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81-531: The code in volume boot records is invoked either directly by the machine's firmware or indirectly by code in the master boot record or a boot manager . Code in the MBR and VBR is in essence loaded the same way . Invoking a VBR via a boot manager is known as chain loading . Some dual-boot systems, such as NTLDR (the boot loader for all releases of Microsoft's Windows NT-derived operating systems up to and including Windows XP and Windows Server 2003 ), take copies of

162-513: A boot manager . When a computer is turned off, its software‍—‌including operating systems, application code, and data‍—‌remains stored on non-volatile memory . When the computer is powered on, it typically does not have an operating system or its loader in random-access memory (RAM). The computer first executes a relatively small program stored in read-only memory (ROM, and later EEPROM , NOR flash ) along with some needed data, to initialize RAM (especially on x86 systems), to access

243-626: A basic shell (as in GNU GRUB), or even games (see List of PC booter games ). Some boot loaders can also load other boot loaders; for example, GRUB loads BOOTMGR instead of loading Windows directly. Usually, a default choice is preselected with a time delay during which a user can press a key to change the choice; after this delay, the default choice is automatically run so normal booting can occur without interaction. They may also handle compression, cryptographic verification, and chain-loading of other bootloaders. The boot process can be considered complete when

324-503: A boot loader reaching over two physical sectors, using 386 instructions for size reasons. At the same time, other vendors managed to squeeze much more functionality into a single boot sector without relaxing the original constraints on only minimal available memory (32 KiB) and processor support (8088/8086). For example, DR-DOS boot sectors are able to locate the boot file in the FAT12, FAT16 and FAT32 file systems, and load it into memory as

405-641: A capability for user software to directly control the hardware was a requirement. In part, this was because a significant subset of all the hardware features and functions was not exposed by the BIOS services. For two examples (among many), the MDA and CGA adapters are capable of hardware scrolling, and the PC serial adapter is capable of interrupt-driven data transfer, but the IBM BIOS supports neither of these useful technical features. Today,

486-440: A crash when the BIOS passes execution to the boot sector assuming that it contains valid executable code. This implies that FAT12 / FAT16 media to be used also by very old versions of DOS must maintain the signature even if they do not contain an operating system or are meant to be bootable on other platforms only; consequently they must contain at least an x86 compatible (dummy) loader as well (for comparison, see examples of FAT on

567-580: A mechanism for passing messages between BIOS and BIOS client software such as an operating system. The messages request data or action from BIOS and return the requested data, status information, and/or the product of the requested action to the caller. The messages are broken into categories, each with its own interrupt number, and most categories contain sub-categories, called "functions" and identified by "function numbers". A BIOS client passes most information to BIOS in CPU registers, and receives most information back

648-400: A mere process within that system, and then irrevocably transfer control to the operating system. The boot loader then terminates normally as any other process would. Most computers are also capable of booting over a computer network . In this scenario, the operating system is stored on the disk of a server , and certain parts of it are transferred to the client using a simple protocol such as

729-422: A new PC today, assuming that the faster speed of execution is acceptable (which it typically is for all but games that use CPU-based timing). Despite the considerable limitations of the services accessed through the BIOS interrupts, they have proven extremely useful and durable to technological change. BIOS interrupt calls perform hardware control or I/O functions requested by a program, return system information to

810-497: A signature of the ASCII string $ PnP in system memory starting from F0000h to FFFFFh at every 16 byte boundary. With partitioned media, when the VBR is launched by the MBR (or different boot loader) rather than the BIOS, many implementation pass additional information to the VBR besides just DL (and sometimes DH and ES:DI as well): Under DR-DOS 7.07 an extended interface may be optionally used by

891-465: A small number of boot loader instructions into memory; a completion signal from the I/O device may then be used to start execution of the instructions by the CPU. Smaller computers often use less flexible but more automatic boot loader mechanisms to ensure that the computer starts quickly and with a predetermined software configuration. In many desktop computers, for example, the bootstrapping process begins with

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972-430: A whole via CHS or LBA, even if the file is not stored in a fixed location and in consecutive sectors. BIOS and UEFI can not only load multiple operating systems from a non-volatile device, they can also initialize system hardware for the loaded operating systems. Examples of first-stage bootloaders include BIOS , UEFI , coreboot , Libreboot , and Das U-Boot . Second-stage bootloaders operate without

