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CSE 2421Autumn 2013

S. Preston

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Y86 programmer-visible state

• The Y86 has: • 8 32-bit registers with the same names as the IA32 32-bit

registers • 3 condition codes: ZF, SF, OF

• no carry flag - interpret integers as signed• a program counter (PC)

• Holds the address of the instruction currently being executed• a program status byte: AOK, HLT, ADR, INS

• State of program execution• memory: up to 4 GB to hold program and data (4096 = 2^12)

%eax

%ecx

%edx

%ebx

%esi

%edi

%esp

%ebp

RF: Program registers

ZF SF OF

CC: Condition codes

PCDMEM: Memory

Stat: Program Status

Condition Codes

• 3 condition codes in y86• ZF – Set if the result of the last arithmetic

operation is 0• SF – Set if the result of the last arithmetic

operation resulted in the sign bit being set• OF – Set if the result of the last arithmetic

operation resulted in an overflow

Conditions

• FLAGS: Zero, Sign, Overflow• Less or Equal

• Z = 1, S = 1, O = X• Less

• Z = 0, S = 1, O = X• Equal

• Z = 1, S = 0, O = X• Not Equal

• Z = 0, S = X, O = X• Greater or Equal

• Z = 1, S = 0, O = X• Greater

• Z = 0, S = 0, O = X

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Simple Addressing Modes• Normal = (R) = Mem[Reg[R]]

• Register Reg specifies memory address • denoted by a register in ( )• Example

ecx = 0x00000120

movl (%ecx),%eax

move the value that is at the address in ecx into eaxMoves 0x11223344 into eax

addr value

0x120

0x11

0x121

0x22

0x122

0x33

0x123

0x44

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Simple Addressing Modes

• Displacement = D(R) = Mem[Reg[R]+D]• Register R specifies start of memory address• Constant displacement D specifies offset

• In bytes•Denoted by displacement(register)ebp = 0x120 movl 2(%ebp),%edxmove the value at ebp (0x120) + 2 into edx

loads 0x33445566 into edx

addr value

0x120

0x11

0x121

0x22

0x122

0x33

0x123

0x44

0x124

0x55

0x125

0x66

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Y86 example program w/ loop

# y86loop.ys.pos 0x0 irmovl $0,%eax # sum = 0 irmovl $1,%ecx # num = 1Loop: addl %ecx,%eax # sum += num irmovl $1,%edx # tmp = 1 addl %edx,%ecx # num++ irmovl $1000,%edx # lim = 1000 subl %ecx,%edx # if lim - num >= 0 jge Loop # loop again halt

CONVERT TO Csum=0;num = 1;do { sum += num; num++;}while (1000 – num >= 0)

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Y86 example program w/ loop

# y86loop.ys.pos 0x0 irmovl $0,%eax # sum = 0 irmovl $1,%ecx # num = 1Loop: addl %ecx,%eax # sum += num irmovl $1,%edx # tmp = 1 addl %edx,%ecx # num++ irmovl $1000,%edx # lim = 1000 subl %ecx,%edx # if lim - num >= 0 jge Loop # loop again halt

Why don’t we just do addl $1, %edx??

Cant! not allowed, only register to register

CONVERT TO Csum=0;num = 1;do { sum += num; num++;}while (1000 – num >= 0)

Y86 Stack

• Stack top address always held in esp

• Stack grows towards lower addresses

%esp

•

•

•

IncreasingAddresses

Stack “Bottom”

Stack “Top”

Y86 Stack - Push

• Pushing• Decrement the stack register by

4 then store new data

//(1)esp = 0x120movl 0xFECA8712, %eaxpush %eax//(1)esp = 0x11C

addr value

0x11B

0x11C

0x11D

0x11E

0x11F

0x120

0x11

0x121

0x22

0x122

0x33

(1) (2)

addr value

0x11B

0x11C

0x12

0x11D

0x87

0x11E

0xCA

0x11F

0xFE

0x120

0x11

0x121

0x22

0x122

0x33

Y86 Stack - Pop

• Pushing• Save new data on stack,

Increment the stack register by 4

//(1) esp = 0x11Cpop eax//(2) esp = 0x120//eax = 0xFECA8712

addr value

0x11B

0x11C

0x12

0x11D

0x87

0x11E

0xCA

0x11F

0xFE

0x120

0x11

0x121

0x22

0x122

0x33

(1) (2)

addr value

0x11B

0x11C

0x11D

0x11E

0x11F

0x120

0x11

0x121

0x22

0x122

0x33

Stack Example 2

==PUSH==subl $4, %espmovl %ebp, (%esp)

