3 Getting Started22BIOS POST Checkpoint CodesThe POST code checkpoints are references and instructions usedduring the BIOS pre-boot process. The following table describesthe checkpoints and associated beep codes, if any, that mayoccur during the POST portion of the BIOS.CheckpointCode Beep Code Description01h One shortbeep beforeboot02h Verify Real Mode. If the CPU is in protectedmode, turn on A20 and pulse the reset line,forcing a shutdown 0.NOTE: Hook routine should not alter DX,which holds the powerup CPUID.03h Disabel Non-Maskable Interrupts (NMI).04h Get CPU type from CPU registers and othermethods.Save CPU type in NVRAM.NOTE: Hook routine should not alter DX,which holds the powerup CPUID.06h Initialize system hardware. Reset the DMAcontrollers, disable video, clear any pendinginterrupts from the real-time clock andsetup port B register.07h Disable system ROM shadow and start toexecute ROMEXEC code from the flash part.This task is pulled into the build only whenthe ROMEXEC relocation is installed.08h Initialize chipset registers to the Initial POSTValues.
