Transcript Basic Computer

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.ht
m
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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
n-1
m+1
opcode
m
0
operand/address
2(n-1)-(m+1) opcodes
2(m+1) addresses
• Example – 16 bit instruction
15
12 11
opcode
operand/address
0
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
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–
–
–
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
Mode bit
Immediate
0 addc
Operand
Direct
3
0
0x33
add
Indirect
0x33
0x42
1
add
0x33
0x33
0x42
0x42
0x88
Operand
Operand
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
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–
–
–
–
–
–
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 unit
111
4096x16
ALU
address
AR
001
PC
010
DR
011
AC
100
IR
101
TR
110
E
INPR
OUTR
clock
16-bit Bus
S2 Access
S1
S0 Select