==POP===movl (%esp), %eaxaddl $4, %esp

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Y86 Instruction Set

Fetch Cycle

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Executing rmmovl

• Fetch• Read 6 bytes

• Decode• Read operand

registers• Execute

• Compute effective address

• Memory• Write to memory

• Write back• Do nothing

• PC Update• Increment PC by 6

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Executing Arithmetic/Logical Ops


• Decode• Read operand registers

• Execute• Perform operation• Set condition codes

• Memory• Do nothing

• Write back• Update register

• PC Update• Increment PC by 2

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Executing popl


• Decode• Read stack pointer

• Execute• Increment stack

pointer by 4• Memory

• Read from old stack pointer

• Write back• Update stack pointer• Write result to

register• PC Update

• Increment PC by 2

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Jumps

• Fetch• Read 5 bytes• Increment PC by 5

• Decode• Do nothing

• Execute• Determine whether to take

branch based on jump condition and condition codes

• Memory• Do nothing

• Write back• Do nothing

• PC Update• Set PC to Dest if branch

taken or to incremented PC if not branch

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Executing Call



• Execute• Decrement stack pointer

by 4• Memory

• Write incremented PC to new value of stack pointer

• Write back• Update stack pointer

• PC Update• Set PC to Dest

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Executing ret



• Execute• Decrement stack pointer

by 4• Memory

• Write incremented PC to new value of stack pointer

• Write back• Update stack pointer

• PC Update• Set PC to Dest

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Instruction encoding practice

• Determine the byte encoding of the following Y86 instruction sequence given “.pos 0x100” specifies the starting address of the object code to be 0x100 (practice problem 4.1)

.pos 0x100 # start code at address 0x100 irmovl $15, %ebx #load 15 into %ebx rrmovl %ebx, %ecx #copy 15 to %ecxloop:

rmmovl %ecx, -3(%ebx)#save %ecx at addr 15-3=12

addl %ebx, %ecx #increment %ecx by 15 jmp loop # goto loop


0x100: 30f30f000000 //irmovl $15, %ebxrrmovl %ebx, %ecxrmmovl %ecx, -3(%ebx)loop:addl %ebx, %ecxjmp loop


0x100: 30f30f000000 //irmovl $15, %ebx0x106: 2031 //rrmovl %ebx, %ecxloop:rmmovl %ecx, -3(%ebx)addl %ebx, %ecxjmp loop


0x100: 30f30f000000 //irmovl $15, %ebx0x106: 2031 //rrmovl %ebx, %ecxloop:0x108: 4013fdffffff //rmmovl %ecx, -3(%ebx)addl %ebx, %ecxjmp loop


0x100: 30f30f000000 //irmovl $15, %ebx0x106: 2031 //rrmovl %ebx, %ecxloop:0x108: 4013fdffffff //rmmovl %ecx, -3(%ebx)0x10e: 6031 //addl %ebx, %ecxjmp loop


0x100: 30f30f000000 //irmovl $15, %ebx0x106: 2031 //rrmovl %ebx, %ecxloop:0x108: 4013fdffffff //rmmovl %ecx, -3(%ebx)0x10e: 6031 //addl %ebx, %ecx0x110: 708010000 //jmp loop

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Instruction encoding practice (cont)

What about disassembling?

