Physics 39 -- September 15, 2009

Physics 39 -- Assembly

September 15, 2009 -- Tuesday

MOVINGDATA

Various sections of an assembly program:

1. data

2. bss - .comm -- common mem area

ex.

.comm symbol, length

- .lcomm -- local common mem area

3. text

Data types/directives

.ascii

.asciz

.byte

double

float

.int

.long

.octa

.quad

.short

.single

example:

*pi:

.float 3.14159

*size

.long 100, 150, 200, 250, 300

#declaration

.section .data

msg:

.ascii " This is a test message"

factors:

.double 37.45, 45.33, 12.30

height:

.int 54

length:

.int 62, 35, 47

.equ factor, 3 #static variable

.equ LINUX_SYS_CALL, 0x80

movl LINUX_SYS_CALL, %eax

buffer - continuous block of bytes

how to show the memory occupied by a file

*vdir -- shows the memory occupied

by a file

*vdir -h - shows the memory

occupied by a file with the memory's

unit

Codes

1. smaller memory

#sizetest1.s

.section .bss

.lcomm buffer, 10000

.section .text

.globl main

main:

movl $1, %eax

movl $0, %ebx

int $0x80

*exit command

gcc -o sizetest1 sizetest1.s

./siztest1

2. bigger memory

#sizetest2.s

.section .data

buffer:

.fill 10000

.section .text

.globl main

main:

movl $1, %eax

movl $0, %ebx

int $0x80

gcc -o sizetest2 sizetest2.s

./sizetest2

MOV instruction

1. movl - 32 bit long word

2. movl - 16 bit word

3. movb - 8 bit byte

1 byte = 8 bits

example:

movl %eax, %ecx

movw %ax, %cx

movb %al, %cl

3. Debugging

#movtest1.s

.section .data

value

.int 1

.section .text

.globl main

main:

nop #no operation

movl value, %ecx

movl $1, %eax

movl $0, %ebx

int $0x80

--debugging--

gcc -o movtest movtest1.s -gstabs

man gcc

/-gstabs

/-gstabs

/-gstabs

q -- (quit)

1. gdb -q movtest1 -- starts the debugging process

2. (gdb) break *main+1 -- command use to begin the

debugging process in the main

3. run --command use to start debugging the main

4. --to print the value of ecx type:

print/x $ecx

5. s - command use to go to the next line

6. info registers -- show all the values which the variables

contain

4. looping

#movtest2.s

.section .data

output:

.asciz "The value is %d\n"

values:

.int 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60

.section .text

.globl main

main:

nop

movl $0, %edi

loop:

movl

values

(, %edi, 4), %eax

pushl %eax

pushl $output

call printf

addl $8, %esp

inc %edi #inc - increment

cmpl $11, %edi #cmp - compare

jne loop #jne - jump not equal

movl $0, %ebx

movl $1, %eax

int $0x80

OUTPUT:

The value is 10

The value is 15

The value is 20

The value is 25

The value is 30

The value is 35

The value is 40

The value is 45

The value is 50

The value is 55

The value is 60

-----------------

*int - 32 bits

referencing

index system

4 things to consider in an

indexed memory mode:

1. base address

2. offset address

to add to the base address

*in movtest2 the offset address = 0.

3. size of data elements

4. index to determine which data element to select

----------------------------------------

in this line taken from movtest2.s

movl $0, %edi

if you change 0 to one the output would be:

The value is 15

The value is 20

The value is 25

The value is 30

The value is 35

The value is 45

The value is 50

The value is 55

The value is 60

-------------------------------------------

*print

pushl %eax

pushl $output

call printf

addl $8, %esp

----------------------------------------

5. largest value in an array

#cmovtest.s

.section .data

output:

.asciz "The largest value is %d\n"

values:

.int 105, 235, 61, 315, 134, 221, 53, 145, 117, 5

.section .text

.globl main

main:

nop

movl values, %ebx

movl $0, %edi

loop:

movl values(, %edi, 4), %eax

cmp %ebx, %eax

cmova %eax, %ebx

inc %edi

cmp $10, %edi

jne loop

pushl %ebx

pushl $output

call printf

addl $8, %esp

pushl $0

call exit

gcc -o cmovtest cmovtest.s

./cmovtest

OUTPUT:

The largest value is 315

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