8086 Microprocessor1

CS601: Microprocessor & Interfacing : UNIT 1

Brief History of 8086• The 8086 is a 16-bit microprocessor chip designed by

Intel and introduced on the market in 1978, which gave rise to the x86 architecture.

• Intel introduced the 8086 as a cost effective way of porting software from the 8080 lines, and succeeded in winning much business on that premise.

• The 8088, a version of the 8086 that used an external 8-bit data bus.

• Following up their 8086 and 8088, Intel released the 80186, 80286 and, in 1985, the 32-bit 80386, cementing their PC market dominance with the processor family's backwards compatibility.

2/27/2010 2Arvind Shrivastava

Features of 8086 Microprocessor

• 8086 has 16-bit ALU; this means 16-bit numbers are directly processed by 8086.

• It has 16-bit data bus, so it can read data or write data to memory or I/O ports either 16 bits or 8 bits at a time.

• It has 20 address lines, so it can address up to 2^20 i.e. 1048576 = 1Mbytes of memory (words i.e. 16 bit numbers are stored in consecutive memory locations).

• Frequency range of 8086 is 6-10 MHz


Features Continued …

• 8086 includes few features, which enhance multiprocessing capability (it can be used with math coprocessors like 8087, I/O processor 8089 etc.

• Operates on +5v supply and single phase (single line) clock frequency.(Clock is generated by separate peripheral chip 8284).

• 8086 comes with different versions. 8086 runs at 5 MHz, 8086-2 runs at 8 MHz, 8086-1 runs at 10 MHz.

• It comes in 40-pin configuration with HMOS technology having around 20,000 transistors in its circuitry.


Features Continued …

• It has multiplexed address and data bus like 8085 due to which the pin count is reduced considerably.

• Higher Throughput (Speed)(This is achieved by a concept called pipelining).

Now a days 8086 is no longer used. But the concept of its principles and structures is very useful for understanding other advanced Intel microprocessors


Pin Diagram of 8086


Internal Architecture of 8086


8086 Architecture Contd..• 8086 has two blocks BIU (Bus Interface Unit) and EU

(Execution Unit).• The BIU performs all bus operations such as

instruction fetching, reading and writing operands for memory and calculating the addresses of the memory operands. The instruction bytes are transferred to the instruction queue.

• EU executes instructions from the instruction system byte queue.

• Both units operate asynchronously to give the 8086 an overlapping instruction fetch and execution mechanism which is called as Pipelining. This results in efficient use of the system bus and system performance.


8086 Architecture Contd…


BUS INTERFACR UNIT: BIU• It provides a full 16 bit bidirectional data bus and

20 bit address bus.• The bus interface unit is responsible for

performing all external bus operations.

Specifically it has the following functions:• Instruction fetch, Instruction queuing, Operand

fetch and storage, Address relocation and Bus control.

• The BIU uses a mechanism known as an instruction stream queue to implement a pipeline architecture.


BIU Contd…• This queue permits prefetch of up to six bytes of

instruction code. When ever the queue of the BIU is not full, it has room for at least two more bytes and at the same time the EU is not requesting it to read or write operands from memory, the BIU is free to look ahead in the program by prefetching the next sequential instruction.

• These prefetching instructions are held in its FIFO queue. with its 16 bit data bus, the BIU fetches two instruction bytes in a single memory cycle.

• After a byte is loaded at the input end of the queue, it automatically shifts up through the FIFO to the empty location nearest the output.


BIU Contd…• The EU accesses the queue from the output end.

It reads one instruction byte after the other from the output of the queue.

• If the queue is full and the EU is not requesting access to operand in memory. These intervals of no bus activity, which may occur between bus cycles are known as Idle state.

• If the BIU is already in the process of fetching an instruction when the EU request it to read or write operands from memory or I/O, the BIU first completes the instruction fetch bus cycle before initiating the operand read / write cycle.


BIU Contd.. Segment registers• 4 Segment registers in BIU are used to hold

the upper 16 bits of starting address of memory

• CS: is a 16-bit register containing address of 64 KB segment with processor instructions. The processor uses CS segment for all accesses to instructions referenced by instruction pointer (IP) register. CS register cannot be changed directly. The CS register is automatically updated during far jump, far call and far return instructions.


