Introduction to x86
Computer Architecture
What is an x86 Processor
• An x86 Processor is now a general term used
to refer to a class of processors that share a
common subset of instructions
• Some x86 microprocessors are: Athlon (AMD),
Opteron (AMD), Pentium IV (Intel), Xeon (Intel),
Pentium Core Duo (Intel), Athlon X2 (AMD)
– Most popular desktop and sever processors
– Knowing instructions for one, you can program
for any other processor
• Some processors have additional instructions that
cannot be used on others!
x86 Philosophy on Instructions
• x86 Processors follow the Complex
Instruction Set Computer (CISC) Philosophy
• It is converse to the Reduced Instruction Set Computer
(RISC) Philosophy
– The data path we discussed in class falls under the category
of RISC
– x86 processors provide a lot of (relatively)
complex instructions
• When you look at the manuals the number of
instructions will strike you.
• Each instruction has its own peculiarity to watch out
for.
CISC vs. RISC
CISC
RISC
Larger number of instructions.
Fewer number of instructions.
Instructions have different
formats.
All instructions have the same
format.
Instructions vary in size. (some Instructions have the same
instructions are just 1 byte while
size.
others are 9 bytes long!)
Data path is very complex and Data path is simpler and
involves multiple clocks.
typically uses a single clock.
Code for software is smaller
Code for software is larger
(example you can multiply or divide
two registers)
(Have to write code for multiplication
or division!)
Puts burden on hardware
Puts burden on software.
CISC vs. RISC (Contd.)
CISC
RISC
Can extract more Instruction
Level Parallelism (ILP) –
complex instructions are
broken internally into smaller
pieces and run in parallel.
Complex instructions enable
better use of other parts of
CPU such as caches.
Compiler development is
harder.
Harder to extract ILP
Athlon, Pentium, Opteron,
Xeon.
ARM, MIPS, Ultra Sparc, Ultra
Sparc T1
Software developer may need
to handle some gritty details.
Streamlines compiler
development.
Microprogramming
• Microprogramming is a computer architecture
design strategy in which complex instructions
are actually implemented as a sequence of
micro (or smaller) instructions.
– Sequence of microinstructions is called a micro
program.
• The are carefully designed and optimized for fastest
execution.
• CPU is actually executing micro programs rather than
the main code and consequently has a RISC type
computing core!
– CISC architectures often use microprogramming.
Micro program Example
• Example of a micro program to perform
operation Add R1, R2 (R2 = R2 + R1)
– Select register R1 as 1st Input to ALU
– Select register R2 as 2nd Input to ALU
– Set carry input of ALU to zero
– Select ALU to perform 2’s complement addition
– Direct result from ALU to register R2
– Update flags as necessary
Micro program storage
• Micro programs need to be retrieved and
processed at high speed!
– They are stored directly on the CPU and are
hardwired to the data path.
• The storage location is called a control store.
– Micro programs are typically just bits that are
applied to inputs of various devices in the data
path.
• Like selection bits for multiplexers and de-multiplexers
Micro instruction implementations
• Microprogramming maybe implemented using
two different strategies
– Vertical micro codes
• One micro instruction at a time just like a conventional
program.
– Horizontal micro codes
• Several or all micro instructions are processed
simultaneously.
Vertical Micro Codes
• This is older/simple approach.
– In this strategy micro codes are executed one
after another in a serial fashion. This is type is
similar to conventional program execution (one
instruction at a time)
00110001
00000001
00001111
11110000
Sequence of short micro codes to
implement a macro instruction
(vertical organization)
Horizontal Micro Codes
• This is a newer and faster approach
• Adopted as transistors started becoming cheaper
– Micro codes are typically executed in parallel
making micro instructions much faster!
– The drawback is that microinstructions become
much wider as they have to accommodate all bits
(even if they are not used) in each instructions.
• All instructions have microinstructions of the same size
00110001 00000001 00001111 11110000
Horizontal organization of micro codes to make a single
microinstruction!
x86 Architecture
• Most x86 processors use little
microprogramming.
• Most instructions are directly executed by a complex
data path
• Some advanced instructions and looping instructions
use microprogramming
– The x86 manual typically provides the
microinstructions to explain the working of each
instruction
– You really don’t have to worry about all the
details when writing assembly language code for
a given processor.
