8085 Instruction Set and Programming

Why it matters

The 8085 microprocessor is a fundamental component in the study of microprocessors and embedded systems. Understanding its instruction set and programming is essential for designing and implementing basic microprocessor-based applications, which are foundational skills for any electronics and telecommunication engineer.

Key ideas

• 8085 Microprocessor: An 8-bit microprocessor with a 16-bit address bus, capable of addressing 64 KiB of memory.
• Instruction Set: Comprises various instructions categorized into data transfer, arithmetic, logical, branching, and control instructions.
• Opcode and Operand: Instructions have an opcode; operands may be explicit or implied, and can identify registers, immediate data or memory.
• Addressing Modes: Includes immediate, direct, register, register indirect, and implied addressing modes.
• Programming: Writing assembly language programs using the 8085 instruction set to perform specific tasks.

Formulas

T = n * (1/f)

• T: Execution duration for n clock periods (seconds)
• n: Number of clock periods (T-states) in the specified operation
• f: Clock frequency (Hertz)

Worked example

Problem: Calculate the time taken to execute an instruction that requires 4 clock periods (T-states) on an 8085 microprocessor with a clock frequency of 3 MHz.

1. Identify given data:

   • Number of clock periods, n = 4
   • Clock frequency, f = 3 MHz = 3 x 10^6 Hz

2. Use the formula: T = n * (1/f)

3. Substitute the values: T = 4 * (1 / (3 x 10^6)) seconds

4. Calculate: T = 4 * (1 / (3 x 10^6)) = 4 / (3 x 10^6) = 1.33 x 10^-6 seconds

5. Final answer: 1.33 µs

A machine cycle such as opcode fetch or memory read contains multiple T-states; an instruction may contain multiple machine cycles. The example explicitly supplies four T-states. Knowing only four machine cycles is insufficient without their lengths and any wait states. Use the processor clock frequency, not an external oscillator frequency without accounting for the clock circuit.

Common mistakes

• Confusing the number of clock cycles with machine cycles.
• Incorrectly calculating the time period by not converting MHz to Hz.
• Misunderstanding the addressing modes and their applications.

For GATE EC

Questions typically involve calculating execution time, understanding instruction formats, and applying addressing modes. Practice writing simple assembly programs and converting between different addressing modes.

Quick check

1. What is the size of the address bus in the 8085 microprocessor?
2. Name two types of instructions in the 8085 instruction set.
3. How many clock periods are required for an instruction if the clock frequency is 2 MHz and the execution time is 2 µs?

Answers: 1. 16-bit, 2. Data transfer and arithmetic, 3. 4 clock periods (T-states).