[BIOS]
The
Basic Input Output System is
the system software embedded on a chip located on the computer’s main board.
The BIOS executes POST to test and initialize the system components and then
boots the operating system. The BIOS also handles the low-level input/output to
the various peripheral devices connected to the computer at run time.
Additionally, most BIOSes have a Setup program that allows the user to configure
the system.
[Option
ROM]
An
Option ROM (Read Only Memory) is a software component located in a ROM chip on
an add-in card or the system board. Its physical address is in system memory
between addresses C0000h and DFFFFh. The BIOS may copy the component to shadow
memory during POST. An Option ROM is characterized by the first two locations
containing a two-byte signature of 55h AAh. Option ROMs are responsible for initializing
their associated hardware, allowing it to be available to the rest of the
system for booting or run time.
[Gate
A20]
Gate
A20 controls 1MB memory wrap-around. When Gate A20 is enabled, it forces memory
accesses to wrap around and fall within the 0-1MB area by forcing address line
20 to be zero. This has the effect of not allowing access to extended memory.
When Gate A20 is disabled, memory accesses beyond 1MB do not wrap around, thus
allowing access to all of extended memory.
[Flat
Real Mode]
Real
mode, also called real address mode, is an operating mode of all x86-compatible
CPUs. Real mode is characterized by a 20-bit segmented memory address space
(giving exactly 1 MB of addressable memory) and unlimited direct software
access to all memory, I/O addresses and peripheral hardware. Real mode provides
no support for memory protection, multitasking, or code privilege levels.
Before the release of the 80286, which introduced Protected mode, real mode was
the only available mode for x86 CPUs. In the interests of backwards compatibility,
all x86 CPUs start in real mode when reset.
[PMM]
The
POST Memory Manager is a software component of the BIOS that provides for the
allocation of memory blocks during system POST.
[Extended BIOS Data Area (EBDA)]
Memory location 40:0e (2 bytes) contains EBDA segment.
Memory location 40:13 (2 bytes) contains size of conventional memory in units of 1K.
Example:
40:0e = 0x8e00 40:0e = 0x8d80
40:13 = 0x266 ==> 40:13 = 0x264
8e00:0 = 0x08(K) 8d80:0 = 0x0A(K)
[POST]
The
Power-On Self-Test is the part of the BIOS that takes control immediately after
the computer is turned on. POST initializes the computer hardware in
preparation for loading the operating system.
[Run-Time]
Execution
of run-time software takes place after the operating system has loaded. BIOS
run-time services are available at POST and remain callable after the operating
system has booted. Application programs and operating systems call BIOS
run-time services for hardware-related functions. The PMM Services are not
callable during run-time, and buffers allocated by the PMM during POST are not
available at runtime.
[Header Structure]
BCV:
Boot Connection Vector
