Computer Architecture and Operating Systems CS 3230 :Assembly Section Lecture 3

1. Computer Architecture and Operating SystemsCS 3230 :Assembly SectionLecture 3 Department of Computer Science and Software Engineering University of Wisconsin-Platteville

2. Assembly files (.asm) We will be using the nasm assembler Program Components Comments Labels Directives Data Main subroutine, which is a global one Instructions: generally the format of an NASM instruction is as follows Label       Instruction    Operands     ;  Comment

3. Program Organization for CS3230 Generally, we will be using a C driver program called driver.c to run our assembler routines Why driver.c ? lets the C system set up the program to run correctly All the segments and their corresponding segment registers will be initialized by C The C library will also be available to be used by the assembly code

4. Program Organization : Skeleton file ; file: skel.asm ; This file is a skeleton that can be used to start assembly programs. %include "asm_io.inc" segment .data ; initialized data is put in the data segment here segment .bss ; uninitialized data is put in the bss segment segment .text global asm_main asm_main: enter 0,0 ; setup routine pusha ; code is put in the text segment. Do not modify the code before ; or after this comment. popa mov eax, 0 ; return back to C leave ret

5. Comments Comments are denoted by semi-colons (;). Everything from the semi-colon to the end of the line is ignored.

6. Labels Labels identify The start of subroutines or locations to jump to in your code Variables are declared as labels pointing to specific memory locations Labels are local to your file/module unless you direct otherwise The colon identifies a label (an address!) Example: NewLabel: To define a label as global we say global NewLabel

7. Directives • Direct the assembler to do something • Define constants • Define memory to store data into • Group memory into segments • Conditionally include source code • Include other files

8. Equ and % define directives • The equ directive • Used to define named constants used in your assembly program • Syntax: symbol equ value • Similar to C’s const directive :(const int symbol = value) • The %define directive • Similar to C’s #define directive (#define name value) • Most commonly used to define constant macros: %define SIZE 100 mov eax, SIZE • Macros can be redefined, and can be more complex than simple constants

9. Data directives • Used in data segments to define room for memory • There are two ways memory can be reserved • Defines room for data without initial value ( segment .bss) • Using : RESXdirective • Defines room for data with initial value (segment .data) • Using : DXdirective • Note: X is replaced with a letter that determines the size of the object as following

10. Example: Data Directives L1 db 0 ;byte labeled L1 w/ initial value 0 decimal L2 dw 1000 ;word labeled L2 w/ initial value 1000 decimal L3 db 110101b ;byte labeled L3 w/ initial value 110101 binary( 53) L4 db 12h ;byte labeled L4 w/ initial value 12 hex (18 decimal) L5 db 17o ;byte labeled L5 w/ initial value 17 octal (15 decimal) L6 dd 1A92h ;doubleword labeled L6 initialized to hex 1A92 L7 resb 1 ;1 uninitialized byte L8 db “A” ;byte initialized to ASCII of A = 65 L9 resw 100 ; reserves room for 100 words • Note: Double quotes and single quotes are treated the same

11. More examples • Sequences of memory may also be defined. L10 db 0, 1, 2, 3 ; defines 4 bytes L11 db "w", "o", "r", ’d’, 0 ; defines a C string = "word" L12 db ’word’, 0 ; same as L11 • For large sequences, NASM’s TIMES directive is often useful. L13 times 100 db 0 ; equivalent to 100 (db 0)’s

12. There are four debugging routines named: dump_regs: this macro prints out the values of the registers (in hexadecimal) Syntax : dump_regs X X: It takes a single integer argument that is printed out as well. This can be used to distinguish the output of different dump regs commands dump_mem: this macro prints out the values of a region of memory (in hexadecimal) and also as ASCII characters Syntax: dump_mem X, label , Y X: This can be used to distinguish the output of different dump_mem commands Label: the starting address of displayed region Y: the number of 16-byte paragraphs to display after the starting address Debugging

13. There are four debugging routines named: dump_stack: this macro prints out the values on the CPU stack Syntax: dump_stack X, Y , Z X: This can be used to distinguish the output of different dump_stack commands Y: the number of double words to display below the address that the EBP register holds Z: the number of double words to display above the address in EBP dump_math: this macro prints out the values of the registers of the math coprocessor Syntax: dump_math X X: This can be used to distinguish the output of different dump_math commands Debugging (cont.)

14. Assembling the code Use PuTTy and WinSCP application to use nasm commands To edit your assembly file nano myfile.asm (or vi or emacs or joe or WinSCP editor ) To assemble your program nasm –f elf myfile.asm To create an executable program gcc –m32 myfile.o driver.c asm_io.o From the above command, you will get a new prompt and a file a.out or a.exe will be created To run the program, give the command ./a.outor ./a.exe