Assembly Language. Assembly language for x86 compatible processors using GNU/Linux operating system

Transcription

1 Assembly Language Assembly language for x86 compatible processors using GNU/Linux operating system x86 refers to the instruction set architecture in most personal computers Derives from model numbers ending in 86 of first 16-bit Intel processors: 8086 (1978) (1982) Modern versions - example 64 bit Intel Core Duo - all backwards compatible to 8086 More on Intel chips ITEC 1000 Introduction to Information Technologies 1

2 Controversy over the name In 1983 Richard Stallman - founder of Free Software Foundation - announced vision of free software. Work began on a unix-like free system In 1991 almost complete except for difficulties developing the unix style kernel - device drivers and system utilities Also in first version of the linux kernel released by Linus Torvalds First GNU superstructure ported to Linux - distribution called Linux/GNU GNU ported linux to their system calling it GNU/Linux Richard Stallman Linux video More on Controversy Linus Trovalds ITEC 1000 Introduction to Information Technologies 2

3 Building Linux Assembly Language Program Following closely the text in the free download book Programming from the Ground up by Jonathan Barlett The first program - on next slide - does nothing but opening and exiting. You first need to: - type the program into a text-editor exactly as it appears in an assembly source file file called exit.s - transfer exit.s to your sit account The program is on the next page ITEC 1000 Introduction to Information Technologies 3

4 Do Nothing Program #PURPOSE: Simple program that exits and returns a # status code back to the Linux kernel # #INPUT: none # #OUTPUT: returns a status code. This can be viewed # by typing # echo $? # after running the program # #VARIABLES: # %eax holds the system call number # %ebx holds the return status #.section.data.section.text.globl _start _start: movl $1, %eax # this is the linux kernel command # number (system call) for exiting # a program movl $0, %ebx # this is the status number we will # return to the operating system. # Change this around and it will # return different things to # echo $? int $0x80 # this wakes up the kernel to run # the exit command ITEC 1000 Introduction to Information Technologies 4

5 After transferring e exit.s to your sit account, compile with the command: as exit.s -o exit.o The command: as exit.s -o exit.o, invokes the assembly language compiler as and creates an object file in machine language called exit.o Most programming projects have many small object files that need to be hooked up (linked) to create an executable file. Done with the linker - a program putting object files together and adding information so the kernel can load and run the executable ITEC 1000 Introduction to Information Technologies 5

6 With only 1 file - linker adds the loading and running information The command is: ld exit.o -o exit which reads - link exit.o and give the resulting file the name exit To run the file exit - type./exit The instruction movl $0, %ebx enters the exit code 0 Type echo $? and you should see the character 0 as response ITEC 1000 Introduction to Information Technologies 6

7 Program Analysis A period before anything does not directly translate to machine code - it indicates an instruction to the assembler as to how to proceed.section indicates a section indicates a section within the program.section.data - refers to a section in which data constants are entered.section.text - refers to the section where program instructions live Within a section there can be other assembler instructions beginning with a period The line.global _start defines a symbol _start later to be used to mark a location of a portion of the program or some data. The word global indicates that the assembler should not forget the value attached to the symbol in compiling with other programs ITEC 1000 Introduction to Information Technologies 7

8 The line _start: indicates the beginning of the program instructions - the colon indicates that the previously defined symbol _start is now a label with a specific address value assigned by the assembler The line mov1 $1, %eax moves the number 1 to the eax register moving 1 to the eax register indicates an exit system call to the Linux kernel - telling the kernel to exit the program use of the $ indicates that the number 1 is to be moved - this is immediate mode addressing - without $ the the contents of address 1 are moved to eax register The exit system call requires that a status code be loaded in the ebx register - hence the line mov1 $1, %ebx The line int $0x80 is an interrupt - it transfers control back to Linux - 0x80 stands for 80 in hex - that is 80H ITEC 1000 Introduction to Information Technologies 8

9 Finding maximum value program Given an arbitrary list of numbers: x1, x2, x3,... xn-1, xn = 0 find the entry with maximum value. Method examines list elements - one by one Steps are, for 1 i n 1. check the current list item xi stored in eax register 2. if xi = 0, exit 3. increase the current list position to i+1 - store i+1 in edi register 4. load xi+1 to eax register 5. compare current value of eax (xi+1) to current highest value stored in ebx 6. if value in eax greater than value in ebx, transfer value in eax to ebx 7. repeat ITEC 1000 Introduction to Information Technologies 9

