Lecture 8 Presented By Dr. Shazzad Hosain Asst. Prof. EECS, NSU

What is Microcomputer?

Typical Microcomputer Architecture

The Microcomputer Bus Address Bus – Information transfer one direction, microprocessor to memory or I/O elements, so unidirectional bus – Typically 20 to 32 bit long – Determines addressable memory, 1 MB to 4 GB

The Microcomputer Bus Data Bus – Bidirectional i.e. to or from microprocessor – Multiplexed / time shared data bus e.g. in bit address is send through 16-bit data bus and 4 status lines – Determines the word size of the microcomputer Control Bus – Used to send control signals to synchronize the operation of individual microcomputer elements – Each microcomputer has a unique set of control signals.

Clock Signals Clock generator generates clock signals Most MPs have external clock generator circuit Each instruction requires a number of clock signals Clock signals determines the speed of MPs The short period cycle means clock speed is large, means high speed of computer

The single chip microprocessor Microprocessor is the CPU of the microcomputer The main areas of microprocessor – Register section – The control unit – The arithmetic and logic unit (ALU)

Basic Microprocessor Registers Instruction Register (IR): stores instructions Program Counter (PC): contains the address of the next instruction to be executed Memory Address Register (MAR): contains address of data Accumulator (A): typically a 8 bit register, used to store result after most ALU operations

Accumulator based vs. general purpose based microprocessor Accumulator based – Intel 8085 and Motorola 6809 are accumulator based. – Accumulator is used as one of the data source – Result goes to accumulator – Example: ADD R1 General purpose based – Intel 8066/../Pentium are of this kind. – The registers contain hold data, memory address, results of ALU so on. – Example: ADD AX, BX, ADD [AX], BX

Use of the Basic Microprocessor Registers Let us add contents of two memory locations For example, add [2010] = 0002 and [2012] = 005 In general [NNNN] represents the content of the memory location NNNN.

Steps of Addition of [2010]+[2012] Load MAR with 2010 Move contents of 2010 into data register, D0, which may be AX/BX etc. Increment MAR by 2 to hold 2012 Add D0 with [2012] and store the result into D0 Instruction set form Motorola 68000

PC L E L E L E L E

Microprogramming the Control Unit Each instruction is executed by a set of microinstructions The microinstructions has their own codes The codes are stored into internal memory, called the control memory (ROM) Let us increment a register by 1

Assembly Language Instruction Format Three address Two address One address Zero address CLD; clear direction NOP; No operation For all microprocessors

References Chapter 6, Fundamental of Digital Logic and Microcomputer Design – by M. Rafiquzzaman – 6.1, 6.2, 6.3, 6.6.3