The document provides an overview of x86 CPU architecture, covering components such as the control unit, arithmetic logic unit, and memory interaction. It outlines different CPU operation modes, general-purpose and segment registers, as well as specific flags in the eflags register. The discussion also touches on the x86-64 bit processors and their features, concluding with an encouragement to explore how basic applications interact with hardware.

1. x86 CPU ARCHITECTURE
SANGRAM KESARI RAY <SHANKAR.RAY030@GMAIL.COM>

2. CPU DESIGN
• C LO C K / / S Y N C H R O N I Z AT I O N F O R O P E R AT I O N S – R E A D I N G F R O M M E M O RY, D O I N G A R I T H M E T I C
I N A L U, E TC .
• C O N T R O L U N I T
• A R I T H M E T I C LO G I C U N I T
• B U S ( PA R A L L E L W I R E S A C R O SS T H E C O M P O N E N TS, E TC H E D O N T H E M OT H E R B O A R D / / B E T W E E N
C P U A N D M E M O RY, E TC .)
• I N S T R U C T I O N E X E C UT I O N :
F E TC H ( I N S T R U C T I O N F R O M M E M O RY - >
D E C O D E ( F R O M B I N A RY B I T PATT E R N ) - >
E X E C UT E ( U P D AT E F LA G S ) - >
S TO R E ( I F A N Y O U T P U T O P E R A N D )

3. PROGRAM EXECUTION
• OS LOADS A PROGRAM INTO MEMORY
• OS EXECUTES THE PROGRAM’ S FIRST INSTRUCTION (ENTRY
POINT)
• FROM HERE THE PROGRAM RUNS BY ITSELF
• WHEN THE PROGRAM ENDS THE OS REMOVES IT FROM
MEMORY

4. CPU OPERATION MODES
• PROTECTED MODE //NO DIRECT ACCESS TO SYSTEM
MEMORY, EACH PROGRAM HAS 2 GB ADDRESS SPACE
• REAL-ADDRESS MODE //DIRECT ACCESS TO SYSTEM
MEMORY, EACH PROGRAM HAS 1 MB ADDRESS SPACE
• VIRTUAL-8086 MODE //CPU SWITCHING FROM PROTECTED
TO REAL MODE
• SYSTEM MANAGEMENT MODE //USED BY SYSTEM FIRMWARE

5. REGISTERS
• GENERAL PURPOSE REGISTERS –
E A X / / A C C U M U L A T O R R E G I S T E R - H O L D S D A T A
E C X / / C O U N T E R R E G I S T E R - L O O P C O U N T E R
E D X / / D A T A R E G I S T E R - H O L D S D A T A
E B X / / B A S E R E G I S T E R - P O I N T E R T O D A T A
E S I / / S O U R C E I N D E X - P O I N T E R T O S O U R C E S T R E A M
E D I / / D E S T I N A T I O N I N D E X - P O I N T E R T O D E S T I N A T I O N S T R E A M
E B P / / B A S E P O I N T E R - P O I N T E R T O B A S E O F T H E S T A C K
E S P / / S T A C K P O I N T E R - P O I N T E R T O T O P O F T H E S T A C K

6. CONT.
• SEGMENT REGISTERS –
C S / / C O D E S E G M E N T - P O I N T E R T O S T A R T I N G A D D R E S S O F P R O G R A M
D S / / D A T A S E G M E N T - P O I N T E R T O S T A R T I N G A D D R E S S O F P R O G R A M D A T A
S S / / S T A C K S E G M E N T - D E F I N E S S T A C K A R E A
E S / / E X T R A S E G M E N T - P O I N T E R T O D E S T I N A T I O N
F S / / G E N E R A L P U R P O S E S E G M E N T R E G I S T E R
G S / / G E N E R A L P U R P O S E S E G M E N T R E G I S T E R

7. INSTRUCTION POINTER
• EIP //POINTER TO NEXT INSTRUCTION
NOTE: SYSTEM SECURITY IS ALL AB OUT CONTROLLING THE EIP REGISTER – DIRECTLY/IN -
DIRECTLY TO CAUSE ABNORMAL CODE EXECUTION.