1053-479: Is an ATA , SCSI , or SATA drive (or in earlier days, an ESDI drive, or an MFM or RLL drive with perhaps a Seagate ST-506 controller, perhaps one of the several Western Digital controller types, or with a different proprietary controller of another brand). The program only needs to identify the BIOS-defined number of the drive it wishes to access and the address of the sector it needs to read or write, and

1134-401: Is loaded at memory location 0000h : 7C00h and with the following CPU registers set up when the prior bootstrap loader (that is, typically the BIOS or MBR, but possibly another boot loader) passes execution to it by jumping to 0000h : 7C00h in the CPU's real mode . Some Compaq BIOSes erroneously use 07C0h:0000h instead. While this resolves to the same location in real mode memory, it

1215-455: Is necessary, because the loading can be precomputed and stored on the ROM when the device is made. Large and complex systems may have boot procedures that proceed in multiple phases until finally the operating system and other programs are loaded and ready to execute. Because operating systems are designed as if they never start or stop, a boot loader might load the operating system, configure itself as

1296-409: Is non-standard and should be avoided, since VBR code assuming certain register values or not written to be relocatible may not work otherwise. Systems with Plug-and-Play BIOS or BBS support will provide a pointer to PnP data in addition to DL: This information allows the boot loader (in the MBR or VBR) to actively interact with the BIOS or a resident PnP / BBS overlay in memory in order to configure

1377-732: Is not normally allowed. While the Disk Parameter Table (DPT/FDPB) is typically set up at 0000h:0078h in memory already, the VBR must move (and possibly fix-up) the DPT pointed to by INT 1Eh vector to this location (INT 1Eh not an interrupt, but a far pointer to the DPT). Some conditions can be relaxed in controlled environments, for example, some boot loaders today assume to have up to 128 KB of memory to work with in normal operation (without querying for more), and some boot loaders using LBA access assume at least an Intel 80188 or 80186 CPU. The VBR

1458-517: Is specified as the parameter of the software interrupt instruction (in Intel assembly language, an "INT" instruction), and the function number is specified in the AH register; that is, the caller sets the AH register to the number of the desired function. In general, the BIOS services corresponding to each interrupt number operate independently of each other, but the functions within one interrupt service are handled by

1539-445: Is that modern operating systems run with the processor in protected (or long ) mode, whereas the BIOS code will only execute in real mode . This means that if an OS running in protected mode wanted to make a BIOS call, it would have to first switch into real mode, then execute the call and wait for it to return, and finally switch back to protected mode. This would be terribly slow and inefficient. Code that runs in real mode (including

1620-425: Is the [inconvenient but inevitable] responsibility of the caller either to avoid this case by not making such calls, or to positively test for an expected effect of the call rather than assuming that the call was effective. Because BIOS has evolved extensively in many steps over its history, a function that is valid in one BIOS version from some certain vendor may not be valid in an earlier or divergent BIOS version from

1701-536: The Atari ST and with MSX-DOS ). Nevertheless, some media for other platforms erroneously contain the signature even without a x86 compatible dummy loader, making the check not 100% reliable in practice. The signature is tested for by most System BIOSes since (at least) the IBM PC/AT (but not by the original IBM PC and some other machines). Even more so, it is also checked by most MBR boot loaders before passing control to

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1782-483: The FAT article .) The presence of an IBM PC compatible boot loader for x86-CPUs in the boot sector is by convention indicated by a two-byte hexadecimal sequence called the boot sector signature ( 55h at fixed offset +1FEh and AAh at +1FFh ) for sector sizes of 512 bytes or more. For 512 byte sectors, the boot sector signature also marks the end of the sector. VBRs on smaller and larger sectors may show signatures at

1863-753: The INT x86 assembly language instruction. For example, to print a character to the screen using BIOS interrupt 0x10, the following x86 assembly language instructions could be executed: A list of common BIOS interrupt classes can be found below. Some BIOSes (particularly old ones) do not implement all of these interrupt classes. The BIOS also uses some interrupts to relay hardware event interrupts to programs which choose to receive them or to route messages for its own use. INT 18h traditionally jumped to an implementation of Cassette BASIC (provided by Microsoft) stored in Option ROMs . This call would typically be invoked if

1944-475: The Trivial File Transfer Protocol (TFTP). After these parts have been transferred, the operating system takes over the control of the booting process. As with the second-stage boot loader, network booting begins by using generic network access methods provided by the network interface's boot ROM, which typically contains a Preboot Execution Environment (PXE) image. No drivers are required, but

