52.223 Low Level Programming Lecturer: Duncan Smeed

Recap

Introduction and Recap52223_01/2

The Organisation of a Computer

Organisation of a computer, showing the five classic components:– input– output– memory– datapath and– control (these last two are sometimes combined and called the processor)

Introduction and Recap52223_01/3

Block Diagram of Typical Computer

Processor (CPU)

Input/Output Memory

Direct Memory

Access

I/O Bus Memory Bus

Introduction and Recap52223_01/4

Input/Output (I/O) Subsystem

CPU

Memory

I/O

Interface

I/O

Interface

I/O

Device

I/O

Device

•••

••• •••

•••

•••

I/O Address

I/O Data

I/O Control

Specialised

data and

control lines

Memory

Bus

Introduction and Recap52223_01/5

Memory-mapped I/O Structure

CPU

Memory

I/O

Interface

I/O

Interface

I/O

Device

I/O

Device

•••

••• •••

•••

•••

Address

Data

Control

Specialised

data and

control lines

Introduction and Recap52223_01/6

Memory Structure

• Most, if not all, modern day computers incorporate at least two types of main memory device:– Read Only Memory (ROM) and

– Random Access Memory (RAM).

Introduction and Recap52223_01/7

Read Only Memory (ROM)

• The contents of such memory devices are non-volatile– which means that their contents are not lost when the power is switched

off.

• ROM is normally required 'boot' (start from a power off state) a computer as it must get (at least) its first few instructions from such a device.

• The (rest of the) operating system is then, typically, loaded from a magnetic media such as tape, floppy disk or hard disk.

• Some computers keep the whole of their operating system in ROM although this has the disadvantage that it cannot be as easily updated as changing the (contents of) magnetic media.

Introduction and Recap52223_01/8

Random Access Memory (RAM)

• More accurately called Read/Write Memory (RWM)– the contents of which can be written as well as read.

• Normally volatile in that it loses its contents when power is switched off.

• Most computers have much more RAM than ROM as most programs, including the operating system, are loaded into this type of memory.

Introduction and Recap52223_01/9

Typical Structure of Main Memory

•

•

•

•

•••

Control Circuits

Main Memory

b bits

0

1

2

3

n-4

n-3

n-2

n-1

Address

Bus

Data Bus

Read

Write

CPU

Introduction and Recap52223_01/10

Memory Organisations

7 0

0

1

2

3

4

5

S 6

T 7

R 8

I 9

N 10

G 11

12

65533

65534

65535

Byte-addressable memory for an 8-bit processor…

Introduction and Recap52223_01/11

Memory Organisations - Little-Endian

Byte-addressable memory for a “Little-Endian” 16-bit processor such as the Intel 8086…

15 8 7 0

1 0

3 2

5 4

7 T S 6

9 I R 8

11 G N 10

65533 65532

65535 65534

Introduction and Recap52223_01/12

Memory Organisations - Big-Endian

Byte-addressable memory for a “Big-Endian” 16-bit processor such as the Motorola 68000…

15 8 7 0

0 1

2 3

4 5

6 S T 7

8 R I 9

10 N G 11

65532 65533

65534 65535

Introduction and Recap52223_01/13

CPU Structure and Function

The major structural components of a CPU are:– Control Unit: Controls the operation of the CPU– Arithmetic and Logic Unit: Performs the

computer’s data processing functions– Registers: Provides storage internal to the CPU– CPU Interconnection: Some means of

communication between the CU, ALU, and registers.

Introduction and Recap52223_01/14

…CPU Structure and Function

The function of the CPU is to:– Fetch Instructions– Interpret Instructions– Fetch data (if required)– Process data (if required)– Write data (if required)

Introduction and Recap52223_01/15

Register Organisation

The registers in the CPU serve two functions:– User-Visible Registers: These enable the

programmer to minimise external memory references by optimising usage of registers.

– Control and Status Registers: These enable the control unit to control the operation of the CPU.

Introduction and Recap52223_01/16

User-Visible Registers

These can be categorised as follows:

– General Purpose

– Data

– Address

– Condition Codes

In some m/cs General Purpose registers may either be used for Data or Address. In a completely orthogonal instruction set (IS) any GP register can contain the operand(s) for any opcode. Often, however, there are restrictions.

Introduction and Recap52223_01/17

Control and Status Registers

These, in general, are not visible to the user.

Four registers are essential to instruction execution:– Program Counter (PC): Contains the address of an instruction to be

fetched.

– Instruction Register (IR): Contains the instruction most recently fetched.

– Memory Address Register (MAR): Contains the address of a memory location.

– Memory Buffer Register (MBR): Contains a word to be written to memory or the word most recently read.

Typical CPU designs include a register often called the Program Status Word (PSW) that contains status information such as condition codes plus other status information.

Introduction and Recap52223_01/18

Instruction Types

Any program written in a high-level language (HLL) must be translated into machine language in order to be executed. Thus the set of m/c instructions must be sufficient to express any of the instructions from a HLL. Thus we can categorise the following types:

– Data Processing: Arithmetic & Logic– Data Storage: Memory instructions– Data Movement: I/O instructions– Control: Test & Branch instructions

Introduction and Recap52223_01/19

Instruction Repertoire - types of operation

The number of different opcodes varies widely from machine to machine. A typical categorisation is:– Data Transfer

– Arithmetic

– Logical

– Conversion

– I/O

– System Control

– Transfer of Control