Assembly programming with x86
• In order to do assembly programming with
x86 you need to know the following:
– Registers provided by x86 processors
– Instruction set
• Refer to x86 instruction set manuals available from
Intel.
– http://www.intel.com/design/pentium4/manuals/253666.htm
– http://www.intel.com/design/pentium4/manuals/253667.htm
– A text editor to type out your assembly program
– An assembler & linker to convert assembly to
binary
x86 Registers
• x86 registers are grouped into two categories
– General purpose registers
• Used for doing arithmetic and logic operations
• Used like temporary variables in programs
– Reserved (or specialized) registers
• These are used by the microprocessor internally for
performing certain operations
• The values in these registers are typically not modified
by programs
– They are set and handled by the operating system that loads
and runs your programs.
General purpose x86 Registers
• 4-basic 32-bit registers that can be reused as 16-bit
or 8-bit registers
• 32-bit registers are called: eax, ebx, ecx, and edx
• Low 16-bit parts of 32-bit registers are correspondingly called: ax,
bx, cx, and dx
• 8-bits parts are called: ah, al, bh, bl, ch, cl, dh, & dl
Bit positions
32
16 15
8 7
0
EAX
AH
AL
AX
EBX
BH
BL
BX
ECX
CH
CL
CX
EDX
Cannot use high 16bits independently!
DH
DL
DX
Names for
low 16-bits
Using parts of registers
• Note that registers eax, ax, ah, and al all
share the same register space!
– They are names for parts of the same register!
– Altering any one correspondingly alters other
registers as well!
• Example
Now AH = 1, AL = 1 but
EAX and AX = 257!
Change
Set EAX
AHtoto1 1
EAX
00
00
00
01
01
Other general purpose registers
• The x86 processor also provides other
general purpose registers.
– They cannot be accessed as bytes!
Register
Names for
low 16-bits
• Smallest unit is lower 16-bits
Bit positions
32
16 15
8 7
EBP (Base Pointer)
BP
ESI (Source Index)
SI
EDI (Destination Index)
DI
ESP (Stack Pointer)
Cannot use high 16bits independently!
SP
0
Segment Registers
• Segment registers (16-bits) are special registers
– 4 Special segment registers
• CS – Code Segment register
– Indicates an area of memory in which instructions are stored.
• DS – Data Segment register
– Indicates an area of memory in which R/W data is stored.
• SS – Stack Segment register
– Area of memory in which stack for a program is stored.
– Stack is used for calling methods/functions/subprograms
• ES – Extra Segment register
– Used for copying data from one segment to another.
– They are typically set by the operating system
– You will never change them (in this course)
– We will discuss segment registers further in the course
EFLAGS Register
• EFLAGS register is a special (32-bit) register
that contains flags (bits) generated from
results of ALU operations.
– Typically, the only way to change the values is to
perform an ALU operation.
• Usually a set of individual flags (or bits) are selectively
changed by each instruction.
• You have to know which instruction changes which bit!
– Initially it is hard but with some practice you will get the hang
of it.
• Each bit can be indirectly inspected using suitable
instruction
– Typically a conditional jump instruction!
Flags in EFLAGS
• Certain flags in EFLAGS are frequently used
– ZF: Zero Flag
• Set if output from ALU is zero. Cleared otherwise.
– CF: Carry Flag
• Set if arithmetic operation generates a carry or borrow.
• This flag indicates overflow for unsigned arithmetic.
– PF: Parity Flag
• Set if least significant byte (8-bits) of result from ALU
has an even number of 1s.
– SF: Sign Flag
• Set to most significant bit of the result (1 indicates
negative result while 0 indicates positive result)
Instruction Pointer (IP)
• It is a special 32-bit register that indicates the
address of the next instruction to be executed
by the microprocessor.
– The register is called EIP in x86 processors
– Value is typically initialized by the OS when
programs are loaded into memory.
– Changed by conditional or unconditional jumps
– By function/method calls
Reading the Manual
• Now that you know about registers, it is now
time to start exploring instructions supported
by x86 processors.
– For this task you have to refer to the Intel
manuals for details of instructions.
– First you need to understand the notations used
• I will explain some of the notation using an example.
• You have to learn other notations by reading the
manual.
The MOV instruction
• One of the most overloaded instruction in the
x86 manual for assignment operation
– Copy value from one location to another
• Location can be memory, register, or constant!