2025-507: The Windows Boot Manager . In file systems such as FAT12 (except for in DOS 1.x), FAT16 , FAT32 , HPFS and NTFS , the VBR also contains a BIOS Parameter Block (BPB) that specifies the location and layout of the principal on-disk data structures for the file system. (A detailed discussion of the sector layout of FAT VBRs, the various FAT BPB versions and their entries can be found in

2106-413: The boot sector ) of the most promising device, typically starting at a fixed entry point such as the start of the sector. A first-stage bootloader is a compact 512-byte program that resides in the master boot record (MBR) and executes when a computer starts. Running in 16-bit real mode at address 0x7C00, it performs minimal hardware initialization , sets up a basic execution environment , and locates

2187-458: The extended BIOS parameter block on FAT12 and FAT16 volumes since DOS 4.0, whereas the FAT32 EBPB introduced with DOS 7.1 requires even 87 bytes, leaving only 423 bytes for the boot loader when assuming a sector size of 512 bytes. Microsoft boot sectors, therefore, traditionally imposed certain restrictions on the boot process. For example, the boot file had to be located at a fixed position in

2268-530: The kernel . Many implement modular designs supporting loadable modules for additional functionality. These choices can include different operating systems (for dual or multi-booting from different partitions or drives), different versions of the same operating system (in case a new version has unexpected problems), different operating system loading options (e.g., booting into a rescue or safe mode ), and some standalone programs that can function without an operating system, such as memory testers (e.g., memtest86+ ),

2349-589: The master boot record in order to leave room for the default 64-byte partition table with four partition entries and the two-byte boot signature , which the BIOS requires for a proper boot loader — or even less, when additional features like more than four partition entries (up to 16 with 16 bytes each), a disk signature (6 bytes), a disk timestamp (6 bytes), an Advanced Active Partition (18 bytes) or special multi-boot loaders have to be supported as well in some environments. In floppy and superfloppy volume boot records , up to 59 bytes are occupied for

2430-515: The BIOS in a new PC still supports most, if not all, of the BIOS interrupt function calls defined by IBM for the IBM AT (introduced in 1984), along with many more newer ones, plus extensions to some of the originals (e.g. expanded parameter ranges) promulgated by various other organizations and collaborative industry groups. This, combined with a similar degree of hardware compatibility, means that most programs written for an IBM AT can still run correctly on

2511-584: The BIOS was unable to identify any bootable disk volumes on startup. At the time the original IBM PC (IBM machine type 5150) was released in 1981, the BASIC in ROM was a key feature. Contemporary popular personal computers such as the Commodore 64 and the Apple II line also had Microsoft Cassette BASIC in ROM (though Commodore renamed their licensed version Commodore BASIC), so in a substantial portion of its intended market,

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2592-499: The BIOS will take care of translating this general request into the specific sequence of elementary operations required to complete the task through the particular disk controller hardware that is connected to that drive. The program is freed from needing to know how to control at a low level every type of hard disk (or display adapter, or port interface, or real-time clock peripheral) that it may need to access. This both makes programming operating systems and applications easier and makes

2673-483: The BIOS) is limited to accessing just over 1 MiB of memory, due to using 16-bit segmented memory addressing . Additionally, the BIOS is generally not the fastest way to carry out any particular task. In fact, the speed limitations of the BIOS made it common even in the DOS era for programs to circumvent it in order to avoid its performance limitations, especially for video graphics display and fast serial communication. Beyond

2754-486: The BIOS, this difference is nearly invisible. (As a good example of the other side of this issue, a significant share of the PC programs in use at the time the PCjr was introduced did not use the keyboard through BIOS exclusively, so IBM also included hardware features in the PCjr to emulate the way the original IBM PC and IBM PC XT keyboard hardware works. The hardware emulation is not exact, so not all programs that try to use

2835-529: The CGA-compatible text display hardware has only one global display cursor and cannot automatically advance the cursor, use the cursor position to address the display memory (so as to determine which character cell will be changed or examined), or interpret control characters. For another example, the BIOS keyboard interface interprets many keystrokes and key combinations to keep track of the various shift states (left and right Shift , Ctrl , and Alt ), to call

