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Making a Computer

• Binary number system → Boolean functions• Boolean functions → Combinational circuits• Combinational circuits → Sequential circuits• Sequential/Combinational circuits →

Functional units• Functional units → Computer architecture

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Defining a Computer

• Computer Architecture– Bus– Registers– Register transfer language– Microoperations– Instruction set– Timing and control

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Instruction Set

• Instruction (Assembly language statement)– Binary code– Consists of an operation code and operand(s)– Specifies a sequence of microoperations to be

executed• One high level language (e.g. C++/Java) statement specifies

a sequence of instructions

– Stored in memory– Note that we are talking about a level higher than RTL

but lower than Java, C++, etc.

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Exercise

• Start Visual Studio .NET• Select “New Project”

– Visual C++ Projects• Win32 Console Application

– Type in a project name (e.g. AssemblyExample)– Browse to a directory (e.g. Desktop)– Press “OK”– Press “Finish”

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Exercise

• Modify your main function to look like this:

int _tmain(int argc, _TCHAR* argv[]){

int x, y;

x = 1;y = 2;x = x + y;

return 0;}

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Exercise

• Set a break-point on the “int x, y;” line by clicking on that line and pressing F9– A red circle should appear to the left of the

line

• Run the program by pressing F5 and answering “yes” to the question

• Open the Debug menu – Select Windows → Disassembly– Select Windows → Registers

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Exercise

• In the Watch1 window (lower left) select an empty line (turns blue) and type &x

• In the Watch1 window (lower left) select an empty line (turns blue) and type &y

• In the Memory1 window (select tab) enter the smaller of the two hexadecimal numbers next to the &x and &y into the Address field

• If you want to see the binary (hex) codes for the instructions type an instruction address (next to an assembly language instruction) into the Memory1 window’s address field– Notice that instructions are of varying lengths

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Exercise

• Note the value of the EIP register in the Registers window – it matches the value next to the yellow arrow at the left of the assembly code

• Press F10 and watch the Memory1 window• Press F10 again and watch the Memory1 window• Press F10 again and watch the EAX register• Press F10 again and watch the EAX register• Press F10 again and watch the Memory1 window

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Exercise

• What you witnessed was the modification of Pentium processor registers and memory as it executed individual assembly language instructions

• Note that the Pentium assembly language is extremely complex as it is a powerful processor

• If you want to learn more, manuals can be downloaded from

http://developer.intel.com/design/pentium4/manuals/253665.htm

Basic Computer

• The following discussions are based on a fictitious computer called “Basic Computer” by the author of the textbook

• It’s a much better way to learn computer architecture concepts than trying to understand the Intel Pentium architecture

Assembly Language

• Every computer architecture (or family of architectures) has its own unique assembly language

• Unlike Java, you should not learn assembly language syntax, data types, etc.

• You should learn to program/think at the assembly language level– It’s a way of thinking that requires intimate knowledge

of the underlying hardware architecture

Assembly Language Instructions

• Each instruction has two basic parts– Operation code (opcode)

• What the instruction wants the processor to do

– Operand(s) (registers, memory addresses)• Data location that the instruction wants the

processor to manipulated

• Some operands will be explicit while others will be implicit (implied by the opcode)

Assembly Language Instructions

• n-bit instruction format

• Example – 16 bit instruction

opcode operand/address0m+1 mn-1

opcode operand/address012 1115

2(n-1)-(m+1) opcodes

2(m+1) addresses

24 = 16 opcodes

212 =4096 addresses

Assembly Language Instructions

• Instructions within the same Assembly language may be of differing lengths– i.e. not all instructions utilize the same

number of bits as we saw with the Pentium

Internal Operation

• To execute an assembly language instruction the processor goes through 4 steps– Fetch an instruction from memory– Decode the instruction– Read the operands from memory/registers– Execute the instruction

• This is often referred to as the Fetch-Execute cycle or the Instruction cycle

• To execute a program the processor repeats this cycle until a halt instruction is reached