8. EFLAGS
• A 32-BIT REGISTER STORES RESULTS OF A CERTAIN OPERATION IN THE C PU
0-BIT //CARRY FLAG -IF AN ARITHMETIC OPERATION CAUSED CARRY/B ORR OW
2-BIT //PARITY FLAG -SET IF NUMBER OF BITS IN LSB IS EVEN
4-BIT //ADJUST FLAG -IF A CARRY OCCURS FROM LOWER NIBBLE
6-BIT //ZERO FLAG -SET IF RESULT OF AN OPERATION IS ZERO
7-BIT //SIGN FLAG -SET IF RESULT OF AN OPERATION IS NEGATIVE
8-BIT //TRAP FLAG -SET IF STEP -BY-STEP DEBUGGING

9. CONT.
9 - B I T / / I N T E R R U P T I O N F L A G - S E T I F I N T E R R U P T S A R E E N A B L E D
1 0 - B I T / / D I R E C T I O N F L A G - I F S E T R E A D I N G M E M O R Y B A C K WA R D S
1 1 - B I T / / O V E R F L O W F L A G - I F A N O U T P U T I S L A R G E R F O R A R E G I S T E R TO H O L D
1 2 - 1 3 - B I T / / I / O P R I V I L E G E L E V E L F I E L D S - I / O P R I V I L E G E L E V E L O F C U R R E N T P R O C E S S
1 4 - B I T / / N E S T E D TA S K F L A G - S E T I F C U R R E N T P R O C E S S I S L I N K E D TO T H E N E X T P R O C E S S
1 6 - B I T / / R E S U M E F L A G - I F S E T, R E S P O N D TO D E B U G E X C E P T I O N S
1 7 - B I T / / V I RT U A L- 8 0 8 6 M O D E - S E T I F I N 8 0 8 6 C O M PA B I T I B I L I TY M O D E
1 8 - B I T / / A L I G N M E N T- F L A G - S E T I F C P U D E T E C T S A N U N -A L I G N E D M E M O R Y O P E R A N D , S TO R I N G
A W O R D I N A N O D D - B Y T E A D D R E S S

10. CONT.
19-BIT //VIRTUAL INTERRUPT FLAG - SET AF TER HANDLING A PENDING I NTERRUPT
20-BIT //VIRTUAL INTERRUPT PENDING FLAG - SET IF AN INTERRUPT IS PENDING
21-BIT //IDENTIFICATION FLAG -IF SET, CPU SUPPORT CUPID INSTRUCT ION
ALL OTHER BITS ARE RESERVED FOR CPU

11. MMX REGISTERS
• 8 64 -BIT REGISTERS //THEY ARE LIKE GENERAL PURPOSE REGISTERS
• MM0, MM1, MM2, MM3, MM4, MM5, MM6, MM7

12. XMM REGISTERS
• 8 128 -BIT REGISTERS //THEY ARE LIKE GENERAL PURPOSE REGISTERS
• XMM0, XMM1, XMM2, XMM3, XMM4, XMM5, XMM6, XMM7

13. FLOATING-POINT UNIT
• T O B E U S E D F O R F L O A T I N G - P O I N T A R I T H M E T I C
• 8 8 0 - B I T D A T A - R E G I S T E R S - S T 0 , S T 1 , S T 2 , S T 3 , S T 4 , S T 5 , S T 6 , S T 7
• 2 4 8 - B I T P O I N T E R R E G I S T E R S -
F I P ( F P U I N S T R U C T I O N P O I N T E R ) ,
F S C ( F P U D A T A P O I N T E R )
• 3 1 6 - B I T C O N T R O L R E G I S T E R S -
F T A G ( T A G R E G I S T E R )
F C T R L ( C O N T R O L R E G I S T E R )
F S T A T ( S T A T U S R E G I T E R )
• 1 1 1 - B I T O P C O D E R E G I S T E R / / S T O R E S O P C O D E O F T H E L A S T N O N - C O N T R O L I N S T R U C T I O N , L I K E A D D I T I O N , E T C

14. X86-64 BIT PROCESSORS
• 64-BIT LONG ADDRESSES
• CURRENTLY ONLY LOWER 48 -BITS ARE USED
• HAS TWO MODES – COMPATIBILITY MODE TO RUN 16/32 -BIT OPERANDS AND 64- BIT MODE
TO RUN 64 - BIT OPERANDS
• 16 64 -BIT GENERAL PURPOSE REGISTER- RAX, RBX, RCX, RDX, RDI , RSI , RBP, RSP, R8 -R15
• 64-BIT RFLAGS REGISTER //UPPER 32 -BITS ARE NOT USED, FLAGS WORK SAME AS 32 -BIT

15. HOW
COMPUTER
WORKS
Q > I encourage the reading to figure
out how typing something in notepad
actually gets displayed on the screen?
BRAVE ARE THOSE WHO COULD MAKE
IT THIS FAR! Next time when we meet,
we’ll start with Assembly
programming using Visual Studio and
MASM Assembler. Bye!
APPLICATION
OS
BIOS
HARDWARE