Class Notes cs250 Part I: Introduction to Intel 8086/8088 Microprocessor




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Class Notes

CS250


Part I: Introduction to Intel 8086/8088 Microprocessor


Microprocessor is a processor (cpu) that is contained on a single silicon chip.

  1. A Historical Background of Microprocessors (Intel)


  • Intel 4004: the world’s first microprocessor developed by Intel Corporation in 1971.

    • 4-bit cpu

    • 4096 4-bit memory locations

    • 45 instructions

    • Speed: 50k 1ps (instructions per second)

      • Other Intel Microprocessors

    1. 8008 (later in 1971): and extended 8-bit version of 4004, memory size: 16k bytes, 48 instructions, 50k 1ps.

    2. 8080 (1973): 8-bit cpu, 64k byte memory, 500k 1ps.

    3. 8088/8086 (1977,1978): 16bit cpu, 1M byte memory, 2.5M 1ps.

    4. 80186: a similar version of 8086, it has more instructions.

    5. 80286 (1983): an extended 16M-byte memory of 8086, 8MHz cpu, 4M 1ps.

    6. 80386 (1986): fully 32-bit cpu, up to 4G byte memory, hardware memory management and memory assignment. Pipelined instruction execution, 33M 1ps.

    7. 80486 (DX, SX) (1989): an improvement of 80386, 50M 1ps, with built-in math processor (for floating-point and extended-precision number operations)

    8. Pentium (1993): P5 or 80586, 60-133M2, 16k cache, 4G memory, 2 integer units.

    9. Pentium with MMX

    10. Pentium Pro (1995): P6, 150-166 M2, 16k cache, 156k-second level cache, three integer units.

    11. Pentium 4





  1. 8086/8088 Microprocessor Fundamentals


2.1 Introduction

The 8086/8088 refers to microprocessors developed by Intel Corporation, which was used by IBM personal computers (XT, 1981). 8088 and 8086 are functionally identical (8-bit cpu) except that 8088 uses 8 dada lines in its data bus while 8086 uses 16 data lines in its data bus.


2.2 Fundamental components of 8086/8088

The 8086/8088 microprocessor consists of four functional units.



  1. Execution unit (EU): decodes and executes machine instructions.

  2. Arithmetic and logic unit (ALU): performs math and logical operations on command by the EU.

  3. Internal storage (sometime called registers): is used for internal data storage.

  4. Bus interface unit (BIU): handles all communications with the I/O via the system bus and maintains instruction queue.


2.3 Execution Unit (EU)

The Execution Unit controls the activity within the processor. It is responsible for retrieving binary machine-language instructions from the Instruction Queue maintained by the Bus Interface Unit deciphering them, and seeing to it that the correct steps are performed within the processor to carry out each instruction. If an instruction requires the use of data that is stored internally, in a register, then the Execution Unit retrieves the data from the correct register; if the instruction requires external data, from memory perhaps, then the EU requests the data from the Bus Interface Unit.


Whenever an instruction calls for an arithmetic or logical function, the EU passes the data to Arithmetic and Logic Unit together with a command telling the ALU what to do with the data. The EU then accepts the resulting data from the ALU and sees to it that it is stored into correct location (register or memory), as designated by the instruction.
2.4 Arithmetic and Logic Unit (ALU)

The ALU contains circuitry that is capable to perform arithmetic (+, -, *, /) operations and logic operations (AND, OR, NOT, XOR) operations. As the ALU completes a requested operation, it also controls individual bits of Flags register (sometimes called the program status register). It sets (1) or clear (0) the correct bits to reflect specific characteristics of the result of the operation, such as whether the result was zero or non-zero, whether it was positive or negative, or if the result is too big to be stored in a byte or word. The EU checks the status of those Flag bits whenever executing conditional jump instructions.


2.5 Bus Interface Unit (BIU)

BIU is a circuitry to response memory access and communicate with I/O devices. In most microprocessors, these functions are performed by EU, and some times the EU has to be idle due to different speed and serial execution.