Internal Operation

• All this is under the control of the Control Unit• This is the component that decodes the

instruction and sends out microoperations to the rest of the hardware– The control unit can be hardwired

• Made up entirely of sequential circuits designed to do precisely the fetch-execute steps – fixed instruction set

– The control unit can be microprogrammed• A small programmable processor within the processor –

programmable instruction set

• More on this later

Addressing Modes

• In designing a computer architecture the designer must specify the instruction set– Opcode/operand pairs

• In specifying operands there are a number of alternatives– Immediate instructions– Direct address operands– Indirect address operands

Immediate Instruction

• The 2nd part of the instruction is the operand (rather than the address of the operand)

• An example might be an instruction that adds a constant to a register

add 3

– The “3” is the value we want to add, not an address in memory

Direct Address Instruction

• The 2nd part of the instruction is the memory address of operand

• An example might be an instruction that adds a value in memory to a register

add 0x30213

– The “0x30213” is the memory address of the value that we want to add

Indirect Address Instruction

• The 2nd part of the instruction is the memory address of the location that holds the memory address of the operand

• An example might be an instruction that adds a value in memory to a register

add 0x30213

– The “0x30213” is a memory address that holds the memory address of the value that we want to add

Addressing Modes

I opcode address

0 addc 3 0 add 0x33 1 add 0x33

0x420x33 0x420x33

0x880x42

Operand

Operand

Operand

Immediate Direct IndirectMode bit

Addressing Modes

• The term effective address refers to the actual address of the operand– For the previous example

• Immediate address mode– Effective address is the instruction itself

• Direct address mode– Effective address is the memory location 0x33

• Indirect addressing mode– Effective address is the memory location 0x42

Addressing Modes

• Something in the instruction word will specify which addressing mode is applicable– The operand itself (for immediate instructions)– A designated bit (for direct vs. indirect

address instructions)

Addressing Modes

• Indirect addressing is a convenient way to implement arrays (which are nothing more than pointers to blocks of contiguous memory)

• Some architectures define additional modes such as “read location then increment”– These are all derivations of the three defined

here

Registers

• In designing a computer architecture the designer must specify the register set

• There are essentially two categories– Special purpose registers– General purpose registers

Special Purpose Registers

• Program Counter (PC)– Holds the memory address of the next instruction of

our program

• Memory Address Register (AR)– Holds the address of a location in memory that we

want to access (read/write)

• The size of (number of bits in) these two registers is determined by the number of memory addresses in our architecture

Special Purpose Registers

• Instruction Register (IR)– Holds the instruction (opcode/operand) we are about

to execute

• Data Register (DR)– Holds the operand read from memory to be sent to

the ALU

• Accumulator (AC)– Holds an input to the ALU and the output from the

ALU

Special Purpose Registers

• Input Register (INPR)– Holds data received from a specified external

device

• Output Register (OUTR)– Holds data to be sent to a specified external

device

General Purpose Registers

• Temporary Register (TR)– For general usage either by our program or

the architecture

Registers• These registers (shown previously) are specified for the fictitious

architecture given in the textbook• All architectures will have these in some form• Most architectures will have more than just these

– More general purpose registers– Stack pointers– Interrupts– Program status bits– Multiple I/O ports– Timers– etc.

• To effectively program the architecture (in assembly language) you need to be aware of all the available registers and their usage

• High level language compilers possess this knowledge

Bus

• In designing a computer architecture the designer must specify the bus layout– The size of the bus (in bits)– What is connected to the bus– Access control to the bus

• Recall that a bus is an efficient alternative to lots of wires when it comes to transferring data between registers, control units, and memory locations

Bus Architecture

Memory unit4096x16

AR

PC

DR

AC

INPR

IR

TR

OUTR

ALUE

16-bit Bus

address

clock

S2S1S0

111

001

010

011

100

101

110

AccessSelect