Microprocessors and Interfacing
8086 Pin Summary; 8086 Addressing Modes; 8086 Instruction Set Categories
C-CAT
8086 Pin Summary
The 8086 is available in 40-pin (8086) and 68-pin (8088/8086 variants) packages. Key signals:
| Pin(s) | Signal | Description |
|---|---|---|
| AD0–AD15 | Address/Data | Multiplexed lower 16 address / 16 data lines |
| A16/S3 – A19/S7 | Address/Status | High address bits multiplexed with status |
| BHE/S7 | Bus High Enable | Enables upper 8 data bits D8–D15 |
| RD' | Read | Active LOW — read cycle |
| WR' | Write | Active LOW — write cycle |
| ALE | Address Latch Enable | Latches address during T1 |
| DEN' | Data Enable | Enables bus transceivers (8286/8287) |
| DT/R | Data Transmit/Receive | Direction of data flow |
| M/IO' | Memory/I/O | HIGH = memory, LOW = I/O |
| READY | Ready | Wait-state input from memory/peripheral |
| HOLD / HLDA | Hold / Hold Acknowledge | DMA bus request |
| INTR | Interrupt Request | Maskable interrupt |
| NMI | Non-Maskable Interrupt | Edge-triggered, non-maskable |
| INTA' | Interrupt Acknowledge | Response to INTR |
| RESET | Reset | Active HIGH system reset |
| CLK | Clock | 5 MHz typical |
| MN/MX' | Mode select | Minimum vs Maximum mode |
| VCC / GND | Power | +5 V and ground |
8086 Addressing Modes
28.1 Immediate Mode
MOV CX, 4374H ; 4374H is the immediate operand
28.2 Direct (Memory) Mode
MOV BL, [4374H] ; Data at absolute address 4374H → BL
28.3 Register Mode
MOV CX, AX ; Copy AX to CX
ADD BX, DX ; Add DX to BX
28.4 Register Indirect Mode
MOV AL, [BX] ; AL ← Memory[DS×10H + BX]
MOV AL, CS:[BX] ; Override default segment with CS
Valid indirect registers: BX, BP, SI, DI
28.5 Based Addressing
MOV AL, [BP+10H] ; Effective address = BP + 10H
28.6 Indexed Addressing
MOV AX, [SI] ; DS:SI (default segment DS for SI)
MOV AX, [DI] ; ES:DI (default segment ES for DI)
28.7 Based Indexed Addressing
MOV AX, [BX][SI] ; Effective address = BX + SI
MOV AX, [BP][DI] ; Effective address = BP + DI
28.8 Register Relative (Displacement) Mode
MOV AX, 50H[BX] ; Effective address = BX + 50H
MOV AX, 1234H[SI] ; 16-bit displacement + SI
Combined formula:
Physical Address = Segment × 10H + Base + Index + Displacement
Example:
DS = 2000H, BX = 1000H, SI = 0050H
PA = 2000H × 10H + 1000H + 0050H = 20000H + 1050H = 21050H
28.9 Branch Addressing Modes
| Mode | Range | Example |
|---|---|---|
| Intrasegment (near) | ±128 (short) or ±32K (near) | JMP SHORT NEXT |
| Intrasegment indirect | Target in register/memory | JMP [BX] |
| Intersegment (far) | Cross segment boundary | JMP 5000H:2000H |
| Intersegment indirect | Far pointer in memory | JMP FAR [2000H] |
8086 Instruction Set Categories
| Category | Examples | Purpose |
|---|---|---|
| Data transfer | MOV, PUSH, POP, XCHG, IN, OUT, LEA, LDS, LES | Move data |
| Arithmetic & logical | ADD, SUB, MUL, DIV, INC, DEC AND OR, XOR, NOT, CMP | Computation |
| Branch | JMP, CALL, RET, Jcc (JE, JNE, JG, JL, …) | Control flow |
| Loop | LOOP, LOOPE/LOOPZ, LOOPNE/LOOPNZ | CX-based iteration |
| Machine control | HLT, WAIT, LOCK, NOP | Processor control |
| Flag manipulation | CLC, STC, CMC, CLD, STD, CLI, STI | Set/clear flags |
| Shift & rotate | SHL, SHR, SAR, ROL, ROR, RCL, RCR | Bit manipulation |
| String | MOVS, CMPS, SCAS, LODS, STOS, REP | Block memory ops with SI/DI |
String instructions use DF flag for direction and CX as counter with optional REP prefix.
Continue learning
Related notes
Definition of AI; Need of AI
Artificial Intelligence
Introduction to Data Engineering; Big Data — The 5 V's; Types of Data
Big Data and Data Engineering
Introduction to C Programming; C Program Structure; Data Types and Variables
C Programming
What Is a Computer?; Machine Cycle: Fetch–Decode–Execute; CPU Organization
Computer Architecture
Put this topic into timed practice
Open mock tests when you want full-exam pacing, or keep drilling in practice mode.