How it works
The 8085 has a 8-bit data bus (AD0–AD7, multiplexed), 8 dedicated upper address lines (A8–A15), and a 16-bit address bus overall giving 64 KB addressable memory space. Internal registers: six 8-bit general-purpose registers B, C, D, E, H, L (usable as three 16-bit pairs BC, DE, HL); accumulator A; flag register F (S, Z, AC, P, CY flags); 16-bit stack pointer SP; 16-bit program counter PC. The ALU performs 8-bit arithmetic and logic. The instruction register holds the fetched opcode; the internal data bus connects all units. Clock frequency is typically 3 MHz for 8085AH with a 6 MHz crystal.
Key points to remember
The 8085 has 246 instructions, 5 flag bits (Sign, Zero, Auxiliary Carry, Parity, Carry), and operates on a 5V supply at 3 MHz (AH version) or 6 MHz (AH-2 version). Memory address space = 2¹⁶ = 64 KB; I/O address space = 2⁸ = 256 ports. The 8085 is an 8-bit processor but has 16-bit address capability through the multiplexed bus and A8–A15 lines. Serial I/O is supported through SID (Serial Input Data) and SOD (Serial Output Data) pins. Five interrupt inputs: TRAP (non-maskable), RST 7.5, RST 6.5, RST 5.5, and INTR. The HOLD and HLDA pins support DMA operation by relinquishing bus control.
Exam tip
The examiner always asks you to draw the 8085 internal architecture block diagram and explain the function of ALE — show the AD0–AD7 lines connecting to the 74LS373 latch with ALE as the latch enable, because that demultiplexing circuit is a guaranteed 10-mark question.