Making a Computer

1
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

2
Defining a Computer

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

3
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.

4
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

5
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
6
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

7
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 windows address field
• Notice that instructions are of varying lengths

8
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

9
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/manua
  ls/253665.htm

10
Basic Computer

• The following discussions are based on a
  fictitious computer called Basic Computer by
  the author of the textbook
• Its a much better way to learn computer
  architecture concepts than trying to understand
  the Intel Pentium architecture

11
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
• Its a way of thinking that requires intimate
  knowledge of the underlying hardware architecture

12
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)

13
Assembly Language Instructions

• n-bit instruction format
• Example 16 bit instruction

2(n-1)-(m1) opcodes
2(m1) addresses
24 16 opcodes
212 4096 addresses
14
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

15
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

16
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

17
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

18
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

19
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

20
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

21
Addressing Modes
I
opcode
address
Mode bit
Immediate
Direct
Indirect
0
addc
3
0
add
0x33
1
add
0x33
0x33
0x33
0x42
0x42
0x42
0x88
22
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

23
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)

24
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

25
Registers

• In designing a computer architecture the designer
  must specify the register set
• There are essentially two categories
• Special purpose registers
• General purpose registers

26
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

27
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

28
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

29
General Purpose Registers

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

30
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

31
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

32
Bus Architecture
S2
Access Select
Memory unit 4096x16
111
S1
S0
address
001
AR
010
PC
011
16-bit Bus
DR
E
ALU
100
AC
INPR
101
IR
110
TR
OUTR
clock