In the design of the 8086/8088, Intel introduces BIU in order to avoid idle time and hold an instruction queue such that instructions can be executed in a pipeline fashion.
2.6 System Bus and Other Support Hardware Devices in IBM PC

2.6.1 In order to build a complete computer, there are a number of other hardware devices (chips) in the IBM PC. Some of them are:


Memory

Up to one megabyte of system memory, including that which is on the system board, the video adapter card, and any add-on memory boards.



Timer chip

Generates an external interrupt every fifty-five milliseconds to allow the PC to keep track of real time (system data and time).


8259 Interrupt Controller chip

Processes hardware (external) interrupts.


6845 CRT Controller

Located on the video adapter card. Controls the video signals to the monitor.


NEC D765 or Intel 8272 Floppy Disk Controller
Located on the disk adapter card. Acts as an interface between the processor and the disk drive.
Inter 8237 Direct Memory Access (DMA) Controller

Located on the system board. Used by the disk controller to transfer data between disk and memory.

8250 Asynchronous Communications Element or Universal Asynchronous Receiver/Transmitter (UART)

Located on each serial communications adapter card (COM1, COM2, etc.).




      1. System Bus: is a series of conductive traces of signal lines on the system board, which is used to communicate between 8086/8088 (cpu) and all other devices.

The system bus is made up of three functional parts:



    1. Data bus

    2. Address bus

    3. Control bus

System bus



8086/8088

Memory

I/O port1

I/O portn

BIU

To read from memory, for example, the Bus Interface Unit puts the correct memory address onto the Address Bus and puts the command to read from memory onto the Control Bus. All devices connected to the bus see this address and command simultaneously, but only the memory-control circuitry respond to it. The memory-control circuitry is then responsible for decoding the address, retrieving the data from the appropriate memory chips, and placing the data onto the Data Bus for retrieval by the BIU.


To write to memory, the BIU puts the memory address onto the Address Bus, the byte of data onto the Data Bus, and a command to write data onto memory of the control bus. The memory-control circuitry decodes the command and the address, retrieves the data from the Data Bus, and stores it into the correct memory chips. All other circuitry simply ignores the command.
Communication with the many special-purpose microprocessors attached to the System Bus is accomplished through I/O ports. I/O ports are used for the transfer of data between the 8086/8088 processor and the other support hardware within the system. The In and OUT instructions tell the processor to input or output data through the I/O ports.
When executing an IN instruction, the BIU puts the I/O port address onto the Address Bus and puts the command to input data onto the Control Bus. The circuitry on some peripheral device attacked to the bus recognizes the red command and decodes the I/O port as the address of some register within the peripheral device. It then retrieves the data from the register and places it onto the Data Bus from which it is retrieved by the BIU and fed to the Execution Unit within the processor.
To execute an OUT instruction, the BIU puts the command to output data onto the Control Bus, and I/O address onto the Address Bus, and the data to be output onto the Data Bus. The circuitry of a peripheral device is then responsible for recognizing the output command and the I/O address and for retrieving the data from the data bus.

  1. Memory and Internal Storage


    1. Storage Elements

  • Bits: The smallest storage element on any computer is the bit, which can have a value of 0 or 1.

  • Nibbles: A nibble is a sequence of 4 bits.

e.g.

bit 3 bit 0


  • Bytes: A byte is a sequence of 8 bits.

e.g.

bit 7 bit 0


  • Words: in 8086/8088, a word consists of 16 bits that can be viewed as:

      1. A horizontal sequence of 16 bits or.

e.g.

bit 15 bit 0




      1. A vertical sequence of 2 bytes e.g..


bit 15 bit 8

bit 7 bit 0

Note: in memory, the least significant bye (bits 0-7) is stored in lower numbered memory location, and the most significant byte (bits 8-15) is stored in the next higher numbered location.