0x200: a06f80080200000030f30a000000900x400: 6113730004000000

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0x200: a06f push %esi0x202: 80080200000030f30a000000900x400: 6113730004000000

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0x200: a06f pop %esi0x202: 8008020000 call 0x2080x208: 30f30a000000900x400: 6113730004000000

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0x200: a06f pop %esi0x202: 800802000000 call 0x2080x208: 30f30a000000 irmovl $a0, %ebx0x20F: 900x400: 6113730004000000

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0x200: a06f pop %esi0x202: 800802000000 call 0x2080x208: 30f30a000000 irmovl $a0, %ebx0x20F: 90 ret0x400: 6113730004000000

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0x200: a06f pop %esi0x202: 800802000000 call 0x2080x208: 30f30a000000 irmovl $a0, %ebx0x20F: 90 ret0x400: 6113 subl %ecx, %ebx0x402: 730004000000

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0x200: a06f pop %esi0x202: 800802000000 call 0x2080x208: 30f30a000000 irmovl $a0, %ebx0x20F: 90 ret0x400: 6113 subl %ecx, %ebx0x402: 73004000000 je 0x400

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Y86

int Sum(int *Start, int Count){ int sum = 0; while (Count) {

sum += *Start;Start++;Count--;

}}

x86

• 8 32 bit registers• General Purpose Registers

• eax• ebx• ecx• edx• esi• edi

• Stack Register• esp

• Base Register• ebp

x86 - Registers

x86 Registers

• eax, ebx, ecx, edx• Registers can be accessed in parts• Rrx – Referes to the 64 bit register• erx – Refers to 32 bit register• rx – Referes to the lower 16 bits of erx• rh – Refers to the top 8 bits of the rx bit

register• rl – Refers to the lower 8 bits of the rx

register

Condition Flags

• CF – Carry – Last arithmetic resulted in a carry

• PF – Parity – Last arithmetic/logical operation results in even parity (even number of 1’s)

• ZF – Zero – Last arithmetic/logical operation resulted in a zero

• SF – Sign – Last arithmetic operation resulted in the sign bit being set

• OF – Overflow – Last arithmetic resulted in an overflow

Condition Flags

• AF – Adjust – Last arithmetic results in a carry out of the lowest 4 bits (Used for BCD arithmetic)

• TF – Trap – Enables CPU single step mode – Used for debugging

• IF – Interrupt Enable – Enables cpu to handle system interrupts

• DF – Direction – Sets the direction of string processing from R->L

Verbiage

• Opcode operand1, operand2

• operand1 and/or operand2 are not always required, depending on the opcode

• mov eax, ebx• Opcode – mov• operand1 – eax• operand2 - ebx

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Operand Specifiers

• Source operand• Constants, registers, or memory

• Destination operand• Registers or memory

• Cannot do Memory-Memory transfer with a single instruction

• 3 types of operands• Immediate• Register • Memory

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IA32 – Intel Architecture

• 32-bit address bus• normal physical address space of 4 GBytes (232

bytes)• addresses ranging continuously from 0 to

0xFFFFFFFF• Data formats

• Primitive data types of C• Single letter suffix

• denotes size of operand• No aggregate types

• Arrays, structures

C Declaration Suffix Size

char B 8 bits

short W or S 16 bits

int L 32 bits

* (pointer) L 32 bits

float S 32 bits

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Addressing Modes

• An addressing mode is a mechanism for specifying an address.

• Immediate • Register• Memory

• Absolute• specify the address of the data

• Indirect• use register to calculate address

• Base + displacement• use register plus absolute address to calculate address

• Indexed• Indexed

• Add contents of an index register• Scaled index

• Add contents of an index register scaled by a constant

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Operand addressing exampleOperand ValueComment

%eax0x100 Register

0x104 0xAB Absolute Address - memory

$0x1080x108 Immediate

(%eax) 0xFF Address 0x100 - indirect

4(%eax) 0XAB

Address 0x104 - base+displacement(4+register)

9(%eax,%edx) 0X11Address 0x10C – indexed(9 + eax+edx)

0x104(%ecx,%edx) 0X13

Address 0x108 – indexed(0x104+ecx+edx)

0xFC(,%ecx,4) 0XFFAddress 0x100 - scaled index(0xfc+0+ecx*4)

