Q25Microprocessor and Microcontroller

a) Memory in 8085 µP

The 8085 microprocessor utilizes a strict 16-bit address bus ($A_0-A_{15}$). Mathematically, $2^{16}$ yields $65,536$ unique addressable locations. Since the data bus is exactly 8 bits wide (1 Byte), the absolute maximum memory capacity the 8085 can natively address is 64 Kilobytes (64KB). The entire memory architecture is linearly mapped from address $0000H$ up to $FFFFH$. Any attempt to interface more than 64KB requires highly complex hardware bank-switching techniques. The memory is physically split between ROM (for permanent execution code) and RAM (for volatile data storage).

b) RIM and SIM Instructions

These are the most complex control instructions in the 8085, utilized specifically to mathematically manage the hardware interrupt mask and serial I/O. - SIM (Set Interrupt Mask): This instruction explicitly reads the 8-bit data currently residing in the Accumulator and violently programs the internal interrupt hardware. It dictates exactly which hardware interrupts (RST 7.5, 6.5, 5.5) are masked (disabled) and which are active. It is also used to transmit a single serial data bit out via the SOD (Serial Output Data) pin. - RIM (Read Interrupt Mask): The exact opposite operation. It reads the current, real-time hardware status of all interrupts (pending and masked) and the current state of the SID (Serial Input Data) pin, loading this critical status byte directly into the Accumulator for software analysis.

c) Counters and Time Delays

Microprocessors operate at ultra-high megahertz speeds. Often, software must be purposefully slowed down to interface with slow physical hardware (like an LED blink or a motor turn). Because the 8085 lacks a built-in hardware timer, engineers must utilize software "Time Delays." A software time delay is mathematically constructed using a "Counter." A general-purpose register (e.g., Register C) is loaded with a massive hexadecimal number. The program then executes an endless, useless loop, aggressively decrementing the register (DCR C) and jumping back (JNZ) until the register mathematically hits zero. Because every single instruction consumes a strictly known, exact number of clock cycles (T-states), the engineer can calculate the exact microscopic time required to execute the loop, and mathematically tune the initial register value to generate a precise 1.00-second delay.