– Handles all possible combinations using different
op-codes
• Mnemonic is the same but op-codes are different!
– Refer to page 635 of Instruction Set reference
• http://download.intel.com/design/Pentium4/manuals/25
366620.pdf
Snippet from Manual
Opcode
Instruction
Description
88 /r
Mov r/m8, r8
Mov r8 to r/m8.
89 /r
Mov r/m16, r16
Mov r16 to r/m16.
89 /r
Mov r/m32, r32
Mov r32 to r/m32.
C6 /0
Mov r/m8, imm8
Mov imm8 to r/m8.
C7 /0
Mov r/m16, imm16 Mov imm16 to r/m16.
All you need to know is what do the
notations (r/m32 or imm8) actually
imply.
Understanding notation
• Review section 3.1.1.2 (Page 51) of the instruction
manual
• http://download.intel.com/design/Pentium4/manuals/25366620.pdf
– rN:A N-bit register (EAX etc. for 32-bit, AX etc. for 16-bit,
AL/AH etc. for 8-bit)
– immN: An N-bit immediate or constant value (represented
as unsigned or 2’s complement)
– mN: A N-bit memory address from where K-bytes are to
be stored. Value of K depends on size of value being R/W
– r/mN (r/m8, r/m16, r/m32) : A N-bit register or N-bit
memory address to read/write data to.
Manual to Assembly Conversion
• The assembler we are going to use uses a
different notation
– Instructions are written with destination registers
last
– Register names are prefixed in % sign
– Constant values are prefixed with $ sign
• Example: $31, $3.142
• Hexadecimal constants are written as $0x7F
– Hexadecimal constants cannot use fractions like $0x7F.A
• Octal constants have leading 0 as $071
• Binary constants have a ‘b’ at the end like $01110b
Example Translation
• Examples of various MOV instructions:
Comments
Suffix
Interpretation
Manual
Assembly
Mov r/m8, r8
Movb %al, %ah
AH = AL
b – byte
*
Movb k , %bl
BL=k, Loads 1 byte from k
w – word (2-bytes)
Movl %ebx,%eax
EAX = EBX
l – int/long (4-bytes)
Movl k, %eax
EAX=k, Loads 4 bytes!
Mov r/m32,
r32
Mov r/m32,
imm32
Movl $10, %eax
EAX = 10 (4 bytes!)
Movl $-20, %ebx
EBX = -20 (4 bytes!)
* where k is symbol (or variable) for a memory address.
Variables
• In assembly symbols (or variables) are typically
used to refer to addresses
– Variables have a type associated with them which defines
the following attributes:
• Size (in bytes): Most important in assembly
– Once defined you cannot change the size!
• Data type: Weak concept in assembly
– You can always reinterpret values in different ways
– Valid types are:
•
•
•
•
•
byte (1-byte)
word (2-bytes)
int or long (4-bytes)
float (4-bytes)
string (array of bytes)
Defining Variables
• Variables are defined by defining a symbol
– With a given type
– And a default initial value.
var1: .int -32
/* Java: int var1 = -32;
*/
var2: .byte 0
/* Java: byte var2 = 0;
*/
var3: .float -3.142 /* Java: float var3 = -3.142; */
Comments are delimited
by /* and */ character
sequences!
Putting it all together!
• The first almost complete assembly:
/* Program to swap values of variables var1 & var2 */
.text
/* Every thing below goes in Code Segment */
.global _start /* Global entry point into program */
_start:
movl
movl
movl
movl
.data
val1:
val2:
/* Indicate where program starts */
val1, %eax /* eax = val1 */
val2, %ebx /* ebx = val2 */
%eax, val2 /* val2 = eax */
%ebx, val1 /* val1 = ebx */
/* Everything below goes in data segment */
.int 10
.int 20
Problem with earlier code
• The previous assembly is actually valid
– It will compile, link, run do swapping of integers
– However, it does not terminate!
• How to make the program terminate?
• One way is to stop the microprocessor from
processing further instructions.
– Have to tell the OS to stop the program (or
process) from running further
• Need to interact with OS for this task.
OS Interactions
• Example code to stop a process (or program)
from running
– Invoke an OS routine or “system call”
.text
/* Every thing below goes in Code Segment */
Read as call Interrupt 80 hex! (Transfer
.global _start /* Global control
entry to
point
*/ Will
OS). into
This isprogram
OS specific!
