coreboot consists of multiple stages that are compiled as separate binaries and are inserted into the CBFS with custom compression. The bootblock usually doesn’t have compression while the ramstage and payload are compressed with LZMA.
Each stage loads the next stage a given address (possibly decompressing it).
Some stages are relocatable and can be placed anywhere in DRAM. Those stages are usually cached in CBMEM for faster loading times on ACPI S3 resume.
Supported stage compressions:
The bootblock is the first stage executed after CPU reset. It is written in assembly language and its main task is to set up everything for a C-environment:
- Cache-As-RAM for heap and stack
- Set stack pointer
- Clear memory for BSS
- Decompress and load the next stage
On x86 platforms that includes:
- Microcode updates
- Timer init
- Switching from 16-bit real-mode to 32-bit protected mode
The bootblock loads the romstage or the verstage if verified boot is enabled.
The Cache-As-Ram, also called Non-Eviction mode, or CAR allows to use the
CPU cache like regular SRAM. This is particullary usefull for high level
C, which need RAM for heap and stack.
The CAR needs to be activated using vendor specific CPU instructions.
The following stages run when Cache-As-Ram is active:
The verstage is where the root-of-trust starts. It’s assumed that it cannot be overwritten in-field (together with the public key) and it starts at the very beginning of the boot process. The verstage installs a hook to verify a file before it’s loaded from CBFS or a partition before it’s accessed.
The verified boot mechanism allows trusted in-field firmware updates combined with a fail-safe recovery mode.
The romstage initializes the DRAM and prepares everything for device init.
- Early device init
- DRAM init
To leave the CAR setup and run code from regular DRAM the postcar-stage tears down CAR and loads the ramstage. Compared to other stages it’s minimal in size.
The ramstage does the main device init:
- PCI device init
- On-chip device init
- TPM init (if not done by verstage)
- Graphics init (optional)
- CPU init (like set up SMM)
After initialization tables are written to inform the payload or operating system about the current hardware existance and state. That includes:
- ACPI tables (x86 specific)
- SMBIOS tables (x86 specific)
- coreboot tables
- devicetree updates (ARM specific)
It also does hardware and firmware lockdown:
- Write-protection of boot media
- Lock security related registers
- Lock SMM mode (x86 specific)