CSC231-Assembly · 12 00000000 48656C6C6F20746865- Hello db "Hello there!", 10, 10 13 00000009 7265210A0A 14 HelloLen equ $-Hello ... Two Tips for Assembly Language Programmers

mith College

Computer Science

Dominique Thiébaut [email protected]

CSC231-AssemblyWeek #4 Fall 2019

D. Thiebaut, Computer Science, Smith College

Logistics

• Midterm Exam

• Oct 21st.

• In class.

• On paper.

• Closed books, closed notes, closed computers.


Outline• More Hexadecimal Notation

• Creating a listing with opcodes

• Seg Faults

• Hexdump

• mov & add

• Registers


Remem

ber…

Nerd Alert!Nerd Alert!Remem

ber…


11 section .data 12 00000000 48656C6C6F20746865- Hello db "Hello there!", 10, 10 13 00000009 7265210A0A 14 HelloLen equ $-Hello 15 16 section .text 17 global _start 18 _start: 19 20 ;;; print message 21 00000000 B804000000 mov eax, 4 ; write 22 00000005 BB01000000 mov ebx, 1 ; stdout 23 0000000A B9[00000000] mov ecx, Hello ; 24 0000000F BA0E000000 mov edx, HelloLen ; 25 00000014 CD80 int 0x80 26 27 ;;; exit 28 00000016 BB00000000 mov ebx, 0 29 0000001B B801000000 mov eax, 1 30 00000020 CD80 int 0x80


Two Tips forAssembly Language Programmers

• Seeing invisible characters

• Segmentation faults


Seeing Invisible Characters







Two Tips forAssembly Language Programmers

• Seeing invisible characters

• Segmentation faults


segFault


segFault


Segments

RAM

Code

DataAd

dres

ses


Hexdump


Visualizing Bytes in RAM(Almost!)

emacs hello.asm

nasm -f elf hello.asmld -melf_i386 hello. -o hello./hello

nasm -f elf hello.asmld -melf_i386 hello. -o hellohexdump -v -C hello


Hexdump




Our Goal forThis Weekint x, y, sum;

x = 3;y = 5;sum = x + y;


Sub-Outline

• mov & add instructions

• Registers

• Memory storage directives


You already knowsome of this material…

• https://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html

https://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html

https://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html


The mov instruction

mov dest, source


The mov instruction

mov dest, source

copy


The mov instruction

mov dest, source


copy


Quick Look Back at Simulator


Pentium Registers

eax

ebx

ecx

edx

Pentium Processor


Operands

• mov reg, reg

• mov reg, mem

• mov mem, reg

• mov reg, imm

• mov mem, imm


section .data a dd 1234

section .text mov eax, 34 mov ebx, 12345

mov edx, eax mov ecx, ebx

eax

ebx

ecx

edx



section .text mov eax, dword[a] mov ebx, eax

mov eax, 1234 mov dword[a], eax

eax

ebx

ecx

edx


section .data hello db "Hi!" helloL equ $-hello a dd 1234

section .text mov eax, 123456789 mov dword[a], eax

mov dword[a], 0 mov ecx, dword[a]

eax

ebx

ecx

edx


The add instruction

add dest, source


The add instruction

add dest, source

dest = dest + source



section .text mov eax, 3 mov ebx, 5

add eax, ebx

eax

ebx

ecx

edx



section .text mov eax, dword[a] add eax, 1

mov dword[a], eax

eax

ebx

ecx

edx



section .text add dword[a], 1

eax

ebx

ecx

edx


Exerciseint x, y, sum;

x = 3;y = 5;sum = x + y;

Translate this into Assembly


We stopped here last time…We stopped here last time…


A Quick Introductionto the next Homework

Assignment…

getcopy hw4_1_skel.asm

cp hw4_1_skel.asm hw4_1.asm

getcopy 231Lib.asm

nasm -f elf hw4_1.asm

nasm -f elf 231Lib.asm (do only once!)

ld -melf_i386 hw4_1.o 231Lib.o -o hw4_1


eax

ebx

ecx

edx

ax

bx

cx

dx

Pentium Registers


eax

ebx

ecx

edx

ax

bx

cx

dx

ah al

bh bl

ch cl

dh dl

Pentium Registers


Think of ah and al as boxes inside

a bigger one called ax, and ax as

half of a bigger box still,

called eax.


Pentium Registers

11111111 11111111 11111111 1111111132 bits 4,294,967,295

11111111 1111111116 bits 65,535

111111118 bits 255

11114 bits 15

largest unsigned


Pentium Registers

F F F F F F F F32 bits 4,294,967,295

F F F F 16 bits 65,535

F F 8 bits 255

F 4 bits 15

largest unsigned0000 00001 10010 20011 30100 40101 50110 60111 71000 81001 91010 A1011 B1100 C1101 D1110 E1111 F