2916-469: The CPU executing software contained in ROM (for example, the BIOS of an IBM PC or an IBM PC compatible ) at a predefined address (some CPUs, including the Intel x86 series , are designed to execute this software after reset without outside help). This software contains rudimentary functionality to search for devices eligible to participate in booting, and load a small program from a special section (most commonly

2997-626: The CPU in Protected mode or Long mode generally do not use the BIOS interrupt calls to support system functions, although they use the BIOS interrupt calls to probe and initialize hardware during booting . Real mode has the 1MB memory limitation, modern boot loaders (e.g. GRUB2 , Windows Boot Manager ) use the unreal mode or protected mode (and execute the BIOS interrupt calls in the Virtual 8086 mode , but only for OS booting) to access up to 4GB memory. In all computers, software instructions control

3078-494: The IBM PC needed BASIC to compete. As on those other systems, the IBM PC's ROM BASIC served as a primitive diskless operating system, allowing the user to load, save, and run programs, as well as to write and refine them. (The original IBM PC was also the only PC model from IBM that, like its aforementioned two competitors, included cassette interface hardware. A base model IBM PC had only 16 KiB of RAM and no disk drives of any kind, so

3159-575: The ISR finishes, the CPU continues with the main program. On x86 CPUs, when an interrupt occurs, the ISR to call is found by looking it up in a table of ISR starting-point addresses (called "interrupt vectors") in memory: the Interrupt vector table (IVT). An interrupt is invoked by its type number, from 0 to 255, and the type number is used as an index into the Interrupt Vector Table, and at that index in

3240-482: The MBR and in conjunction with LOADER: In conjunction with GUID partition tables (GPT), an Enhanced Disk Drive Specification (EDD) 4 Hybrid MBR proposal recommends another extension to the MBR to VBR interface: Boot manager A bootloader , also spelled as boot loader or called bootstrap loader , is a computer program that is responsible for booting a computer. If it also provides an interactive menu with multiple boot choices then it's often called

3321-425: The above factors, problems with BIOS functionality include limitations in the range of functions defined, inconsistency in the subsets of those functions supported on different computers, and variations in the quality of BIOSes (i.e. some BIOSes are complete and reliable, others are abridged and buggy). By taking matters into their own hands and avoiding reliance on BIOS, operating system developers can eliminate some of

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3402-540: The boot order etc., however, this information is ignored by most standard MBRs and VBRs. Ideally, ES:DI is passed on, but PnP-enabled operating systems typically also have fallback methods to retrieve the PnP BIOS entry point later on so that most operating systems do not rely on this. Information in ES:DI can be used as a hint - according to the PnP BIOS specification, "$ PnP" installation check structure could be found by searching for

3483-435: The boot sector by reducing the reported INT 12h memory accordingly, so that they do not get overwritten by the MBR's and VBR's actions). The BIOS Boot Specification allows for 64 KB of memory and explicitly recommends 0000h:7C00h to 0000h:FFFFh as memory space for MBR and/or VBR. The boot code must not assume better CPUs than the original Intel 8088 or 8086 (used in the original PC) and make no assumptions in regard to

3564-539: The boot sector. Some BIOSes (like the IBM PC/AT) perform the check only for fixed disk / removable drives, while for floppies and superfloppies it is enough to start with a byte greater or equal to 06h and the first nine words not to contain the same value, before the boot sector is accepted as valid, thereby avoiding the explicit test for 55h , AAh on floppies. Since old boot sectors (i.e. very old CP/M-86 and DOS media) sometimes do not feature this signature despite

3645-414: The bootstrap code that individual operating systems install into a single partition's VBR and store them in disc files, loading the relevant VBR content from file after the boot loader has asked the user which operating system to bootstrap. In Windows Vista , Windows Server 2008 and newer versions, NTLDR was replaced; the boot-loader functionality is instead provided by two new components: WINLOAD.EXE and

3726-462: The cassette interface and BASIC in ROM were essential to make the base model usable. An IBM PC with less than 32 KiB of RAM is incapable of booting from disk. Of the five 8 KiB ROM chips in an original IBM PC, totaling 40 KiB, four contain BASIC and only one contains the BIOS; when only 16 KiB of RAM are installed, the ROM BASIC accounts for 4/7ths of the total system memory.) As time went on and BASIC

3807-450: The computer is ready to interact with the user, or the operating system is capable of running system programs or application programs. Many embedded systems must boot immediately. For example, waiting a minute for a digital television or a GPS navigation device to start is generally unacceptable. Therefore, such devices have software systems in ROM or flash memory so the device can begin functioning immediately; little or no loading

