Microprocessors and Microcontrollers
What this domain tests
Questions target 8085 and 8051 fundamentals: architecture, registers, addressing modes, flags, interrupts, memory interfacing, and simple instruction tracing. The exam tests understanding, not assembly-language programming depth.
Core concepts
- The 8085 is an 8-bit microprocessor with a 16-bit address bus, 40 pins, and multiplexed lower address/data lines AD0-AD7. ALE latches the lower address byte.
- 8085 registers include A, B, C, D, E, H, L, the stack pointer, program counter, and flags: sign, zero, auxiliary carry, parity, and carry.
- The 8051 is an 8-bit microcontroller with on-chip ROM/RAM, four 8-bit ports, timers/counters, serial port, and interrupt structure. It integrates CPU and peripherals.
- Addressing modes include immediate, register, direct, indirect, and implied. Interrupts transfer control to a vector address; the 8085 stack grows downward in memory.
Worked example
In an 8085, A = 9AH and B = 2AH. ADD B gives 9AH + 2AH = C4H. Carry = 0 because the sum is below 100H. Sign = 1 because the MSB of C4H is 1. Zero = 0. Auxiliary carry = 1 because the low nibbles A + A = 14H, producing a carry from bit 3 to bit 4.
Traps
Do not treat ALE as an address line; it is a latch-enable pulse. Remember active-low control signals such as RD and WR. The 8085 is not a microcontroller, and the 8051 is not a 16-bit processor. Stack operations use the stack pointer, not the program counter.
Key points
- 8085: 8-bit CPU, 16-bit address bus.
- AD0-AD7 are multiplexed; ALE demultiplexes them.
- 8051 has on-chip ports, timers, and serial interface.
- 8085 flags: S, Z, AC, P, CY.
- Stack grows downward in 8085 memory.