CS 380 Lab 2 - Boot Process Assigned 1/28/10 Points: 10

Transcription

1 CS 380 Lab 2 - Boot Process Assigned 1/28/10 Points: 10 In this lab we will examine the booting process for the Intel family of processors. We will create a simple boot strap program that will display a simple message on the screen. Tools Needed NASM Diskprobe Intel Bootstrap Process There are several ways of starting an operating system. Each approach uses a boot strap mechanism that loads an operating system into memory and then transfers control to it. A computer consists of hardware and software. The software may reside in RAM (Random Access Memory) or ROM (Read Only Memory). The BIOS (Basic Input/Output System) resides in ROM when the machine is turned on. The normal sequence for booting an operating system is as follows. After completion of the system's Power On Self Test (POST), the BIOS bootstrap routine generates an INT 19. The interrupt tries to read the boot sector from the first bootable drive. The boot sector is the first sector on the device. If this is a value boot sector the program residing in the boot sector will load the OS. If the sector is not a valid boot sector, the BIOS will load the first sector of the next bootable device. This sector is called the Master Boot Sector (MBR). If the MBR is valid, the boot loader in the MBR will load the OS. If the MBR is invalid, the BIOS will execute INT 18 which will report an error on most systems and then hang. Booting from a Thumb Drive There are three requirements for booting from a thumb Drive: The machine must have a USB port The BIOS select the drive as a bootable device The thumb drive must be bootable The boot sector contains 256 bytes. The last two bytes of a valid boot sector contain 55AA After completion of the system's Power On Self Test (POST), the BIOS bootstrap routine generates an INT 19. If a boot sector is found on the drive, the boot sector is read into memory at location 0000:7C00. The BIOS then checks to see if the last two bytes of the 1

2 sector contain a hexadecimal 55AA. If those two bytes are present then control is transferred to 0000:7C00. The boot loader program is assumed to be present at that location. The boot loader is responsible for finding and loading the kernel into memory. Once the kernel is loaded, control is then transferred to the kernel. INT 19 is also called when the Crt-Alt-Del keys are pressed. Assignment 1 Use the files found on the web site to: Install the NASM assembler Copy and assemble the boot.asm file Transfer it to a thumb drive using Disk Probe Boot up a computer using the thumb drive Boot Loader The boot program is listed below: ; Simple Boot Strap Program ; Uses nasm command: nasm boot.asm -o boot.bin ; Boot.asm [org 0] start: mov ax,07c0h mov ds,ax ; The BIOS puts us at 07c000h, so we need to set the DS ; register to that value mov ax,9000h ; A stack segment address of 90000h is used mov ss,ax ; mov sp,0ffffh ; The whole segment is used hang: mov si, msg call message jmp hang ; The offset of msg is place in the SI register ; The message procedure is called ; We have nothing else to do so we wait ; Displays the string at DS:SI to the screen 2

3 message: lodsb ; Loads the byte at DS:SI into AL or al,al ; Zero out the upper bits of the AX register jz done ; Test if character is 0 in which case we are done mov ah,0eh ; Sets up the BIOS write char in teletype mode mov bx,0007 ; Sets the page number and attribute int 10h ; Call BIOS jmp message done: ret msg DB "CS 380 Boot Strap Program - Version 1.0",0 times 510-($-$$) db 0 ; Fills the rest of the sector with zeros dw 0AA55h ; Sets the last two bytes to AA55 end The boot loader needs to be place in the first sector of the thumb drive. When the boot sector is loaded into memory the DL, CS, and IP are set. DL - Boot Drive CS - 0 IP - 0x7C00 The DL register holds a value indicating which device was used for booting: 0x01 - Diskette 0x80 - Primary hard drive The BIOS boots into 16-bit real mode. It is necessary to set up any segment registers that the boot loader will need. An easiest way of doing this looks like this: mov ax, 7C0h mov ds, ax It is sometimes necessary to set up the program stack. The SS register needs to be placed into memory somewhere. SP needs to be assigned a value. SP is decremented when it is pushed. 3

4 mov ax,9000h ; Better make sure this area is not used for other purposes mov ss,ax mov sp,0ffffh ; Using the whole segment The boot strap program will normally load and transfer control to the kernel. In this lab, we need to simply display a message that we have loaded. mov si, msg call message ; The offset of msg is place in the SI register ; The message procedure is called msg DB "CS 380 Boot Strap Program - Version 1.0",0 After the message has been displayed we will do nothing: hang: jmp hang ; We have nothing else to do so we wait The call instruction invokes the following function to display a message: ; Displays the string at DS:SI to the screen message: lodsb ; Loads the byte at DS:SI into AL or al,al ; Zero out the upper bits of the AX register jz done ; Test if character is 0 in which case we are done mov ah,0eh ; Sets up the BIOS write char in teletype mode mov bx,0007 ; Sets the page number and attribute int 10h ; Call BIOS jmp message done: ret The boot strap program will be written to the boot sector of a thumb drive. The last two bytes of this sector must contain the bytes AA 55. We can place these bytes at that 4

5 location using the following code sequence. times 510-($-$$) db 0 ; Fills the rest of the sector with zeros dw 0AA55h ; Sets the last two bytes to AA55 end The boot strap program can be assembled using NASM. This is done using a command prompt with the command: nasm boot.asm -o boot.bin This command can be placed in the make.bat file for your convenience. Disk Probe To write a sector to a thumb drive using Disk Probe: 1) Select and activate the thumb drive Select the menu Drives - Logical Volume Under the Logical Volumes list box, double click on thumb drive letter: There will be a slight delay as the application identifies thumb drive. 5

6 For Handle 0, remove the Read Only check Next click on the Set Active button. Next, click on the OK button to close the window 2) Open and then write the boot file to the thumb drive Use the Open File menu command to locate an select boot.bin. Use the Sectors Write menu command to bring up the Write Sector dialog box: Click on the Write it button. Click on Yes for the Disk Probe overwite message: The thumb drive can now be used to boot the PC. It is sometimes necessary to close Diskprobe and then reopen it when the thumb drive is modified or the sector to be written is re-assembled. Diskprobe will also interfere with Window seeing the thumb drive. Assignment 2 Modify the prompt to display your OS name and demonstrate it to me. 6