CSC231 - Assembly · 2017-02-08 · D. Thiebaut, Computer Science, Smith College 11 section .data...

mith College

Computer Science

Dominique Thiébaut dthiebaut@smith.edu

CSC231 - AssemblyWeek #3

D. Thiebaut, Computer Science, Smith College

mov registers

hexdumplistings

number systems

memory

Let's Finish Last Week's Material…

section .data Hello db "Hello there!", 10, 10 HelloLen equ $-Hello section .text global _start _start:

;;; print message mov eax, 4 ; write mov ebx, 1 ; stdout mov ecx, Hello ; address of message to print mov edx, HelloLen ; # of chars to print int 0x80

;;; exit mov ebx, 0 mov eax, 1 int 0x80

Listing

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

Listing

Hexdump

Our Goal for This Weekint x, y, sum;

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

• Mov instruction

• Registers

• Memory storage options

You already know some of this material…

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

The mov instruction

mov dest, source

Operands• mov reg, reg

• mov reg, mem

• mov mem, reg

• mov reg, imm

• mov mem, imm

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.

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

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

Summary of important concepts just seen• Numbers

• Op Codes

• Machine Language

• Hexadecimal

• Executable Files

Back to the mov instruction

mov dest, source

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

section .text; put lf in al

Test Cases

section .text; put al in ch

Test Cases

section .text; put a in bx

; put bx in b

; put bx in ax

; put 0 in cx

Test Cases

section .text; put x in eax

; put y in ecx

; put ecx in edx

; put ex into y

Test Cases

section .text; put 0 in ah

; put 3 in cx

; put 5 in edx

; put 0x12345678 into eax

Test Cases

We understand mov!

The add instructionadd dest, source

section .text; add 3 to ch

; add 100 to b

; add -1 to edx

; add x to y

Test Cases

Reminder: Our Goal is…

int x, y, sum;

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

Reminder: Our Goal is…

int x, y, sum;

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

Translate this into Assembly

Exercisesection .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.

Exercisesection .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.

Exercisesection .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.

typical

midterm

question!

Exercisesection .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.

typical

midterm

question!

Follow a step by step execution

of the program

CSC231 - Assembly · 2017-02-08 · D. Thiebaut, Computer Science, Smith College 11 section .data...

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