Declaring Variables


db, dw, dd

• db: define byte storage

• dw: define word storage

• dd: define double-word storage


Examples: db

msg db "Hello", 10

a db 0b db 'H' ; also 72 or 0x48c db 255d db 0x80


Examples: dw

x dw 0y dw 1z dw 255t dw 0x1234


Examples: dd

alpha dd 0beta dd 255gamma dd 0x12345678


Summary ofimportant concepts just seen

• Numbers

• Op Codes

• Machine Language

• Hexadecimal

• Executable Files


More Examples ofThe mov Instruction

mov dest, source


section .datalf db 10c db 0a dw 0x1234b dw 0x dd 0y dd 0x12345678

section .text; put char lf in al

Exercise 1

eax

ebx

ecx

edx

ax

bx

cx

dx

ah al

bh bl

ch cl

dh dl



section .text; copy al to Variable c

Exercise 2

eax

ebx

ecx

edx

ax

bx

cx

dx

ah al

bh bl

ch cl

dh dl



section .text; put a in bx

; put bx in b

; put bx in ax

; put 0 in cx

Exercise 3

eax

ebx

ecx

edx

ax

bx

cx

dx

ah al

bh bl

ch cl

dh dl



section .text; put x in eax

; put y in ecx

; put ecx in edx

; put ex into y

Exercise 4

eax

ebx

ecx

edx

ax

bx

cx

dx

ah al

bh bl

ch cl

dh dl



section .text; put 0 in ah

; put 3 in cx

; put 5 in edx

; put 0x12345678 into eax

Exercise 5

eax

ebx

ecx

edx

ax

bx

cx

dx

ah al

bh bl

ch cl

dh dl


We understand mov!

We understand add!


More add Addressing Modes

add dest, source


section .datalf db 10ch db 0a dw 0x1234b dw 0x dd 0y dd 0x12345678

section .text; add 3 to Variable c

; add 100 to b

; add -1 to edx

; add x to y

Example

eax

ebx

ecx

edx

ax

bx

cx

dx

ah al

bh bl

ch cl

dh dl


Reminder: Our Goal was…

int x, y, sum;

x = 3;y = 5;sum = x + y;



int x, y, sum;

x = 3;y = 5;sum = x + y;

Translate this into Assembly,

once more



short x, y, sum;

x = 3;y = 5;sum = x + y;

Translate this new program into Assembly

Typical

Midterm

Question


Exercise 1section .dataa db 10b db 0c dw 0x1234d dw 0e dd 0f dd 0x12345678

section .text

Swap a and b. Then c and d. Then e and f.

Do on

your own!

Share result

on Piazza.

Correct if faulty


Exercise 2section .dataa db 10b db 0c dw 0x1234d dw 0e dd 0xcdeff dd 0x12345678

section .text

Set the least significant byte of e and f to 00.

Do on

your own!

Share result

on Piazza.

Correct if faulty


Exercise 3section .dataa db b db c dw d dw e dd f db

section .text

reconstruct the declarations of a, b, c, d, e

and f.

99

88

77

66

55

44

33

22

1f

1a

11

00 a

typical

midterm

question!

hex

Do on

your own!

Share result

on Piazza.

Correct if faulty


Exercise 4section .dataa db b db c dw d dw e dd f db

section .text


and f.

99

88

00

00

55

44

33

22

31

26

11

00 a

typical

midterm

question!

dec

Do on

your own!

Share result

on Piazza.

Correct if faulty


Same with Bytes

byte x, y, sum;

x = 3;y = 5;sum = x + y;



With an Array!

int A[3];

A[0] = 3;A[1] = 5;A[2] = A[0]+A[1]



Exercisesection .dataa db b db c dw d dw e dd f db

section .text


and f.

99

88

77

66

55

44

33

22

31

26

11

00 a

typical

midterm

question!

decimal


Follow a stepby step execution

of the program


8 9 section .data 10 00000000 03000000 a dd 3 11 00000004 05000000 b dd 5 12 00000008 00000000 sum dd 0 13 14 ;;; 15 ;;; code area 16 ;;; 17 18 section .text 19 global _start

20 00000000 A1[00000000] _start: mov eax, dword[a] ;eax <-- a 21 00000005 0305[04000000] add eax, dword[b] ;eax <-- eax+b = a+b 22 0000000B 83E801 sub eax, 1 ;eax <-- eax-1 = a+b-1 23 0000000E A3[08000000] mov dword[sum], eax ;sum <-- eax = a+b-1

24 ;;; exit() 25 00000013 B801000000 mov eax,1 26 00000018 BB00000000 mov ebx,0 27 0000001D CD80 int 0x80 ; final system call


a dd 3b dd 5sum dd 0

100 mov eax, dword[a]105 add eax, dword[b]10A sub eax, 110E mov dword[sum], eax

eax

ebx

ecx

edx

eip




eax

ebx

ecx

edx

eip



eax

ebx

ecx

edx

eip

Tick!




eax

ebx

ecx

edx

eip



eax

ebx

ecx

edx

eip



eax

ebx

ecx

edx

eip

Tick!




eax

ebx

ecx

edx

eip



eax

ebx

ecx

edx

eip



eax

ebx

ecx

edx

eip



eax

ebx

ecx

edx

eip

Tick!


Frequency: 3.2 GHz

cycle: 1/3.2 GHz =0.3125 ns

sec ms us ns


Arduino

;hello.asm; turns on an LED which is connected to PB5 (digital out 13)

.include "./m328Pdef.inc"

ldi r16,0b00100000out DDRB,r16out PortB,r16

Start:rjmp Start

Clock speed: 16 MHz

~1/200 speed of Pentium

http://www.science.smith.edu/dftwiki/index.php/Comparing_Different_Computers_with_N_Queens_Program



Raspberry Pi

/* -- first.s *//* This is a comment */.global main /* 'main' is our entry point and must be global */ main: /* This is main */ mov r0, #2 /* Put a 2 inside the register r0 */ bx lr /* Return from main */

Clock speed: 1.4 GHz

~1/3 speed of Pentium



Documents

CSC231-Assembly · 12 00000000 48656C6C6F20746865- Hello db "Hello there!", 10, 10 13 00000009 7265210A0A 14 HelloLen equ $-Hello ... Two Tips for Assembly Language Programmers