assembly language IBM pc microprocessor, Lecture notes of Microcomputers

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Chapter 1

Microcomputer Systems

Components of a Microcomputer System

• (^) System Unit
  • (^) Contains circuit boards
  • (^) Motherboard
    • (^) main circuit board
    • (^) contains expansion slots for additional add-in boards
  • (^) CPU - Microprocessor - "brain of the computer"
  • (^) Memory circuits - store information
  • (^) I/O Circuits - Communicate with I/O Devices
• (^) I/O Devices - peripheral devices
  • (^) Keyboard
  • (^) Display Screen - Monitor
  • (^) Disk Drives
  • (^) Printer

Memory Address

• (^) Each memory address refers to 8 bits or 1 byte of data
• (^) On 8086 microprocessor, each memory address is a 20 bit number (80286 uses a 24 bit address!)
• (^) Thus 2^20 or 1,048,576 bytes = 1 megabyte of memory can be addressed with an 8086 microprocessor
• (^) Note: The address of a byte in memory and the contents of a byte in memory are not the same thing

Bit Positions

Byte 7 6 5 4 3 2 1 0 Word 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 High Byte Low Byte Note: When a word is stored in memory, the low byte is stored in the lower address and the high byte is stored in the higher address.

Buses

• (^) A set of wires connecting different computer components that allow the CPU to communicate with memory and I/O circuits via signals
• (^) Address bus
  • (^) holds the address of the memory location to be processed
• (^) Control bus
  • (^) CPU sends control signals on the control bus to control various part of a computer system
• (^) Data bus
  • (^) Receives data sent by memory circuits
• (^) Refer to Figure 1.5 in the text

CPU

• (^) Executes program instructions expressed in machine language - a series of binary bits
• (^) Instructions are from 1 to 6 bytes long
• (^) The instruction set for each CPU is unique - consists of a series of very basic operations
• (^) 8086 Microprocessor Organization (Ref: Figure 1.6)
  • (^) Execution Unit (EU)
  • (^) Bus interface Unit (BIU)

Bus Interface Unit (BIU)

• (^) Facilitates communications between EU and I/O circuits or memory
• (^) Responsible for transmitting addresses, data, and control signals on the buses
• (^) Five registers - hold addresses of memory locations
  • (^) CS - Code Segment
  • (^) DS - Data Segment
  • (^) SS _ Stack Segment
  • (^) ES - Extra Segment
  • (^) IP - Instruction Pointer - Contains the address of the next instruction to be executed by EU
• (^) Internal Bus - connects EU with BIU
• (^) Instruction prefetch
  • (^) while EU is executing an instruction, the BIU fetches up to 6 bytes of the next instruction and places them in the instruction queue - speeds up processing

I/O Ports

• (^) I/O devices are connected to the computer through I/O circuits
• (^) Each I/O circuit contains registers called I/O ports - some are used for data and others are used for control commands
• (^) I/O ports are connected to the bus system and have an I/O address - these I/O addresses can only be referred to in input or output instructions
• (^) CPU communicates with the I/O ports which in turn communicate with the I/O devices
• (^) Serial Ports
  • (^) transfer data 1 bit at a time (Used for keyboards and a few slow printers)
• (^) Parallel Ports
  • (^) transfer data 8 bits at a time (Used for disk drives and most printers)

Clock

• (^) A clock circuit controls the processor by generating clock pulses
• (^) The time between two pulses is the clock period
• (^) The number of pulses per second is the clock rate or the clock speed - measured in Megahertz ( 1 million pulses per second)
• (^) On the 8086 - a memory read takes 4 clock periods while a multiplication takes more than 70 clock periods!
• (^) The faster the clock circuit, the faster the processor operates until it ultimately reaches its maximum clock speed

Common I/O Devices

• (^) Floppy Disks
  • (^) 5 1/4 inch - low density (360K) high density (1.2M)
  • (^) 3 1/2 inch - low density (720K) high density (1.44M)
• (^) Hard disks - fixed disk
• (^) CD ROMS
• (^) Keyboard
  • (^) has its own microprocessor that sends coded signals to computer
• (^) Monitor
  • (^) controlled by video adapter circuit
• (^) Printers
  • (^) give hardcopy output

Machine Language

• (^) Each CPU can only execute its own machine language
• (^) Machine Language is stored in binary code - a series of 0's and 1's
• (^) Very difficult to write ML programs!
• (^) Each family of CPU's has its own distinct machine language

Assembly Language

• (^) Uses symbolic names to represent operations, registers, and memory locations
• (^) Assembler
  • (^) a program that converts assembler language statements to machine language
• (^) Still difficult because instruction set is so primitive
• (^) One ALC statement usually translates to 1 ML statement
• (^) Just as each computer has its own Machine Language - each computer has its own Assembler Language

Advantages of Assembly Language

• (^) Efficiency
  • (^) Usually produces a faster, shorter machine language program
• (^) I/O to certain specific locations can be done easily in ALC but is impossible in some high level languages
• (^) Most high level languages accept ALC subroutines
• (^) By studying assembler language, you can gain a feeling for how the computer “works” and why things happen the way they do inside a computer

Sample Program

TITLE PGM1_1: SAMPLE PROGRAM ;----------------------------> IN YU'S BOOK ON PAGE 14 .MODEL SMALL .STACK 100h .DATA A DW 2 B DW 5 SUM DW? .CODE MAIN PROC ;----------------------------> INITIALIZE DS MOV AX,@DATA MOV DS,AX ;----------------------------> ADD A AND B - PLACE RESULT IN SUM MOV AX,A ADD AX,B MOV SUM,AX ;----------------------------> EXIT TO DOS MOV AX,4C00h INT 21h MAIN ENDP END MAIN