BIU Contd ….• SS: is a 16-bit register containing address of 64KB

segment with program stack. By default, the processor assumes that all data referenced by the stack pointer (SP) and base pointer (BP) registers is located in the stack segment. SS register can be changed directly using POP instruction.

• DS: is a 16-bit register containing address of 64KB segment with program data. By default, the processor assumes that all data referenced by general registers (AX, BX, CX, DX) and index register (SI, DI) is located in the data segment. DS register can be changed directly using POP and LDS instructions.


BIU Contd …• ES: is a 16-bit register containing address of 64KB

segment, usually with program data. By default, the processor assumes that the DI register references the ES segment in string manipulation instructions. ES register can be changed directly using POP and LES instructions.

• Instruction Pointer (IP): is a 16-bit register. It holds 16-bit address or offset of the next code byte within code segment.


BIU Contd…• The BIU also contains a dedicated adder which is

used to generate the 20bit physical address that is output on the address bus. This address is formed by adding an appended 16 bit segment address and a 16 bit offset address.

• For example: The physical address of the next instruction to be fetched is formed by combining the current contents of the code segment CS register and the current contents of the instruction pointer IP register.

• The BIU is also responsible for generating bus control signals such as those for memory read or write and I/O read or write.


Execution unit• The Execution unit is responsible for decoding

and executing all instructions.• The EU extracts instructions from the top of the

queue in the BIU, decodes them, generates operands if necessary, passes them to the BIU and requests it to perform the read or write bus cycles to memory or I/O and perform the operation specified by the instruction on the operands.

• During the execution of the instruction, the EU tests the status and control flags and updates them based on the results of executing the instruction.


Execution Unit Contd…• If the queue is empty, the EU waits for the

next instruction byte to be fetched and shifted to top of the queue.

• When the EU executes a branch or jump instruction, it transfers control to a location corresponding to another set of sequential instructions.

• Whenever this happens, the BIU automatically resets the queue and then begins to fetch instructions from this new location to refill the queue.


8085 Compatible Flags

EU : Flag Registers

• OF : set if the result is too large positive number, or is too small negative number to fit into destination operand.

• DF : String Direction Flag, if set then string manipulation instructions will auto-decrement index registers. If cleared then the index registers will be auto-incremented.

• IF : setting this bit enables maskable interrupts.• TF : if set then single-step interrupt will occur after the next

instruction.• SF : set if the most significant bit of the result is set.

U U U U O D I T S Z U A U P U C

F F F F F F F F F

Undefined


EU : Flag registers…

• ZF : set if the result is zero.• AF : Auxiliary Carry Flag, set if there was a

carry from or borrow to bits 0-3 in the AL register.

• PF : Parity Flag, set if parity (the number of "1" bits) in the low-order byte of the result is even.

• CF : Carry flag, set if there was a carry from or borrow to the most significant bit during last result calculation.


EU Contd…: General Purpose registers• EU has eight general purpose

registersAH,AL,BH,BL,CH,CL,DH,DL• These registers can be used to store 1 byte of

data• AX is also called accumulator, Accumulator can be

used for I/O operations and string manipulation.• Certain pairs of these GP regs can be used to

store 16-bit words ex. : AH/AL -> AXBH/BL-> BX, CH/CL-> CX, DH/DL-> DX

• Advantage of using internal regs to store data is that data can be accessed much more quickly


EU Contd…


EU Contd…• Stack Pointer (SP) is a 16-bit register pointing to program

stack.• Base Pointer (BP) is a 16-bit register pointing to data in

stack segment. BP register is usually used for based, based indexed or register indirect addressing.

• Source Index (SI) is a 16-bit register. SI is used for indexed, based indexed and register indirect addressing, as well as a source data address in string manipulation instructions.

• Destination Index (DI) is a 16-bit register. DI is used for indexed, based indexed and register indirect addressing, as well as a destination data address in string manipulation instructions.


Memory Segmentation• Memories of 8086: Program, data and stack

memories occupy the same memory space.• The total addressable memory size is 1MB. As

the most of the processor instructions use 16-bit pointers the processor can effectively address only 64 KB of memory.

• To access memory outside of 64 KB the CPU uses special segment registers to specify where the code, stack and data 64 KB segments are positioned within 1 MB of memory.


Memory Segmentation


Memory Segmentation & Addressing

The real mode memory-addressing scheme, using a segment address plus an offset.


