8085 Microprocessor Architecture & Operations

The 8085 is an 8-bit microprocessor that executes instructions by fetching, decoding, and executing them using its internal architecture, including registers, ALU, and control units. Understanding its components and operations is key to embedded systems programming.

Core Principles

• The 8085 architecture comprises functional blocks like the Accumulator, ALU, Flag Register, General Purpose Registers, Program Counter, and Stack Pointer.
• The Accumulator (8-bit) is central to arithmetic and logical operations.
• The ALU performs arithmetic (add, subtract) and logical (AND, OR, XOR) operations.
• The Flag Register (8-bit) stores the status of operations (Sign, Zero, Carry, etc.).
• The Program Counter (16-bit) holds the address of the next instruction.
• The Stack Pointer (16-bit) manages the stack memory for temporary storage and subroutine calls.

Key Terms

• Accumulator: An 8-bit register used for arithmetic and logical operations.
• ALU: Arithmetic and Logic Unit; performs calculations and logical operations.
• Flag Register: Stores status bits (flags) after an operation (e.g., Zero, Carry).
• Program Counter (PC): 16-bit register holding the address of the next instruction.
• Stack Pointer (SP): 16-bit register pointing to the top of the stack memory.
• Addressing Mode: Method specifying how operands are accessed in an instruction.
• Interrupt: Signal that temporarily halts normal program execution for urgent tasks.

Pro Tips

• Leverage register pairs (BC, DE, HL) for efficient data manipulation.
• Understand flag register implications for conditional branching.
• Utilize the stack for managing subroutine calls and interrupts effectively.

Pitfalls to Avoid

• Confusing 8-bit and 16-bit register operations.
• Incorrectly implementing stack operations (PUSH/POP order).
• Misunderstanding the role of specific flags in conditional jumps.
• Using invalid instructions (e.g., MOV A, 35H instead of MVI A, 35H).
• Ignoring address decoding logic in memory interfacing.

Myth vs Reality

• All interrupts in the 8085 have the same priority.: The 8085 has a defined priority order for interrupts, with TRAP having the highest and INTR the lowest.
• The stack grows upwards in memory.: The stack in the 8085 grows downwards in memory; PUSH decreases the Stack Pointer, and POP increases it.
• MOV A, 35H is a valid instruction.: The correct instruction to load immediate data 35H into the accumulator is MVI A, 35H.

Real World Examples

• Adding two numbers.: The ALU performs the addition, and flags (like Zero or Carry) are set based on the result.
• Calling a subroutine.: The CALL instruction pushes the return address onto the stack, and RET pops it back to resume execution.
• Generating a square wave.: Using the SOD pin with MVI and SIM instructions to toggle output HIGH and LOW.

Timeline

• 1974: Intel introduces the 8080 microprocessor.
• 1976: Intel releases the 8085 microprocessor, an improved version of the 8080.
• Late 1970s - 1980s: The 8085 becomes widely used in microcomputers, embedded systems, and educational kits.
• 1980s: Development of more advanced microprocessors like the 8086 and Z80.
• Present: The 8085 remains relevant in specific embedded applications and as a foundational learning tool.

People

• Federico Faggin: Key designer of the Intel 8080 and 8085 microprocessors.
• Marcian 'Ted' Hoff: Led the team that developed the Intel 4004, the first commercial microprocessor.
• Stanley Mazor: Contributed significantly to the architecture of early Intel microprocessors, including the 8080 and 8085.

Quiz

• Which register stores the address of the next instruction to be executed in the 8085?: Program Counter (PC)
• What is the primary function of the ALU in the 8085?: Perform arithmetic and logical operations
• Which addressing mode is used when the operand is directly specified within the instruction itself?: Immediate Addressing