3888-406: The earlier IBM PC and compatibles, a boot sector should typically work with 510 bytes of code (or less) and in only 32 KiB (later relaxed to 64 KiB ) of system memory and only use instructions supported by the original 8088 / 8086 processors. The first stage of PC boot loaders (FSBL, first-stage boot loader) located on fixed disks and removable drives must fit into the first 446 bytes of

3969-621: The end of the actual sector size as well, however, the semantics described herein apply to the 16-bit signature at +1FEh only. This signature indicates the presence of at least a dummy boot loader which is safe to be executed, even if it may not be able to actually load an operating system. It does not indicate the presence of a (or even a particular) file system or operating system, although some old versions of DOS prior to 3.3 relied on it in their process to detect FAT -formatted media (newer versions do not). Boot code for other platforms or CPUs should not use this signature, since this may lead to

4050-521: The exact state of the hardware, the interrupt system (interrupts can be enabled or disabled) or the location and size of the stack. Although the original IBM BIOS initializes the DS, ES, and SS CPU registers to segment 0000h and maintains the initial stack at SS : SP = 0000h:0400h , this is not a condition to rely on, as not all BIOSes and MBR codes follow this convention. Registers not mentioned below must be treated as not initialized. Direct hardware access

4131-464: The extent that programs are written to use the BIOS exclusively) from being constrained to maintain exact hardware compatibility with old systems when designing new systems, in order to maintain compatibility with existing software. For example, the keyboard hardware on the IBM PCjr works very differently than the keyboard hardware on earlier IBM PC models, but to programs that use the keyboard only through

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4212-612: The facilities of the firmware on IBM PC compatible computers. Traditionally, BIOS calls are mainly used by DOS programs and some other software such as boot loaders (including, mostly historically, relatively simple application software that boots directly and runs without an operating system—especially game software). BIOS runs in the real address mode (Real Mode) of the x86 CPU, so programs that call BIOS either must also run in real mode or must switch from protected mode to real mode before calling BIOS and then switching back again. For this reason, modern operating systems that use

4293-565: The fact that they can be booted successfully, the check can be disabled in some environments. This also reflects the fact that floppies can be formatted to use smaller sector sizes than 512 bytes. If the BIOS or MBR code does not detect a valid boot sector and therefore cannot pass execution to the boot sector code, it will try the next boot device in the row. If they all fail it will typically display an error message and invoke INT 18h . This will either start up optional resident software in ROM ( ROM BASIC ), attempt to remote boot via network, reboot

4374-496: The first call. Many modern operating systems (such as Linux and Windows ) do not use any BIOS interrupt calls at all after startup, instead choosing to directly interface with the hardware. To do this, they rely upon drivers that are either a part of the OS kernel itself, ship along with the OS, or are provided by hardware vendors. There are several reasons for this practice. Most significant

4455-617: The form (usually stack-based) used by the high-level language to the register-based form required by BIOS, then back to the HLL calling convention after the BIOS returns. In some variants of C, BIOS calls can be made using inline assembly language within a C module. (Support for inline assembly language is not part of the ANSI C standard but is a language extension; therefore, C modules that use inline assembly language are less portable than pure ANSI standard C modules.) Invoking an interrupt can be done using

4536-585: The hardware and avoid BIOS completely, when programming the early IBM PC models (prior to the PS/2). From the beginning, programmers had the choice of using BIOS or not, on a per-hardware-peripheral basis. IBM did strongly encourage the authorship of "well-behaved" programs that accessed hardware only through BIOS INT calls (and DOS service calls), to support compatibility of software with current and future PC models having dissimilar peripheral hardware, but IBM understood that for some software developers and hardware customers,

4617-443: The hardware and initializes its state; finds, loads, and runs the boot program (usually, an OS boot loader, and historical ROM BASIC ); and provides basic hardware control to the software running on the machine, which is usually an operating system (with application programs) but may be a directly booting single software application. For IBM's part, they provided all the information needed to use their BIOS fully or to directly utilize

4698-440: The instructions and start their execution. These instructions typically start an input operation from some peripheral device (which may be switch-selectable by the operator). Other systems may send hardware commands directly to peripheral devices or I/O controllers that cause an extremely simple input operation (such as "read sector zero of the system device into memory starting at location 1000") to be carried out, effectively loading

