the essential steps to follow when developing a program for the 8085 microprocessor:
Understand the requirements and constraints of the problem you need to solve
Determine the sequence of operations needed to solve the problem
Create a step-by-step procedure that outlines the solution
Visualize the algorithm with a flowchart to better understand the program flow
Convert the algorithm into assembly language instructions for the 8085
Program 8085 in Assembly language to add two 8-bit numbers and store 8-bit result in register C.
The Intel 8085 microprocessor supports several addressing modes that determine how operands are accessed during instruction execution. These addressing modes offer flexibility in programming and allow efficient use of memory and register resources.
The general format of an 8085 assembly language instruction is:
Examples:
In immediate addressing, the operand is specified directly within the instruction itself.
In direct addressing, the operand's memory address is directly specified within the instruction.
In register addressing, the operand is located in one of the microprocessor's registers.
In indirect addressing, the operand's address is stored in a register or memory location, and the data is accessed indirectly.
This is a specific form of indirect addressing where the operand's address is stored in a register.
In indexed addressing, the operand's address is calculated by adding an index value to a base address. The 8085 doesn't have built-in support for indexed addressing, but it can be simulated using other addressing modes.
These addressing modes provide versatility in accessing operands and data, allowing programmers to write efficient and compact code for various tasks. Mastery of addressing modes is essential for effective programming on the Intel 8085 microprocessor, enabling the utilization of its capabilities to their fullest extent.
In the Intel 8085 microprocessor architecture, instructions and data are structured according to specific formats to facilitate their interpretation and execution. The instruction format typically consists of an operation code (opcode) and, in some cases, additional operands.
Instructions are encoded using a variable-length format, with opcodes representing different operations such as data transfer, arithmetic, logical, branching, and control transfer instructions. Each opcode is associated with a specific operation that the microprocessor performs when executing the instruction. Additionally, some instructions may require additional data, which can be specified as immediate values, memory addresses, or register operands, depending on the addressing mode used.
Data in the 8085 architecture can be represented in various formats, including binary, hexadecimal, and ASCII. Binary data consists of sequences of 1s and 0s, which are interpreted by the microprocessor as numerical values or instructions. Hexadecimal notation is commonly used to represent binary data in a more compact and readable format, with each hexadecimal digit representing four bits of binary data. ASCII encoding is used for representing alphanumeric characters and symbols, with each character assigned a unique binary code.
Both instructions and data are stored in the memory of the 8085 microprocessor, with instructions typically residing in program memory and data stored in either program memory or data memory locations. The microprocessor fetches instructions from memory using the program counter (PC) and executes them sequentially, interpreting each opcode and its associated operands according to the instruction set architecture.
8085 Instruction set can be classified according to size (in bytes) as:
One-byte instructions in the Intel 8085 microprocessor architecture are compact instructions that occupy only one byte of memory. These instructions typically represent simple operations that can be executed quickly by the microprocessor.
One-byte instructions are often used for basic arithmetic, logical operations, data transfer between registers, and control transfer instructions such as NOP (no operation) and HLT (halt). Since they are encoded in only one byte, they are easy to decode and execute, contributing to faster program execution and efficient memory utilization.
Examples of one-byte instructions in the 8085 architecture include:
Two-byte instructions in the Intel 8085 microprocessor architecture are instructions that require two bytes of memory to represent. These instructions are typically more complex than one-byte instructions and may involve additional operands or data. Two-byte instructions provide more functionality and flexibility, allowing for a wider range of operations to be performed by the microprocessor.
Some examples of two-byte instructions in the 8085 architecture include:
In the Intel 8085 microprocessor architecture, three-byte instructions are instructions that require three bytes of memory to represent. These instructions are typically more complex and involve additional operands or data compared to one-byte or two-byte instructions. Three-byte instructions provide even greater functionality and flexibility, enabling a wider range of operations to be performed.
Some examples of three-byte instructions in the 8085 architecture include:
Block data transfer involves moving a block of data from one memory location to another. This is a common operation in assembly programming, especially when dealing with arrays or large data sets. The following program demonstrates how to transfer a block of data from one memory location to another using the 8085 microprocessor.
This unit explores assembly language programming based on Intel 8085, focusing on basic tasks like data transfer, arithmetic operations, and shifts. It progresses to more intricate programs, demonstrating the use of loops and comparisons for code conversion, character coding, and finding the largest value in an array.
Assembly language programs are machine-dependent and require a deep understanding of underlying hardware architecture. Development in assembly language often demands more effort and expertise, potentially increasing development time and costs.
The assembler is a crucial component of computer design, enabling smooth communication between memory modules and the central processing unit (CPU). It handles the tedious operation of converting symbolic code into object code, which can be stored as paper tape or diskette files using the Intellect development system's text editor.
Memory interfacing is essential for ensuring smooth data communication between the CPU and memory, and memory interface strategies must change to provide larger capacities, quicker speeds, and new memory modules as memory technology grows.
In summary, understanding the programming model and its functions is crucial for effectively using the microprocessor's capabilities and creating assembly language programs. The Intel 8085 microprocessor architecture consists of the Program Counter (PC), a 16-bit register that stores the memory address of subsequent instructions. It automatically increases after each instruction execution, allowing the CPU to execute instructions sequentially until a branch or jump instruction modifies the value.
The Stack Pointer (SP) Register oversees the stack memory region, following the Last-of-First-Out (LIFO) principle. The instruction set of the 8085 microprocessor includes data transfer, arithmetic, logical, branching, and control transfer instructions. The 8085 assembly language allows for problem analysis, program logic development, algorithm creation, and program instruction writing.
The microprocessor supports several addressing modes, allowing for flexibility in programming and efficient use of memory and register resources. Instructions and data are structured according to specific formats, with instructions encoding in variable-length format and data represented in binary, hexadecimal, and ASCII formats. The 8085 instruction set can be classified as 1-byte Instructions, 2-byte Instructions, and 3-byte Instructions. Assembly language defines instructions, while memory interfacing is used for efficient memory utilization.