Memory Segmentation Cont…• 16-bit pointers and data are stored as:

address: low-order byteaddress+1: high-order byte

• 32-bit addresses are stored in "segment:offset" format as:address: low-order byte of segmentaddress+1: high-order byte of segmentaddress+2: low-order byte of offsetaddress+3: high-order byte of offsetPhysical memory address pointed by segment:offset pair is calculated as:address = (<segment> * 16) + <offset>


Generation of 20 bit physical address

• The 20-bit Physical address is often represented as, Segment Base : Offset

• OR CS : IPCS 3 4 8 0 0 Implied Zero+IP 1 2 3 4 -----------------------

3 5 A3 4 H


Microprocessor architecture Contd..

• The programming model of the 8086 through the Pentium II microprocessor.

• Some registers are general-purpose or multipurpose registers, while some have special purposes.


Programming Model

Figure 1.1 programming

Model

Figure 1.1 The programming model of the Intel 8086 through the Pentium II.


Registers

• Registers hold various data sizes (bytes, words, or doublewords) and are used for almost any purpose as dictated by a program.

• Multipurpose Registers• Special-purpose Registers• Segment Registers


Real mode memory addressing• Real mode operation allows the microprocessor

to address only the first 1M byte of memory space - even the Pentium microprocessor.

• The first 1M byte of memory is called either the real memory or conventional memory system.

• Real mode operation allows application software written for the 8088/8086, which contain only 1M byte of memory, to function in the 80286 and above without changing the software.

• In all cases, each of these microprocessors begins operation in the real mode by default whenever power is applied or the microprocessor is reset.


Segments and offsets

• A combination of a segment address and an offset address access a memory location in the real mode.

• The segment address, located within one of the segment registers, defines the beginning address of any 64K-byte memory segment.

• The offset address selects any location within the 64K-byte memory segment.


Table 2.1 Example segment addresses


Segments and offsets• A 20-bit real mode address allows one to access the

start of a segment at any 16-byte boundary within the first 1M byte of memory.

• Any real mode segments can only begin at a 16-byte boundary in the memory system and this boundary is often called a paragraph.

• In the 80286 (with special external circuitry) and the 80386 through the Pentium Pro, an extra 64K minus 16 bytes of memory is addressable when the segment address is FFFFH and the HIMEM.SYS driver is installed in the system.

• This area of memory (0FFFF0H-10FFEFH) is referred to as high memory.


Default segment and offset registers

• The microprocessors has a set of rules that apply to segments whenever memory is addressed.

• These rules, which apply in either the real or protected mode, define the segment register and offset register combination used by certain addressing modes.


Default Segment and Offset Registers

8086-80486 and Pentium-Pentium II default 16-bit segment and offset address combinations.



• The code segment register defines the start of the code segment and the instruction pointer locates the next instruction within the code segment.

• Stack data are references through the stack segment at the memory location addressed by either the stack pointer (SP/ESP) or the base pointer (BP/EBP).


Default Segment 7 Offset registers

80386, 80486, Pentium, Pentium Pro, and Pentium II default 32-bit segment and offset address combinations.



• One can think of segments as windows that can be moved over any area of memory to access data and code.

• A program can have a lot of segments, but can only access four (in 8086-80286) or six (in 80386 and above) segments at a time.


Addressing Modes• Immediate Addressing: the data is provided in

the instruction, MOV CX, 437AH• Direct Addressing (Segment Override prefix): the

instruction operand specifies the memory address where data is located, MOV BL, [437AH]

• Register Addressing: references the data in a register or in a register pair, MOV CS, AX

• Register Indirect Addressing : instruction specifies a register containing an address, where data is located. This addressing mode works with SI, DI, BX and BP registers.


TutorialQ1. Describe the sequence of signals that occurs on the

address bus, the control bus and the data bus when a simple microcomputer fetches an instruction?

A1. The Sequence of signals is as follows:- Address is sent out on address bus- Memory read signal is sent out on control bus- Memory sends instruction byte/s on data bus- CPU reads in the instruction and decodes it


Tutorial Contd ….Q2. What is the main difference between 8086

and 8088?