(%eax,%edx,4) 0X11Address 0x10C - scaled index(eax+edx*4)*scaled index multiplies the 2nd argument by the scaled value (the 3rd argument)

which must be a value of 1, 2, 4 or 8 (sizes of the primitive data types)

Address

Value

0x100 0xFF

0x104 0xAB

0x108 0x13

0x10C 0x11

Register

Value

ax 0x100

cx 0x01

dx 0x03

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Operand Combinations example

Source Dest Src,Dest* C analog

Immediate Register movl $0x4, %eax temp = 0x4;

Immediate Memory movl $-147, (%eax) *p = -147;

Register Register movl %eax, %edx temp2 = temp1;

Register Memory movl %eax, (%edx) *p = temp;

Memory Register movl (%eax), %edx temp = *p;

• Each statement should be viewed separately.• REMINDER: cannot do memory-memory transfer with a single instruction.• The parentheses around the register tell the assembler to use the register

as a pointer.

Size Directive

• Typically you can infer the size being operated on by which variation of the register is in use

• mov (ebx), %eax• Destination EAX, implies moving 4 bytes

• mov (ebx), %ax• Destination AX implies moving only 2 bytes

• mov (ebx), %ah• Destination ah implies moving only 1 bytes

Size Directive

• Sometimes it is unclear though. • mov $2, (%ebx)• How many bytes do you move into memory

@ address ebx. 2? 4? 8?• Have to explicitly specify size when

dealing with immediate values• movw $2, (%ebx)

• Explicitly move 2 bytes• movl $2, (%ebx)

• Explicitly move 4 bytes

C Declaration Suffix Size

char b 8 bits

short w or s 16 bits

int l 32 bits

* (pointer) l 32 bits

float s 32 bits

long q 64 bits

x86 Instructions

• Three categories• Data Movement• Arithmetic/Logic• Control-Flow

• We will not cover ALL x86 instructions, there are numerous obscure ones

• We will cover all of the common instructions• For full list of operands see Intel’s

instruction set reference

x86 Instructions

<reg32> Any 32-bit register (EAX, EBX, ECX, EDX, ESI, EDI, ESP, or

EBP) <reg16> Any 16-bit register (AX, BX, CX, or

DX) <reg8> Any 8-bit register (AH, BH, CH, DH,

AL, BL, CL, or DL) <reg> Any register <mem> A memory address (e.g., [eax], [var +

4], or dword ptr [eax+ebx]) <con32> Any 32-bit constant <con16> Any 16-bit constant <con8> Any 8-bit constant <con> Any 8-, 16-, or 32-bit constant

Data Movement

• mov source, destination• Move data from source to destination• Syntax

mov <reg>,<reg>mov <reg>,<mem>mov <mem>,<reg>mov <const>, <reg>mov <const>, <mem>

• Examplesmov %eax, %ebx — copy the value in eax into ebx

Data Movement

• push• add 4 bytes on top of the stack• Syntax

push <reg32>push <mem>push <con32>

• Examplespush %eax — push eax on the stack

Data Movement

• pop• remove top 4 byte value from stack• Syntax

pop <reg32>pop <mem>

• Examplespop %eax — pop off stack into eax

Data Movement

• lea – Load Effective Address• loads the address of the source into the

registers in the second operand• Syntax

lea <mem>, <reg32>• Exampleslea (var), %eax — address of var is placed into eax

Arithmetic and Logic• add• adds together the two operands and stores result

in the second operand• Syntax

add <reg>,<reg>add <reg>,<mem>add <mem>,<reg>add <imm>,<reg>add <imm>,<mem>Examplesadd $10, %eax — add 10 to the current value in eax, and store result in eax

Arithmetic and Logic• sub• subtracts the two operands and stores result in the

second operand• Syntax

sub <reg>,<reg>sub <reg>,<mem>sub <mem>,<reg>sub <imm>,<reg>sub <imm>,<mem>Examplessub $10, %eax — subtracts 10 from the current value in eax, and store result in eax

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Technology

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