Crafting a boot time program How we can utilize some standard real mode routines that reside in the PC s ROM BIOS firmware Our bare machine If we want to do a hands on study of our CPU without any operating system getting in our way we have to begin by exploring Real Mode it s the CPU s startup state We will need to devise a mechanism by which our program code can get loaded into memory since we won t have an OS This means we must write a boot loader Our bare machine If we want to do a hands on study of our CPU without any operating system getting in our way we have to begin by exploring Real Mode it s the CPU s startup state We will need to devise a mechanism by which our program code can get loaded into memory since we won t have an OS So we utilize the ROM BIOS boot loader What s a boot loader After testing and initializing the machine s essential hardware devices the startup program in the ROM BIOS firmware will execute its boot loader to read a small amount of code and data into memory from some type of non volatile storage medium e g floppy diskette hard disk magnetic tape CD ROM etc and then jump to that location in memory PC ROM BIOS BOOT LOCN Vendor s Firmware ROM BIOS No installed memory Video Display Memory Volatile Program Memory VRAM RAM 0x00007E00 0x00007C00 BOOT LOCN 512 bytes IVT and BDA 8086 memory map 1 MB Some Requirements A boot loader has to be 512 bytes in size because it has to fit within a disk sector Must begin with executable machine code Must end with a special boot signature Depending on the type of storage medium it may need to share its limited space with certain other data structures such as the partition table on a hard disk or the Bios Parameter Block on a MS DOS diskette Writing a boot sector program Not practical to use a high level language We need to use 8086 assembly language We can use the Linux development tools The vi editor The as assembler The ld linker The dd copy and convert utility Using ROM BIOS functions Our system firmware provides many basic service functions that real mode programs can invoke this includes boot loaders Video display functions Keyboard input functions Disk access functions System query functions A machine re boot function ROM BIOS documentation The standard ROM BIOS service routines are described in numerous books that deal with assembly language programming for Intel based Personal Computers They are also documented online e g see the link on our class website to Ralf Brown s Interrupt List A typical calling convention A service function ID number is placed in the processor s AH register Other registers are then loaded with the values of any parameters that are needed Then a software interrupt instruction is executed which transfers control to the code in the Read Only Memory ROM Often upon return the carry flag is used for indicating that the function succeeded Example Write String function Setup parameters in designated registers AH function ID number e g 0x13 AL cursor handling method e g 0x01 BH display page number e g 0x00 BL color attributes e g 0x0A CX length of the character string DH DL row number column number ES BP string s starting address seg off Call BIOS via software interrupt int 0x10 Real Mode 8086 8088 had only one execution mode It used segmented memory addressing Physical memory on 8086 was subdivided into overlapping segments of fixed size The length of any memory segment was fixed at 64KB until the processor has been switched out of its initial real mode and its segment registers reprogrammed 64KB Memory Segments Fixed size segments partially overlap Segments start on paragraph boundaries Segment registers serve as selectors stack data code SS DS CS Real Mode Address Translation Logical address 0x12340 0x06789 0x18AC9 16 bit segment address 16 bit offset address 0x1234 0x6789 x 16 20 bit bus address Physical address 0x18AC9 Real Mode Memory Map Vendor s Firmware ROM BIOS 64 kbytes No installed memory Video Display Memory Top of RAM 0x000A0000 Volatile Program Memory 0x00007E00 0x00007C00 0x00000500 0x00000400 0x00000000 Video ROM VRAM 128 kbytes Extended BIOS Data 1 MB RAM BOOT LOCN 512 bytes RBDA IVT 256 bytes 1024 bytes Downloading a class demo You can download a program source file from our CS 630 course website to your own present working directory by using the Linux file copy command like this cp home web cruse cs630 bootdemo s Here the final period character is the Linux shell s symbol for your current directory Compiling and Installing Compiling our bootdemo s using as is a twostep operation and requires use of a linkerscript named ldscript as bootdemo s o bootdemo o ld bootdemo o T ldscript o bootdemo b Installing our bootdemo b into the starting sector of a hard disk partition is very simple dd if bootload b of dev sda4 Executing our bootdemo You ll need to perform a system reboot Our classroom machines will load GRUB the Linux GRand Unified Boot loader GRUB will display a menu of Boot Options You can choose boot from a disk partition In class exercise Can you modify our bootdemo s program so that it will display some information that is more useful For example can you display the Vendor Identification String and the Brand String Remember you code and data must fit within a single 512 byte disk sector You can use fileview to check on space
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