  • Double words: Two words represents a double word which is viewed as

    1. A horizontal sequence of 32 bits or

    2. A vertical sequence of 2 words or

    3. A vertical sequence of 4 bytes

Note: in memory, its 0-7 bits would come first; bits 8-15 would be in the next higher numbered byte and so on.




  • Quadword: A quadword is 2 double words, hence 4 words or 8 bytes.

  • Tenbytes: A tenbyte is a sequence of 10 bytes.




    1. Memory

Computer memory consists of an ordered sequence of storage units (8-bits called bytes), each with its own address.

  • 8086/8088’s memory is bytes-addressable, which means that each byte has its own address, e.g. if A is the address of a word (16 bits), then A is actually the address of the first byte (bits 0-7) of the word, A+1 is the address of the second byte (bits 8-15) of the word, and A+2 is the address of the next word.

  • Memory address space: (MAS): 220= 1,048,576= 1M(bytes) (address space: 0—220- 1, i.e.00000-fffff: in hexadecimal)

  • Map of the memory address space (MAS)

The whole MAS is organized by a 216 * 24 matrix: there are 216 paragraphs, each of which with 16 (24) bytes.
Paragraph numbers:


0000


0001

0002

FFFE

FFFF
Figure 1: Map of the memory address space (MAS) of the 8086/8088.




  • Hexadecimal number system

The hexadecimal number system uses sixteen characters to represent umbers:0 through 9 and A through F; A is worth ten, B is worth eleven, …, and F is fifteen. In assembly program, the hexadecimal will be indicated by the letter h.
Example: 7Fh is the number 127(7*16+15*1) and its equivalent binary number is 01111111b.
The conversion between binary and hexadecimal is simple. To covert from hexadecimal to binary: just represent each hexadecimal digit by four binary digits: Example: 74Dh = 011101001101b
To convert from binary to hexadecimal: break the binary number into groups of four digits, and convert each group into one hexadecimal digit:

Example: 101001001110b = A4Fh




  • Character representation

Each character is represented by a byte or two hexadecimal digits according to ASCII code.
For example: ASCII code Letter

41h A


42h B

. .


. .

5Ah Z


61h a

62h b


. .

. .


7Ah z

30h 0


. .

. .


39h 9
In assembly the ASCII representation of character can be defined by enclosing the characters in paired single (‘) or double (“) quotes.

Example: ‘A’ ‘e’ “9”

Note ‘A’ = 41h, ‘a’ = 61h and “9” = 39h


  • Segment-relative addresses.

    1. There are 16 bits in a segment register. The content of the segment register represents the paragraph number of the paragraph at which it begins.

    2. All memory address are formed by segment-relative format which is computed as follows:

Absolute address=(segment register)*16 + offset (16-bit) denoted by segment register: where the offset can be content of a register (16-bit) and a 16-bit value.


e
CS: 1110000000000000

CS*16: 11100000000000000000 (Beginning of code segment)

IP: 0000000000001101 (offset)

--------------------------------------------------------------------------

CS : IP = 11100000000000001101 (Actual location (absolute address) in memory)
.g.
where CS is the code segment register, IP is the instruction pointer register


    1. Program stack

A program stack is a region of memory (RAM), which is defined by the assembly program. The program stack is implemented by setting the SS (stack segment) register and by initializing the SP (stack pointer) register to the byte immediately above the top of the sack segment. Data is moved onto the stack in word- sized units by an operation called a push and retrieved from the stack in word-sized units by an operation called pop in the reversed order (last in- first out).

Figure2: Stack operations


A Push operation decrements the SP register twice (-2) and put the word at SS : SP. A Pop operation retrieves the word at SS: SP and increments the SP register twice (+2)
Example: A program stack is to use 100h bytes and SS= 58A1h, A = 1234h and B = 5678h.

Figure 3 shows the “empty” stock, where the initial value of SP register = 100h and the stack starts at address of 58A10h and ends at address of 58B0Fh (100 bytes). Figure 4 shows the stack after two Push operations: Push A and Push B, and Figure 5 shows the stack after two Pushes and one Pop operations (Pop B).