Not work on Windows!
_start:
/* Indicate where program starts */
/* Rest of the program actually goes here */
movl $1, %eax /* Set eax=1, SysCall Code for exit */
movl $0, %ebx /* Exit code value set to zero */
int $0x80
/* Transfer control to OS */
.data
The Final Assembly
/* Program to swap values of variables var1 & var2 */
.text
/* Every thing below goes in Code Segment */
.global _start /* Global entry point into program */
_start:
movl
movl
movl
movl
/* Indicate where program starts */
val1, %eax /* eax = val1 */
val2, %ebx /* ebx = val2 */
%eax, val2 /* val2 = eax */
%ebx, val1 /* val1 = ebx */
movl $1, %eax
movl $0, %ebx
int $0x80
.data
val1:
val2:
/* Set eax=1, SysCall Code for exit */
/* Exit code value set to zero */
/* Transfer control to OS */
/* Everything below goes in data segment */
.int 10
.int 20
More Instructions
• I will cover some basic, core instructions in class.
– You are expected to review the instruction manuals to
obtain a comprehensive list of instructions!
Instruction
Result
addb $8, %AL
AL += 8;
addl %ebx, %eax
EAX += EBX
addl k, %eax
EAX += k
addl %eax, k
k += EAX
Add Instruction
• Add instruction performs integer addition
– It is applicable for both signed & unsigned numbers
– Sets OF, CF, and SF based on result of addition
Instruction
Result
addb $8, %AL
AL += 8;
addl %ebx, %eax
EAX += EBX
addl k, %eax
EAX += k
addl %eax, k
k += EAX
* Where k is an int variable
Sub Instruction
• Sub instruction performs integer subtraction
– It is applicable for both signed & unsigned numbers
– Sets OF, CF, and SF based on result of addition
Instruction
Result
subb $8, %AL
AL -= 8;
subl %ebx, %eax
EAX -= EBX
subl k, %eax
EAX -= k
subl %eax, k
k -= EAX
* Where k is an int variable
MUL Instruction
• MUL Instruction comes in 2 flavors
• Note: Multiplying 2 n-bit numbers generates a 2n-bit result
– Example: Multiplying 2 32-bit numbers generates a 64-bit result.
– MUL: Performs unsigned multiplication
– IMUL: Performs signed integer multiplication
– Source and destination registers are implied!
• Source: Uses AL, AX, or EAX depending on size of operand
• Destination: Uses DX:AX or EDX:EAX to store 32-bit or 64-bit result!
Instruction
Result
mulb %BL
AX = AL * BL
mull %ebx
imull k
EDX:EAX = EAX * EBX
EDX:EAX = EAX * K
* Where k is an int variable
MUL (Continued)
Operand Size
Operand 1
(Implicit)
Operand 2
Destination
Byte
AL
r/m8
AX
Word
AX
r/m16
DX:AX
Int
EAX
r/m32
EDX:EAX
• OF and CF are set to 0 if upper half of the
result is 0
– Values in SF, ZF, PF are undefined at the end of
the instruction!
DIV Instruction
• DIV Instruction comes in 2 flavors
– DIV: Performs unsigned division
– IDIV: Performs signed integer division
– Source and destination registers are implied!
• Source: Uses AL, AX, or DX:AX or EDX:EAX depending on size of divisor
• Destination: Uses AX or DX:AX or EDX:EAX to store remainder and
quotient respectively
Instruction
Result
divb %BL
AL=(AX/BL); AH=AX%BL
divl %ebx
EAX=(EDX:EAX / EBX);
EDX=(EDX:EAX % EBX)
divl k
EAX=(EDX:EAX / k);
EDX=(EDX:EAX % k)
* Where k is an int variable
DIV (Continued)
Operand Size
Dividend
Divisor
Quotient
Remainder
Word/Byte
AX
r/m8
AL
AH
Word/word
DX:AX
r/m16
AX
DX
Int/int
EDX:EAX
r/m32
EAX
EDX
• Values of CF, OF, SF, ZF, and PF are undefined!
• Before performing idiv use cdq instruction to sign
extend eax value to edx. Otherwise your division
will give incorrect results for negative numbers!