4779-505: The keyboard hardware directly will work correctly on the PCjr, but all programs that use only the BIOS keyboard services will.) In addition to giving access to hardware facilities, BIOS provides added facilities that are implemented in the BIOS software. For example, the BIOS maintains separate cursor positions for up to eight text display pages and provides for TTY -like output with automatic line wrap and interpretation of basic control characters such as carriage return and line feed, whereas

4860-464: The lock-state indicators (LEDs). Operating systems and other software communicate with the BIOS software, in order to control the installed hardware, via software interrupts. A software interrupt is a specific variety of the general concept of an interrupt. An interrupt is a mechanism by which the CPU can be directed to stop executing the main-line program and immediately execute a special program, called an Interrupt Service Routine (ISR), instead. Once

4941-400: The nonvolatile device (usually block device , e.g., NAND flash) or devices from which the operating system programs and data can be loaded into RAM. Some earlier computer systems, upon receiving a boot signal from a human operator or a peripheral device, may load a very small number of fixed instructions into memory at a specific location, initialize at least one CPU, and then point the CPU to

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5022-601: The operating system subsequently initializes itself and may load extra device drivers . The second-stage boot loader does not need drivers for its own operation, but may instead use generic storage access methods provided by system firmware such as the BIOS or Open Firmware , though typically with restricted hardware functionality and lower performance. Second-stage implementations can include interactive user interfaces, allowing boot option selection and parameter modification. They handle kernel loading, including processing of initrd/initramfs images, and can pass boot parameters to

5103-453: The outcome some other way). Sometimes it can also be difficult to determine whether or not a certain BIOS function call is supported by the BIOS on a certain computer, or what the limits of a call's parameters are on that computer. (For some invalid function numbers, or valid function numbers with invalid values of key parameters—particularly with an early IBM BIOS version—the BIOS may do nothing and return with no error code; then it

5184-413: The physical hardware (screen, disk, keyboard, etc.) from the moment the power is switched on. In a PC, the BIOS, pre-loaded in ROM on the motherboard, takes control immediately after the CPU is reset, including during power-up, when a hardware reset button is pressed, or when a critical software failure (a triple fault ) causes the mainboard circuitry to automatically trigger a hardware reset. The BIOS tests

5265-465: The print-screen service when Shift + PrtScrn is pressed, to reboot the system when Ctrl + Alt + Del is pressed, to keep track of the lock states (Caps Lock, Num Lock, and Scroll Lock) and, in AT-class machines, control the corresponding lock-state indicator lights on the keyboard, and to perform other similar interpretive and management functions for the keyboard. In contrast, the ordinary capabilities of

5346-405: The program, or do both. A key element of the purpose of BIOS calls is abstraction - the BIOS calls perform generally defined functions, and the specific details of how those functions are executed on the particular hardware of the system are encapsulated in the BIOS and hidden from the program. So, for example, a program that wants to read from a hard disk does not need to know whether the hard disk

5427-456: The programs smaller, reducing the duplication of program code, as the functionality that is included in the BIOS does not need to be included in every program that needs it; relatively short calls to the BIOS are included in the programs instead. (In operating systems where the BIOS is not used, service calls provided by the operating system itself generally fulfill the same function and purpose.) The BIOS also frees computer hardware designers (to

5508-678: The risks and complications they face in writing and supporting system software. On the other hand, by doing so those developers become responsible for providing "bare-metal" driver software for every different system or peripheral device they intend for their operating system to work with (or for inducing the hardware producers to provide those drivers). Thus it should be apparent that compact operating systems developed on small budgets would tend to use BIOS heavily, while large operating systems built by huge groups of software engineers with large budgets would more often opt to write their own drivers instead of using BIOS—that is, even without considering

5589-477: The root directory of the file system and stored within consecutive sectors, conditions taken care of by the SYS command and slightly relaxed in later versions of DOS. The boot loader was then able to load the first three sectors of the file into memory, which happened to contain another embedded boot loader able to load the remainder of the file into memory. When Microsoft added LBA and FAT32 support, they switched to

5670-444: The same BIOS program and are not independent. (This last point is relevant to reentrancy .) The BIOS software usually returns to the caller with an error code if not successful, or with a status code and/or requested data if successful. The data itself can be as small as one bit or as large as 65,536 bytes of whole raw disk sectors (the maximum that will fit into one real-mode memory segment). BIOS has been expanded and enhanced over

