Computer Labs: Assembly with the Netwide Assembler (NASM) 2o MIEIC Pedro F. Souto ([email protected]) September 28, 2010 NASM Command Line Options -h for usage instructions -o output file name -f output file format I Must be coff always -l generate listing file, i.e. file with code generated -e preprocess only -g enable debugging information Exemplo nasm -g -f coff foo.asm -o foo.o NASM Directives BITS 32 generate code for 32 bit processor mode CPU 386 | 686 | ... restrict assembly to the specified processor SECTION <section_name> specifies the section the assembly code will be assembled into. For COFF can be one of: .text code (program) section .data initialized data section .bss uninitialized data section EXTERN <symbol> declare <symbol> as declared elsewhere, allowing it to be used in the module; GLOBAL <symbol> declare <symbol> as global so that it can be used in other modules that import it via EXTERN Pseudo-Instructions for Data Definition ... in the .data section DB Define byte DW Define word (2 bytes) DD Define double word (4 bytes) IMPORTANT IA-32 architecture is little endian db 0x55 ; just byte 0x55 db 0x55, 0x56, 0x57 ; three bytes in succession db ’a’,0x55 ; character constants are OK db ’hello’,13,19,’$’ ; so are string constants dw 0x1234 ; == db 0x34, 0x12 dw ’a’ ; == db ’a’, 0x0 == db 0x41, 0x0 dw ’ab’ ; == db ’a’,’b’ dw ’abc’ ; == db ’a’,’b’,’c’, 0x0 dd 0x12345678 ; == db 0x78, 0x56, 0x34, 0x12 Pseudo-Instructions for Allocationg Space ... in the .bss section I It makes no sense to define data in the uninitialized section RESB Allocate byte RESW Allocate word (2 bytes) RESD Allocate double word (4 bytes) buffer: resb 64 ; allocate 64 bytes for buffer wordvar: resw 1 ; allocate word larray: resd 10 ; allocate array for 10 longs Other Pseudo-Instructions EQU Define symbolic constant message db ’hello, world’ msglen equ $-message Note The msglen is evaluated once using the value of $ at the point of definition $ evaluates to the assembly position at the beginning of the line containing the expression TIMES Prefix that causes the instruction to be assembled multiple times: zerobuf: times 64 db 0 buffer: db ’hello, world’ times buffer+64-$ db ’ ’ I What does buffer+64-$ evaluate to? NASM vs. MASM NASM is case sensitive I It makes a difference whether you call your label isr, Isr or ISR All memory references must use square brackets I.e. all effective addressses must appear between [ and ]. E.g.: mov eax,[ebx*2+ecx+offset] mov ax,[bp+di+8] NASM does not store variable types This means that you must specify the operand size. E.g.: var dw 0 mov word [var],2 ; and not: mov var, 2 C-Style Function Definition with NASM Macros int foo(int a, int b) { CPU 686 int tmp; BITS 32 tmp = a + b; %include ’macros.mac’ tmp = tmp - a * b; section .text return tmp; proc foo } %arg a: dword b:dword %local tmp: dword uses esi, edi mov esi, [a] mov eax, [b] add eax, esi mov [tmp], eax mov edi, [a] mov eax, [b] mul eax, edi sub [tmp], eax move eax, [tmp] ; return tmp endproc END Pixel() In C: void pixel(int x, int y, int color, char *base, int hres) { *(base + x + y*hres) = color; } In NASM: CPU 686 BITS 32 %include ’macros.mac’ section .text proc pixel %arg x: dword y:dword color:dword, base: dword, hres: dword uses mov ecx, [y] imul ecx, [hres] ; ecx = y*hres mov edx, [base] ; add edx, [x] ; edx = base +x mov al, [color] mov [ecx+dex], al endproc END Further Reading I Dr. Paul Carter, PC Assembly Language I Section 1.3: Assembly Language I Section 1.4: Creating a Program I NASM Manual I Section 2.2: Quick Start for MASM Users I Chapter 3: The NASM Language I Chapter 5: Assembler Directives I Section 8.1: Interfacing to 32-bit C Progarams I Appendix B: x86 Instruction Reference.