10 Maximum value program #VARIABLES: The registers have the following uses: # %edi - Holds the index of the data item being examined # %ebx - Largest data item found # %eax - Current data item # # The following memory locations are used: # data_items - contains the item data. A 0 is used # to terminate the data.section.data data_items: #These are the data items.long 3,67,34,222,45,75,54,34,44,33,22,11,66,0 section.text.globl _start ITEC 1000 Introduction to Information Technologies _start: movl $0, %edi movl data_items(,%edi,4), %eax movl %eax, %ebx start_loop: cmpl $0, %eax je loop_exit incl %edi movl data_items(,%edi,4), %eax cmpl %ebx, %eax jle start_loop bigger movl %eax, %ebx jmp start_loop loop_exit: movl $1, %eax int $0x80 # move 0 into the index register # load the first byte of data # since this is the first item, %eax is the biggest # start loop # check to see if weʼve hit the end # load next value # compare values # jump to loop beginning if the new one isnʼt # move the value as the largest # jump to loop beginning # end the loop # %ebx is the status code for the exit system call # and it already has the maximum number #1 is the exit() syscall 10

11 Program Analysis A list of numbers is declared in the data section of the program - and - the label data_items refers to the beginning position of the list -.long indicates that the each number is a long integer - a binary number in a word 4 bytes long -.global does not precede the label - not necessary - data used only in this small program the line movl $0, %edi moves the number 0 to the edi register - which determines the current item number in the list. If edi contains 0 we refer to the first element of the list. ITEC 1000 Introduction to Information Technologies 11

12 the line movl data_items (, %edi, 4), %eax if i = contents of edi, move the 4 bytes of the i th number to eax - here address of the i th item = (address of data_items) + i x 4 the line movl %eax, %ebx initializes the ebx register with the first item - ebx is to hold the current largest number - since at beginning, the first of the list is the current largest. the line start_loop: is a label indicating the beginning of a loop in which the code looks at each item of the list and compares it to the value in ebx the line cmpl $0, %eax compares the value of the current item in eax to the 0 - which is the last item the line je loop_exit - says that if the in comparison the 2 values are equal - jump to the instructions at loop_exit ITEC 1000 Introduction to Information Technologies 12

13 Otherwise - we are not at the end and we need to increment the instruction counter (the edi register) and load the next item into the eax register done by instructions incl %edi adds 1 to edi register movl data_items(, %edi, 4), %eax Next compare the current largest item held in ebx to the current in eax. If the current largest is still the largest, leave it in ebx and branch to start_loop in order to compare to the next element in the list. The instructions are cmpl %ebx, %eax jle start_loop - saying jump - if, in comparison, contents eax contents ebx ITEC 1000 Introduction to Information Technologies 13

14 Otherwise, contents of eax is greater and becomes the next entry in ebx with instruction movl %eax, %ebx The program continues with branch to start_loop with the unconditional jump instruction jmp start_loop End conditions are the same as before Notice that the exit status code which always is in the %ebx register now contains the maximum value. After compiling run the program with command./ maximum The response is the maximum value ITEC 1000 Introduction to Information Technologies 14

15 Addressing Modes in instructions Direct addressing If an instruction contains a memory address followed by a specified register, the contents of the address are transfered to the register example movl ADDRESS, %exa - contents of memory position specified by ADDRESS is moved to exa register (the l in movl means a piece of data of type long - namely 4 bytes - is to be moved) ITEC 1000 Introduction to Information Technologies 15

16 Indexed addressing: Consists of as many as 4 parameters: an address indicating (pointing to) a position in memory a base register indicating a starting point beyond position indicated by the address an index register that may indicate the block size - (number of bytes) - to be accessed an integer n called multiplier that multiplies the block size Example resulting address: address + value of base register + n(value of index register ) movl data_start(, %ecx, 2), %exa moves to register %exa the long data located at the address of data_start + 2 * (value of %ecx) the base register was not used and had value 0 ITEC 1000 Introduction to Information Technologies 16

17 Indirect addressing If register contains an address, the contents of the addressed portion of memory is moved to another register example: movl (%eax), %ebx the piece of data of length long which is at the position indicated by the address in %eax register is moved to the %ebx register Base pointer addressing If the register %eax contains the address of data, then, for n an integer, n(%eax) indicates a position 4 bytes beyond the address in %eax example: movl 4(%eax), %ebx. ITEC 1000 Introduction to Information Technologies 17

18 Immediate mode Loads a value into a register - very simple example: mov $12, %eax loads the number 12 into the %eax register Register addressing moves data in or out of another register example: movl % ebx, % eax moves the contents of % ebx register to the % exa register. ITEC 1000 Introduction to Information Technologies 18