Figure 3: Empty stack


Figure 4: Stack after two Pushes



Figure 5: Stack after two Pushes and one Pop




    1. Internal Storage (Registers)

Registers: there are 14 registers, each 16 bits (bits 0-15) in length. 14 registers are divided into five groups:

  1. Four segment registers

  2. Four data registers

  3. Two pointer and two index registers

  4. The instruction pointer register

  5. The flag register (status register)




      1. Status register

In 8086/8088 the status register is called the flag register. The flag register contains information about the most recently executed instruction. There are 16 bits in register. But only bits 0, 2, 4 and 6-11 are used
OF IF SF AF PF CF




?

?

?

?



















?




?




?



DF TF ZF


  1. CF (bit 0): the carry flag.

  2. PF (bit 2): the parity flag.

  3. AF (bit 4): the auxiliary flag.

  4. ZF (bit 6): the zero flag.

  5. SF (bit 7): the sign flag.

  6. TF (bit 8): the trap flag.

  7. IF (bit 9): the interrupt flag.

  8. DF (bit 10): the direction flag.

  9. OF (bit 11): the overflow flag.


Figure 6: Register Map of 8086/8088


3.4.2 segment registers

The 8086/8088 has four separate “work area” for the parts of an assembly language program: data, instruction, stack and extra data. Each area is called a segment and has a register to associate with called segment register. Each segment can be up to 65,536 bytes, and two segments my overlap.



  1. Data segment register (DS): indicates the beginning of the data segment.

  2. Code segment register (CS): indicates the beginning of the code segment.

  3. Stack segment register (SS): the stack is a part of memory. The stack is only word-addressable.

  4. Extra data segment register (ES): points to the beginning of the extra data


3.4.3 data registers

There are four 16-bit data registers that are used to store data.



  1. Accumulator: AX (used for arithmetic and logic instruction).

  2. Base register: BX (stored data)

  3. Count register: CX (is altered by the loop instruction).

  4. Data register: DX (used for multiply and divide instructions). An “X” register can be subdivided into a high part (H, 8-15 bits) and a low part (L, 0-7bits).

AX
BX


CX
DX


AH

AL

BH

BL

CH

CL

DH

DL

15 8 7 0
3.4.4 pointer and index registers

The segment registers point to the beginning of the various memory segments. The point registers are used to calculate offsets (displacements) into these segments.




  • There are two pointer registers:

    1. Stack pointer (SP)

    2. Base pointer (BP)

  • Stack pointer (SP): SP is used to hold the current stack location. SS contains the address of the beginning of the stack segment and SP contains the offset.

  • A stack is a pile or list of items that can be accessed one at a time and from only one end, usually the items are removed from the top and added to the top. Thus, the stack is also called last-in first-out (LIFO) list. In8086/8088, the stack is implemented by a segment of the memory, each item is a 16-bit word (2 bytes). There are two registers associated with stack: SS and SP.

  • There are two basic stack operations in 8086/8088. a. PUSH (operand) it decreases the SP by 1 word (2 bytes), then stores a word on the stack e.g. current SP=204. AX=0010. Execution of PUSH AX will result: SP=204-2=202 and store 0010 at memory location at address 202. b. POP (operand it removes the top word from the stack, and then add 1 word (2 bytes) to the SP.

  • The base pointer (BP): BP is also can be used to point to offset within stack segment. Primarily it is used for base indexed addressing modes.

  • There are two index registers:

    1. Source index register (SI)

    2. Destination index register (DI) there is no significance to the words source and destination, and SI and DI can be used interchangeably. Index registers are used in the same way subscripts that are used in high-level language. That enable us to access the elements in an array or table.


3.4.5 the instruction pointer

The instruction pointer (IP) contains the offset address in the code segment of the next instruction to be executed. The absolute address of the next instruction = CS : IP.







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