This page provides a high-level description of some of the major code phases that SeaBIOS transitions through and general information on overall code flow.

SeaBIOS code phases

The SeaBIOS code goes through a few distinct code phases during its execution lifecycle. Understanding these code phases can help when reading and enhancing the code.

POST phase

The Power On Self Test (POST) phase is the initialization phase of the BIOS. This phase is entered when SeaBIOS first starts execution. The goal of the phase is to initialize internal state, initialize external interfaces, detect and setup hardware, and to then start the boot phase.

On emulators, this phase starts when the CPU starts execution in 16bit mode at 0xFFFF0000:FFF0. The emulators map the SeaBIOS binary to this address, and SeaBIOS arranges for romlayout.S:reset_vector() to be present there. This code calls romlayout.S:entry_post() which then calls post.c:handle_post() in 32bit mode.

On coreboot, the build arranges for romlayout.S:entry_elf() to be called in 32bit mode. This then calls post.c:handle_post().

On CSM, the build arranges for romlayout.S:entry_csm() to be called (in 16bit mode). This then calls csm.c:handle_csm() in 32bit mode. Unlike on the emulators and coreboot, the SeaBIOS CSM POST phase is orchastrated with UEFI and there are several calls back and forth between SeaBIOS and UEFI via handle_csm() throughout the POST process.

The POST phase itself has several sub-phases.

  • The "preinit" sub-phase: code run prior to [code relocation](Linking overview#Code relocation).
  • The "init" sub-phase: code to initialize internal variables and interfaces.
  • The "setup" sub-phase: code to setup hardware and drivers.
  • The "prepboot" sub-phase: code to finalize interfaces and prepare for the boot phase.

At completion of the POST phase, SeaBIOS invokes an "int 0x19" software interrupt in 16bit mode which begins the boot phase.

Boot phase

The goal of the boot phase is to load the first portion of the operating system's boot loader into memory and start execution of that boot loader. This phase starts when a software interrupt ("int 0x19" or "int 0x18") is invoked. The code flow starts in 16bit mode in romlayout.S:entry_19() or romlayout.S:entry_18() which then transition to 32bit mode and call boot.c:handle_19() or boot.c:handle_18().

The boot phase is technically also part of the "runtime" phase of SeaBIOS. It is typically invoked immiediately after the POST phase, but it can also be invoked by an operating system or be invoked multiple times in an attempt to find a valid boot media. Although the boot phase C code runs in 32bit mode it does not have write access to the 0x0f0000-0x100000 memory region and can not call the various malloc_X() calls. See [Memory Model](Memory Model) for more information.

Main runtime phase

The main runtime phase occurs after the boot phase starts the operating system. Once in this phase, the SeaBIOS code may be invoked by the operating system using various 16bit and 32bit calls. The goal of this phase is to support these legacy calling interfaces and to provide compatibility with BIOS standards. There are multiple entry points for the BIOS - see the entry_XXX() assembler functions in romlayout.S.

Callers use most of these legacy entry points by setting up a particular CPU register state, invoking the BIOS, and then inspecting the returned CPU register state. To handle this, SeaBIOS will backup the current register state into a "struct bregs" (see romlayout.S, entryfuncs.S, and bregs.h) on call entry and then pass this struct to the C code. The C code can then inspect the register state and modify it. The assembler entry functions will then restore the (possibly modified) register state from the "struct bregs" on return to the caller.

Resume and reboot

As noted above, on emulators SeaBIOS handles the 0xFFFF0000:FFF0 machine startup execution vector. This vector is also called on machine faults and on some machine "resume" events. It can also be called (as 0xF0000:FFF0) by software as a request to reboot the machine (on emulators, coreboot, and CSM).

The SeaBIOS "resume and reboot" code handles these calls and attempts to determine the desired action of the caller. Code flow starts in 16bit mode in romlayout.S:reset_vector() which calls romlayout.S:entry_post() which calls romlayout.S:entry_resume() which calls resume.c:handle_resume(). Depending on the request the handle_resume() code may transition to 32bit mode.

Technically this code is part of the "runtime" phase, so even though parts of it run in 32bit mode it still has the same limitations of the runtime phase.


Internally SeaBIOS implements a simple cooperative multi-tasking system. The system works by giving each "thread" its own stack, and the system round-robins between these stacks whenever a thread issues a yield() call. This "threading" system may be more appropriately described as coroutines. These "threads" do not run on multiple CPUs and are not preempted, so atomic memory accesses and complex locking is not required.

The goal of these threads is to reduce overall boot time by parallelizing hardware delays. (For example, by allowing the wait for an ATA harddrive to spinup and respond to commands to occur in parallel with the wait for a PS/2 keyboard to respond to a setup command.) These hardware setup threads are only available during the "setup" sub-phase of the POST phase.

The code that implements threads is in stacks.c.

Hardware interrupts

The SeaBIOS C code always runs with hardware interrupts disabled. All of the C code entry points (see romlayout.S) are careful to explicitly disable hardware interrupts (via "cli"). Because running with interrupts disabled increases interrupt latency, any C code that could loop for a significant amount of time (more than about 1 ms) should periodically call yield(). The yield() call will briefly enable hardware interrupts to occur, then disable interrupts, and then resume execution of the C code.

There are two main reasons why SeaBIOS always runs C code with interrupts disabled. The first reason is that external software may override the default SeaBIOS handlers that are called on a hardware interrupt event. Indeed, it is common for DOS based applications to do this. These legacy third party interrupt handlers may have undocumented expections (such as stack location and stack size) and may attempt to call back into the various SeaBIOS software services. Greater compatibility and more reproducible results can be achieved by only permitting hardware interrupts at specific points (via yield() calls). The second reason is that much of SeaBIOS runs in 32bit mode. Attempting to handle interrupts in both 16bit mode and 32bit mode and switching between modes to delegate those interrupts is an unneeded complexity. Although disabling interrupts can increase interrupt latency, this only impacts legacy systems where the small increase in interrupt latency is unlikely to be noticeable.

Extra 16bit stack

SeaBIOS implements 16bit real mode handlers for both hardware interrupts and software request "interrupts". In a traditional BIOS, these requests would use the caller's stack space. However, the minimum amount of space the caller must provide has not been standardized and very old DOS programs have been observed to allocate very small amounts of stack space (100 bytes or less).

By default, SeaBIOS now switches to its own stack on most 16bit real mode entry points. This extra stack space is allocated in ["low memory"](Memory Model). It ensures SeaBIOS uses a minimal amount of a callers stack (typically no more than 16 bytes) for these legacy calls. (More recently defined BIOS interfaces such as those that support 16bit protected and 32bit protected mode calls standardize a minimum stack size with adequete space, and SeaBIOS generally will not use its extra stack in these cases.)

The code to implement this stack "hopping" is in romlayout.S and in stacks.c.