Chapter 3 - x86 CPU

.1  x86 Registers

CPU's contain a small set of registers for storing data.  They can contain an instruction, a memory address, or any other type of data.  The x86 CPU's have come in 16-bit, 32-bit, and 64-bit versions and outlined below.  The diagram below shows the 32-bit version of the x86's registers.

8086 (1978)

The first x86 CPU had 16-bit registers.  Four of them (AX, BX, CX, DX) could be accessed as twice as many 8-bit registers.  Since the address bus had 20 pins, there were extra registers (CS, DS, SS, and ES) used for storing 20-bit memory addresses.

80386 (1985) - shown in diagram on right
With the 80386, the general purpose 16-bit registers were expanded to 32-bit and added the prefix "e" for extended.  The 16-bit registers were left for backwards compatibility.

AMD Opteron (2003) and Pentium 4 (2004)
The 32-bit registers were expanded to 64-bits and labeled with the prefix "r" instead of "e".

There are additional registers including the instruction pointer (EIP) and flags.


3.2  8086 CPU Pin Assignments

This is the pin assignments (pinout) of the 1978 Intel 8086 CPU.  The address bus is multiplexed (shared) with the data bus so it could fit in a 40-pin dual in-line package.

Data Bus
It has 16-bit registers and can transfer 16-bits at a time on the data bus (AD0 - AD15).

Address Bus
It has 20-bit memory address bus (AD0 - AD19) so it can address 220 or 1,048,576 bytes of memory.  Since the registers are on 16 bits, it used segment registers CS, DS, SS, ES to store 20 bit addresses.

Interrupt Request
The INTR pin accepts input for triggering an interrupt (such as keyboard press).  The INTA (interrupt acknowledge) pin is activated after a request.

Status Pins
Pins S0, S1, S2 supply control signals for the bus such as I/O and memory reads and writes.

.3  x86 History

The table below is a simplified history of Intel x86 CPU's.  There were many variation for each model and other manufacturers such as AMD.  The 4004, 8008, and 8080 were predecessors to the first x86 CPU - the 8086.  The x86 CPU's are used in Windows PC's and Apple Macintosh computers beginning in 2006.

The data bus width shows what size of data the CPU can process.  For example, a 32-bit CPU generally will have 32-bit registers and 32 external pins to transfer data to other computer components such as memory.   The 32-bit CPU's began with the 80386.  The 32-bit x86 CPU's are backwards compatible with 16-bit instructions, and the 64-bit x86 CPU's are backwards compatible with 32-bit and 16-bit instructions.  Due to technological advances, the number of transistors in CPU's have roughly doubled every two years which is called Moore's Law.

Intel x86 Microprocessors and Predecessors

Year Name Data Bus Width Address Bus Width Addressable Memory Transistors Clock Speed Notes
1971 4004 4 bits 12 bits 4 KB 2,300 740 kHz Used in calculators
1972 8008 8 bits 14 bits 16 KB 3,500 800 kHz Used in Datapoint microcomputer
1974 8080 8 bits 16 bits 64 KB 4,500 2 MHz Used in Altair 8800
1978 8086 16 bits 20 bits 1 MB 29,000 5 MHz First x86 - Used in IBM PC
1982 80286 16 bits 24 bits 16 MB 134,000 6 MHz Used in IBM AT
1985 80386 32 bits 32 bits 4 GB 275,000 16 MHz First 32 bit x86
1989 80486 32 bits 32 bits 4 GB 1.2 million 25 MHz First integrated cache and floating-point unit
1993 Pentium 32 bits 32 bits 4 GB 3.1 million 60 MHz  
1997 Pentium II 32 bits 36 bits 64 GB 7.5 million 300 MHz  
2004 Pentium 4 64 bits 36 bits 64 GB 42 million 3 GHz First Intel 64 bit x86
2006 Core 2 Duo 64 bits 36 bits 64 GB 291 million 2 GHz Two cores
2008 Core i7 64 bits 36 bits 64 GB 731 million 3 GHz Four cores
2017 Core i9 64 bits 36 bits 64 GB 6.5 billion 3.3 GHz Eight cores

Intel 4004 CPU  Intel 8008 CPU  Intel 8086 CPU    Intel 386 CPU
Intel Pentium 4 CPU