8085 Microprocessor Overview and Control Signals

Study Notes

RD (Read): Active low signal. Indicates data is available on the data bus for reading.
WR (Write): Active low signal. Indicates data on the data bus should be written to a selected memory or I/O location.
ALE (Address Latch Enable): Positive going pulse. Indicates that the bits on AD7-AD0 are address bits.

IO/M: Differentiates between I/O and Memory operations. High for I/O, low for Memory.
S1 and So: Status signals that can identify various operations but are rarely used.

Instructions are commands to the microprocessor to perform a specific task.
Each instruction consists of an opcode (operation code) and an operand (data to be operated on).
Instruction types are categorized by word size:
- One-Byte Instructions: Opcode and operand in the same byte.
- Two-Byte Instructions: Opcode in the first byte, operand in the second.
- Three-Byte Instructions: Opcode in the first byte, 16-bit address in the next two bytes.

Immediate Addressing: Operands provided directly within the instruction.
Register Addressing: Operands provided through registers.
Direct Addressing: Accepts data from external devices or sends data to them.
Indirect Addressing: The processor calculates an effective address and uses its contents to form a second address where data is stored.
Implicit Addressing: The data itself specifies the data to be operated on.

Data Transfer Instructions: Transfer data between registers, memory, and I/O devices.
Arithmetic Instructions: Perform addition, subtraction, increment, or decrement operations.
Logical Instructions: Perform logical operations like AND, OR, compare, and rotate.
Branching Instructions: Control program flow with conditional and unconditional jumps, subroutine calls, and returns.
Machine Control Instructions: Control machine functions like Halt, Interrupt, or do nothing.

Multiplying Two 8-bit Numbers:
- Loads immediate data into accumulator (A), registers B and C.
- Loops to add the content of B to the accumulator and decrement C.
- Stores the result in memory location 1000H.
Finding the Largest Number in an Array:
- Loads the address of the first element of the array into the HL pair.
- Loads the count into register B.
- Sets the first element as the largest data.
- Iteratively compares the current element with the largest found so far.
- Stores the largest value in memory location 2000H.

Enables data transfer between memory and I/O subsystems with limited CPU intervention.
Provides a faster alternative to Programmed I/O (PIO).
Requires a DMA controller that can independently access memory.
The DMA device asserts the processor's bus request signal, and the processor relinquishes the buses.
The DMA controller then directly transfers data between source and destination.
The processor resumes control once the DMA operation is complete.

HOLD: Active-high input signal. Indicates a DMA request. The 8085 relinquishes the buses.
HLDA (Hold Acknowledge): Active-high output signal. Indicates that the 8085 has relinquished the buses.