The startup header

The startup header (or boot header) structure is defined in startup.h.

This header file is located in the /src/hardware/startup/lib/public/sys/ directory in the BSP. The structure is named startup_header, is 256 bytes, and its members are populated by mkifs when the bootable image is built. The IPL and/or startup code examine the members of the structure to learn about the contents of the image and how to use it. These members include:

signature, version, flags1, flags2, header_size, machine, startup_vaddr, paddr_bias, image_paddr, ram_paddr, ram_size, startup_size, stored_size, imagefs_paddr, imagefs_size, preboot_size, zero0, zero[3], info[48]

The table below lists the startup_header members and describes their uses. The “Used by” column indicates which components use the member: the IPL, the startup code, both, or N/A.

Member	Type	Description	Used by
`signature`	_Uint32t	The `signature` field is the first 32 bits of the header. It is used to identify the header, and always contains `0x00FF7EEB` in native byte order. Platforms that can be either big-endian or little-endian (bi-endian) typically have a hardware strap that sets the board's endianness. The `signature` member must be set appropriately for the selected endianness.	IPL
`version`	_Uint16t	Version of mkifs that made the image.	IPL
`flags1`	_Uint8t	`flags1` is single-byte to ensure that it's endian-neutral. The following values are defined for `flags1` (`flags2` is currently not used): STARTUP_HDR_FLAGS1_VIRTUAL If this flag is set, the OS is to run with the Memory Management Unit (MMU) enabled. Note: You should always specify a virtual system, by using the `virtual=` attribute in your buildfile, which then sets the STARTUP_HDR_FLAGS1_VIRTUAL flag. STARTUP_HDR_FLAGS1_BIGENDIAN If this flag is set, the IPL is configured for a big-endian processor. If it isn't set, the IPL is configured for a little-endian processor. The IPL should always examine this flag to check that the endianness is correctly set for the processor. STARTUP_HDR_FLAGS1_COMPRESS_NONE The image isn't compressed. STARTUP_HDR_FLAGS1_COMPRESS_ZLIB The image is compressed using libz (gzip). STARTUP_HDR_FLAGS1_COMPRESS_LZO The image is compressed with liblzo. STARTUP_HDR_FLAGS1_COMPRESS_UCL The image is compressed with libucl. This format is chosen when using the `[+compress]` attribute in the mkifs build script. Note: The STARTUP_HDR_FLAGS1_COMPRESS_* constants enumerate compression types. They may set more than one bit, so they aren't strictly flags. The standard startup-* programs are built to understand only the UCL compression method. However, you can adjust the SUPPORT_CMP_* macro definitions in startup/lib/uncompress.c to change to one of the other supported compression methods.	IPL and Startup
`flags2`	_Uint8t	For future use.	N/A
`header_size`	_Uint16t	Size of the startup header, in bytes.	Startup
`machine`	_Uint16t	Machine type, from sys/elf.h.	IPL and Startup
`startup_vaddr`	_Uint32t	Virtual address to jump to after IPL is done.	IPL
`paddr_bias`	_Uint32t	Value to add to a physical address to calculate the value to put into a pointer.	Startup
`image_paddr`	_Uint32t	Set by the IPL to the physical address of the image filesystem.	IPL and Startup
`ram_paddr`	_Uint32t	Physical address in RAM to which the startup code must be copied. The size of the startup code is stored in `startup_size` member.	IPL and Startup
`ram_size`	_Uint32t	Number of bytes the image will occupy when it has been copied into RAM. This size may be greater than `stored_size` if the image was compressed, or smaller than `stored_size` if the image is XIP.	Startup
`startup_size`	_Uint32t	Size of the startup code. Copy this number of bytes from the start of the image into RAM, at the location specified by `ram_paddr`. The startup code is never compressed, so this size is true in all cases.	IPL
`stored_size`	_Uint32t	Size of the image including the header. The `stored_size` member is also used in the copy and extraction routines for non-XIP images.	IPL
`imagefs_paddr`	_Uint32t	Set by the IPL to the physical address of the image filesystem. This value is then used by the startup code.	IPL and Startup
`imagefs_size`	_Uint32t	Size of the uncompressed image filesystem.	Startup
`preboot_size`	unsigned short	Number of bytes from the beginning of the loaded image to the startup header. This value will usually be zero on ARM boards, indicating that nothing precedes the startup portion. On an x86 board with a BIOS or UEFI, this value is non-zero, because there's a small piece of code that gets data from the BIOS or UEFI in real mode, then switches into protected mode and performs the startup. On an x86 board without an BIOS or UEFI, this value is zero.	IPL
`zero0`	_Uint16t	Zero filler; reserved for future expansion.	N/A
`zero[3]`	_Uint32t	Zero filler; reserved for future expansion.	N/A
`info[48]`	_Uint32t	The `info` member is an array of startup_info* structures, which are described in the next section.	IPL and Startup