A2. The 8088 has an 8-bit data bus , so it can write to or read from memory, ports only 8-bits at a time. The 8086 can read/write 8-bit or 16-bits at a time


Tutorial Contd…Q3. What physical address is represented bya. 4370:561EHb.7A32:0028H

A3. 4370:5610H is equivalent to 43700H+561EH= 48D1EH

7A32:0028H is equivalent to 7A320H+0028H= 7A348H


Tutorial Contd…Q4. Write the 8086 asembly language statement which will

perform the following operationsa. Load the number 7986H into the BP registerb. Copy the BP register contents to the SP registerc. Copy the contents of the AX register to the DS registerd. Load the number F3H into the AL register ?

A4. a. MOV BP, 7986Hb. MOV SP, BPc. MOV DS, AXd. MOV AL, F3H


Tutorial Contd…Q5. Describe the difference between the

instructions MOV AX, 2437H and MOV AX, [2437H] ?

A5. MOV AX,2437H: loads the AX register with no. 2437H.

MOV AX,[2437H]: loads the contents of memory location DS+2437H into AL and DS+2437H+1 into AH register


Intel 80286 Microprocessor• The major new feature of the 80286 microprocessor

was protected mode. When switched to this mode, the CPU could address up to 16 MB of operating memory (previous generation of 8086/8088 microprocessors was limited to 1 MB).

• In the protected mode it was possible to protect memory and other system resources from user programs - this feature was necessary for real program multitasking. While this mode was useful for multitasking operating systems, it was of limited use for systems that required execution of existing x86 programs. The protected mode couldn't run multiple virtual 8086 programs.

2/27/2010 Arvind Shrivastava 47

http://www.cpu-world.com/CPUs/8086/index.html

http://www.cpu-world.com/CPUs/8088/index.html

Intel 80286 Contd..• 80286 was a 16-bit microprocessor. Although

in protected mode the CPU could address up to 16 MB of memory, this was implemented using memory segments. Maximum size of memory segment was still 64 KB.

• There was no fast and reliable way to switch back to real mode from protected mode.


Intel 386 Microprocessor• It is an entry-level 32-bit microprocessor designed

for single-user applications and operating systems such as MS-DOS and Windows.Base Architecture the Intel 386 consists of three major components:

• a central processing unit (CPU)

• a memory management unit

-- segmentation unit -- the paging mechanism the Intel has two modes of operation: Real Address Mode (Real Mode), and Protected Virtual Address Mode (Protected Mode).

• a bus interface


Intel 80386 Contd..• The 80386 mode included complete set of 32-bit

registers and 32-bit instructions. • Although in this mode the CPU still used memory

segment architecture similar to the one present in earlier x86 microprocessors, the size of memory segments was increased to 4 GB. This simplified development of 32-bit software, and in most cases applications could run without worrying about switching memory segments.

• It became possible to switch from protected mode back to real-mode without simulating processor reset.


Intel 80386 Contd..• Another new mode in the 80386 CPU was

8086 virtual mode. In this mode the CPU could run old 8086 applications while providing necessary protection of memory and other resources. Introduction of this mode and 80386 protected mode was very significant step. All current 32-bit x86-based operating systems use these modes to run legacy 16-bit and more modern 32-bit applications.


Intel 486 microprocessor The Intel 486 CPU offers the highest performance for DOS,

OS/2, Windows, and UNIX system V/Intel 386 applications. Its 100% binary compatible with the Intel 386 CPU.

Real Mode Architecture. Protected Mode Architecture On-Chip Cache

• 8 KB unified level 1 cache for code and data was added to the CPU. In later versions of the 80486 the size of level 1 cache was increased to 16 KB.


Intel 80486 Contd..• Execution time of instructions was significantly

reduced. Many load, store and arithmetic instructions executed in just one cycle (assuming that the data was already in the cache).

• Intel 486 featured much faster bus transfers - 1 CPU cycle as opposed to two or more CPU cycles for the 80386 bus.

• Floating-point unit was integrated into 80486DX CPUs. This eliminated delay in communications between the CPU and FPU. Furthermore, all floating-point instructions were optimized - they required fewer number of CPU cycles to execute.


Intel 80486 Contd..• Clock-doubling and clock-tripling technology was

introduced in faster versions of Intel 80486 CPU. These i486 processors could run in existing motherboards with 20 - 33 MHz bus frequency, while running internally at two or three times of bus frequency. 80486SX2 and 80486DX2 were clock-doubled version, and 80486DX4 was a clock-tripled version. AMD also produced 80486DX5 or X5 - clock-quadrupled version of the 80486.

• Power management features and System Management Mode (SMM) became a standard feature of the processor.


8086 Microprocessor1

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