5751-402: The same vendor or in a BIOS version—of any relative age—from a different vendor.) Because BIOS interrupt calls use CPU register-based parameter passing, the calls are oriented to being made from assembly language and cannot be directly made from most high-level languages (HLLs). However, a high level language may provide a library of wrapper routines which translate parameters from

5832-430: The same way, but data too large to fit in registers, such as tables of control parameters or disk sector data for disk transfers, is passed by allocating a buffer (i.e. some space) in memory and passing the address of the buffer in registers. (Sometimes multiple addresses of data items in memory may be passed in a data structure in memory, with the address of that structure passed to BIOS in registers.) The interrupt number

5913-446: The second-stage bootloader. Its primary challenge lies in accomplishing these tasks within strict size constraints while handling potential hardware failures. The bootloader must navigate disk structures, often implementing FAT file system support, and manage the delicate transition from the BIOS startup state to a stable environment for the next boot stage. Boot loaders may face peculiar constraints, especially in size; for instance, on

5994-408: The standard PC and PC-AT keyboard hardware are limited to reporting to the system each primitive event of an individual key being pressed or released (i.e. making a transition from the "released" state to the "depressed" state or vice versa), performing a commanded reset and self-test of the keyboard unit, and, for AT-class keyboards, executing a command from the host system to set the absolute states of

6075-867: The strict 512-byte limitation of their first-stage counterparts. They execute in a more sophisticated environment, typically ranging from 8KB to several megabytes in size. This expanded space allows implementation of complex features including multiple filesystem support, runtime configuration, and bootloader menu interfaces. Second-stage bootloaders perform comprehensive hardware initialization. They query and configure various system components including memory controllers , interrupt controllers , and essential peripherals. Modern implementations often handle ACPI tables, USB controller initialization, and preliminary graphics setup. Second-stage boot loaders, such as GNU GRUB , rEFInd , BOOTMGR , Syslinux , NTLDR or iBoot , are not themselves operating systems, but are able to load an operating system properly and transfer execution to it;

6156-485: The system functionality is limited until the operating system kernel and drivers are transferred and started. As a result, once the ROM-based booting has completed it is entirely possible to network boot into an operating system that itself does not have the ability to use the network interface. INT 18h BIOS implementations provide interrupts that can be invoked by operating systems and application programs to use

6237-706: The system via INT 19h after user confirmation, or cause the system to halt the bootstrapping process until the next power-up. From decimal offset 72 to 79, an NTFS PBR contains the partition UUID volume ID serial number. The boot code in the VBR can assume that the BIOS has set up its data structures and interrupts and initialized the hardware. The code should not assume more than 32 KB of memory to be present for fail-safe operation; if it needs more memory it should query INT 12h for it, since other pre-boot code (such as f.e. BIOS extension overlays, encryption systems, or remote bootstrap loaders ) may be present elsewhere in memory as well (and would typically hide themselves from

6318-521: The table is found the address of the ISR that will be run in response to the interrupt. A software interrupt is simply an interrupt that is triggered by a software command; therefore, software interrupts function like subroutines, with the main difference that the program that makes a software interrupt call does not need to know the address of the ISR, only its interrupt number. This has advantages for modularity, compatibility, and flexibility in system configuration. BIOS interrupt calls can be thought of as

6399-434: The years many times by many different corporate entities, and unfortunately the result of this evolution is that not all the BIOS functions that can be called use consistent conventions for formatting and communicating data or for reporting results. Some BIOS functions report detailed status information, while others may not even report success or failure but just return silently, leaving the caller to assume success (or to test

6480-472: Was incompatible with the IBM BIOS. Turbo Pascal , Turbo C and Turbo C++ repurposed INT 18 for memory allocation and paging. Other programs also reused this vector for their own purposes. On DOS systems, IO.SYS or IBMBIO.COM hooks INT 13 for floppy disk change detection, tracking formatting calls, correcting DMA boundary errors, and working around problems in IBM's ROM BIOS "01/10/84" with model code 0xFC before

6561-484: Was no longer shipped on all PCs, this interrupt would simply display an error message indicating that no bootable volume was found (such as "No ROM BASIC", or more explanatory messages in later BIOS versions); in other BIOS versions it would prompt the user to insert a bootable volume and press a key, and then after the user pressed a key it would loop back to the bootstrap loader (INT 19h) to try booting again. Digital's Rainbow 100 B used INT 18h to call its BIOS, which

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