Laboratorio di Sistemi Operativi

TOC(noheading)

Lezione 1: Introduzione

Slide

• Turno 1 (4 Marzo 2009): ​slide (​Versione stampa)
• Turno 2 (6 Marzo 2009): ​slide (​Versione stampa)

Programmi

• Accesso diretto alla macchina fisica mioboot-nobios-simple.asm

  bits 16                         ; 16 bit real mode
  org 0x7C00                      ; origine indirizzo 0000:7C00
  
  start:	
   mov ax, 0xb800	 ; text video memory
   mov ds, ax
   mov eax, 10
  write:
   cmp eax, 0
   jz end
   mov byte [eax], 'm'
   mov byte [eax+1], 0x0F    ; attrib = white on black
   sub eax, 2
   jmp write
  end:
   hlt
  
  times 510-($-$$) db 0 ; 0-padding 
  dw 0xAA55
  

  nasm -l mioboot-nobios-simple.lst -o mioboot-nobios-simple.img mioboot-nobios-simple.asm
  qemu mioboot-nobios-simple.img
  

• Uso dei servizi del BIOS mioboot.asm

  bits 16                         ; 16 bit real mode
  org 0x7C00                      ; origine indirizzo 0000:7C00
          
  start:
          cld                     ; clears direction flag (index regs incremented)
          mov si, boot
          call message
  working:
          mov si, work
          call message
  
          call waitenter
          jmp working
  
  message:
          lodsb                   ; carica un byte da [DS:SI] in AL e inc SI
          cmp al, 0
          jz done
          mov ah, 0x0E            ; write char to screen in text mode
          mov bx, 0               ; BH page number BL foreground color
          int 0x10                ; write AL to screen (BIOS)
          jmp message
  done:   ret
  
  boot: db "Loading unuseful system...." , 10, 13, 0
  work: db "I've done my unuseful stuff!" , 10, 13, 0 
  cont: db "Hit ENTER to continue...", 10, 13, 0
  wow: db "Great! Hello world!" , 10, 13, 0
          
  waitenter: mov si, cont
             call message
             mov ah, 0
             int 0x16                ; Wait for keypress (BIOS)
             cmp al, 'm'
             jz egg
             cmp al, 'b'
             jz basic
             cmp al, 13
             jnz waitenter
             ret
  egg:       mov si, wow
             call message
             jmp waitenter
  basic:     int 0x18		; basic (BIOS)
             hlt
  
          times 510-($-$$) db 0
          dw 0xAA55
  

  nasm -l mioboot.lst -o mioboot.img mioboot.asm
  qemu mioboot.img
  

• Programma che esegue il codice letto da standard input exec.c

  #include <stdio.h>
  
  int main() {
    unsigned char buf[1024];
    void (*ptr)();
    int n;
    unsigned int eax;
  
    n = read(0, buf, 1024);
    buf[n] = '\xc3';
  
    ptr = (void(*)()) buf;
    ptr();
  
    __asm__("mov %%eax, %0" : "=m"(eax));
  
    printf("EAX: %.8x\n", eax);
  
    return 0;
  }
  

  gcc -o exec exec.c
  echo -ne "\xB8\x11\x22\x33\x44" | ./exec
  echo -n "ciao" | ./exec
  ...
  

Link

• ​NASM
• ​PC Assembly Language, by Paul A. Carter
• ​QEmu
• ​Manuali Intel

Lezione 2