80286 & 80386: Architecture, Modes, and Advanced Features

Core Concepts: 80286 vs. 80386 Architecture

The 80286 (1982) and 80386 (1985) were Intel’s leap from 16-bit to 32-bit processing, introducing protected memory, virtual addressing, and multitasking support. While the 80286 retained backward compatibility with 8086, the 80386 added paging, floating-point units (FPU), and 32-bit data buses.

1. Internal Architecture: Key Components

Both CPUs share a 4-functional-unit design but differ in addressing modes and data width:

A real die shot showing the 80386’s 275K transistors and 4 main units. (Image: Pauli Rautakorpi, CC BY 3.0, via Wikimedia Commons)

2. Addressing Modes: Real vs. Protected vs. Virtual

A. Real Address Mode (Backward Compatibility)

• Same as 8086: 20-bit physical address space (1 MB), segment:offset addressing.
• Limitation: No memory protection; crashes can corrupt the OS.
• Used in: Legacy DOS applications, bootloaders.

B. Protected Mode (80286+)

Introduces:

• 24-bit physical address bus (16 MB address space).
• Segment descriptors (stored in GDTR/LDTR registers) define base, limit, and access rights.
• Privilege levels (0–3): Ring 0 (kernel), Ring 3 (user apps).
• Virtual 8086 mode: Runs DOS apps safely in Ring 3.

How it works:

1. CPU fetches a segment selector (e.g., CS=0x10).
2. Segment descriptor table (GDT/LDT) maps it to a base address and limit.
3. Offset is added to the base to get the linear address.
4. Paging unit (80386+) translates linear → physical.

sequenceDiagram
    participant CPU
    participant GDT
    participant Memory
    CPU->>GDT: Load CS=0x10 (segment selector)
    GDT-->>CPU: Return base=0x200000, limit=0xFFFF
    CPU->>Memory: Linear Address = Base + Offset
    Memory-->>CPU: Data fetched

C. Virtual 8086 Mode (80286)

• Runs 8086 code in protected mode with memory isolation.
• Used by Windows 3.x, DOS emulators, and virtual machines.
• Example: Running a DOS-based accounting app on Windows 10 (via NTVDM).

3. Memory Management: Segmentation vs. Paging

Paging in 80386

• Page tables map linear addresses to physical frames.
• Page size: 4 KB (configurable in later x86).
• Example: A process at 0x100000 (linear) might map to physical 0x400000 via a page table.

Linear Address: 0x100000
|---------------------|
| Page Directory (32-bit) | Page Table (32-bit) | Offset (12-bit) |
|---------------------|
       0x00001       0x00002       0x0000

• Page Directory Entry (PDE) at index 0x1 points to a page table.
• Page Table Entry (PTE) at index 0x2 gives the physical frame number (0x400000).
• Offset (0x0000) selects the byte within the page.

4. Pipelining in 80386

The 80386 introduced a 5-stage pipeline to overlap instruction execution:

1. Fetch: Load instruction from memory.
2. Decode: Determine opcode and operands.
3. Execute: ALU/FPU operation.
4. Memory Access: Read/write data.
5. Writeback: Store result to register.

Advantage: ~3x throughput vs. non-pipelined 80286. Disadvantage: Pipeline stalls on branches or data hazards.

5. Advanced Features

A. Floating-Point Unit (FPU) in 80387

• Dedicated 80387 math coprocessor (later integrated into 80486).
• Supports 32/64/80-bit floating-point (IEEE 754).
• Example: Calculating compound interest for a bank loan:

  ; 80387 FPU code to compute (1 + r)^n
  FLD1          ; Load 1.0
  FLD r         ; Load interest rate (e.g., 0.05)
  FADD ST0, ST1 ; 1 + r
  FLD n         ; Load years (e.g., 5)
  FPOW         ; (1 + r)^n
  

B. Virtual 8086 Mode (80286)

• Use Case: Running DOS games (e.g., Doom) on Windows.
• How it works:
  • CPU switches to Ring 3 (user mode).
  • Interrupts (e.g., timer ticks) are trapped by the OS.
  • Memory access is checked against segment limits.

6. Real-World Applications

A. Banking Systems (Nepal: Nabil Bank, Global: Chase)

• Protected Mode: Ensures multi-user transactions don’t corrupt each other.
• Paging: Isolates customer accounts in separate memory pages.
• Example: When you transfer Rs. 50,000 from your Nabil account to Global IME, the OS uses segment descriptors to validate access and paging to lock the transaction in memory.

B. Operating Systems (Windows NT, Linux)

• Windows NT (1993): First OS to use 80386 protected mode for preemptive multitasking.
• Linux: Relies on 80386 paging for memory isolation between processes.

C. Virtualization (VMware, VirtualBox)

• Virtual 8086 Mode: Lets you run old DOS software (e.g., Turbo C++)* on modern Windows.
• Example: A Khalti merchant testing old POS software in a VM.

7. Worked Example: Address Translation in 80386

Problem: A program accesses CS:0x1234. Given:

• CS = 0x10 (selector for GDT entry 1).
• GDT Entry 1: Base = 0x200000, Limit = 0xFFFFF.
• Page Table: Linear 0x201234 → Physical 0x401234.

Steps:

1. Segment Translation:
   • CS:Offset = 0x10:0x1234 → Linear = Base + Offset = 0x200000 + 0x1234 = 0x201234.
2. Paging:
   • Split 0x201234 into:
     • Page Directory Index: 0x201234 >> 22 = 0x00001.
     • Page Table Index: (0x201234 >> 12) & 0x3FF = 0x00002.
     • Offset: 0x201234 & 0xFFF = 0x234.
   • Lookup PDE at 0x00001 → points to Page Table.
   • Lookup PTE at 0x00002 → gives Physical Frame 0x400000.
   • Final Physical Address: 0x400000 + 0x234 = 0x401234.

CS:0x1234 → Linear: 0x201234 → Physical: 0x401234
GDT Entry 1: Base=0x200000, Limit=0xFFFFF
Page Table: 0x201234 → 0x401234

8. Comparison: 8086 vs. 80286 vs. 80386

Exam Tip

1. Draw the block diagram of 80286/80386 and label all 4 units (BIU, EU, ID, CU).
2. Explain protected mode with a segment descriptor table example.
3. Compare Real vs. Protected vs. Virtual 8086 modes in a table.
4. Solve address translation problems step-by-step (segment → linear → physical).
5. Mention real-world uses:
   • Banks use protected mode for transaction isolation.
   • Windows/Linux use paging for memory management.
   • Virtualization (VMware) uses Virtual 8086 mode.
6. For programming questions:
   • Know FPU instructions (FLD, FADD, FPOW).
   • Understand pipeline stalls (e.g., branch mispredictions).

A real photo of the 80386 chip used in early PCs. (Image: contri, CC BY-SA 2.0, via Wikimedia Commons)