CS33: Introduction to Computer Organization - PowerPoint Presentation

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CS33: Introduction to Computer OrganizationWeek 2 – Discussion Section  

Atefeh Sohrabizadeh

atefehsz@cs.ucla.edu

10/11/19

Agendamov Quizzes

Control Flags and Conditional Operations

Stack

Procedure Call

Passing Argument

BombLab

Notes

mov Quizzesmovl

%

eax

, %bx

Invalid: can’t move 32 bit to a 16-bit container

movl %ax, %ebxInvalid: can’t move 32 bit from a16-bit containermovb %al, %ebxInvalid: movb wants to move a byte, but dst is a 32-bit containermovb %al, %bl%rbx = 0x89abcdefmovzbl %al, %ebx %rbx = 0x000000efmovsbl %al, %ebx%ebx = 0xffffffefmovsbq %al, %ebxInvalid: moving 64-bit to 32-bit container

rax

0x123456ef

rbx

0x89abcd78

Additional notesThe size of the register operands must match the size of prefix designated by the instruction

Memory references can work with all sizes:

E.g.:

movb

%al, (%

rbx)What about movb %al, (%ebx)?Not meaningful on a 64-bit machine, since you are giving a 32 bit address

Control FlagsSingle bit values that change with some instructions

SF (Sign Flag)

Is set if the

most recent

operation set MSB to 1

ZF (Zero Flag)Is set if the most recent operation produce zero (or is zero)Usually used for checking equality or if the result is zeroCF (Carry Flag)Is set if the operation has carry outUsually used for unsigned overflow (or comparison)OF (Overflow Flag)Is set if (a>0 && b>0 && t<0) || (a<0 && b<0 && t>=0)Usually used for signed overflow (or comparison)What bit will “addq %rax, %rbx” set if there is an overflow?Both CF and OF. Then based on programmer’s intention, it will interpret the flags

Control Flags (Cont.)Some instructions only set the flags:

cmp

a, b : compute b-a and set the flags accordingly

testq

a, b : compute

a&b and set the flags accordinglyNote that it doesn’t change the value of a or bControl flags usage:Set value of registers using setX family:E.g.: sete %al → sets %al to 1 if ZF=1Use for conditional operations:Conditional moveE.g.: cmove src, dst → dst=src if ZF=1Conditional jumpsE.g.: je L1 → jump to label L1 if ZF=1

Notes on jump operationsjmp

L1

Jump to the instruction labeled as L1

jmp

0x400020

Jump to the instruction at address 0x400020jmp *%raxJump to the instruction that its address is in raxjmp *(%rax)Jump to the instruction that its address is in memory location %raxThe address of the memory location is the value in raxYou can define the address in D(Rb, Ri, S) format

StackA storage for local variables

It grows downwards

%

rsp

contains lowest stack address

Use push and pop to modify itpushq srcDecrement %rsp by 8Write operand src at address given by %rsppopq dstStore value addressed by

%

rsp

in

dst

Increment

%

rsp

by 8

Increasing

Addresses

Stack “Top”

Stack “Bottom”

Stack Pointer

:

%

rsp

Procedure Call%rip stores the instruction pointer

%

rsp

stores the top of current stack frame

%

rbp stores the base of current stack frameCalling the procedure: (callq label)Push return address to stackReturn address is the instruction pointer to the next instruction after function callJump to labelReturning from the procedure: (retq)Pop return address from stackJump to the return address

Passing ArgumentsThe main procedure passes arguments to the called procedure:

For the first 6

args

: %

rdi

, %rsi, %rdx, %rcx, %r8, %r9 respectivelyThe rest of the args are pushed to stackArg 7 is at top of stackThe returned value is at:%raxAfter passing arguments, the procedure call happensThe called procedure can modify these registers

Saving ValuesWhen P calls Q:

Q is responsible for saving registers %

rbx

, %

rbp

, and %r12-%r15Q must ensure these registers have the same value when they return to PEither by not modifying them Or pushing their value to stack before modification and pop them before returningP is responsible for saving all other registers except %rspAs Q may change the rest of the registers, P should push the value of the registers it is using to stack before making the call

BombLab NotesDon’t start in TUI mode as it may show garbled output

Start GDB with “$

gdb

bomb”

Run “layout

asm” in gdb to enter TUI mode and see assembly codeDon’t use “run” command in TUI mode as it may show garbled outputCheckout GDB section in the course websiteLocated at: https://polyarch.github.io/cs33/06-resources/Start early!

Useful Commands When Debugging in AssemblyUse

si

instead of s

stepi

(or

si) execute one machine instruction (follows a call)step (or s) execute one C-program statement (steps into functions)Use ni instead of nInfo register (or “i r”)Shows the value of registersi r $rax: show only value of raxOr just use: print $raxuntil *0x400e58Continue execution until instruction at address 0x400e58x/nfu addressPrints value at the specified memory addressn: how many units to printf: display format (x, d, u, o, t, a, c, f, s)u: unit (b, h, w, g)x/4xw 0x84200: prints the four words starting at address 0x84200 in hex formatCheck week 0 slides and page 280 of the book for more