boot-image-header.rst (2301B)
1================================= 2Boot image header in RISC-V Linux 3================================= 4 5:Author: Atish Patra <atish.patra@wdc.com> 6:Date: 20 May 2019 7 8This document only describes the boot image header details for RISC-V Linux. 9 10TODO: 11 Write a complete booting guide. 12 13The following 64-byte header is present in decompressed Linux kernel image:: 14 15 u32 code0; /* Executable code */ 16 u32 code1; /* Executable code */ 17 u64 text_offset; /* Image load offset, little endian */ 18 u64 image_size; /* Effective Image size, little endian */ 19 u64 flags; /* kernel flags, little endian */ 20 u32 version; /* Version of this header */ 21 u32 res1 = 0; /* Reserved */ 22 u64 res2 = 0; /* Reserved */ 23 u64 magic = 0x5643534952; /* Magic number, little endian, "RISCV" */ 24 u32 magic2 = 0x05435352; /* Magic number 2, little endian, "RSC\x05" */ 25 u32 res3; /* Reserved for PE COFF offset */ 26 27This header format is compliant with PE/COFF header and largely inspired from 28ARM64 header. Thus, both ARM64 & RISC-V header can be combined into one common 29header in future. 30 31Notes 32===== 33 34- This header can also be reused to support EFI stub for RISC-V in future. EFI 35 specification needs PE/COFF image header in the beginning of the kernel image 36 in order to load it as an EFI application. In order to support EFI stub, 37 code0 should be replaced with "MZ" magic string and res3(at offset 0x3c) should 38 point to the rest of the PE/COFF header. 39 40- version field indicate header version number 41 42 ========== ============= 43 Bits 0:15 Minor version 44 Bits 16:31 Major version 45 ========== ============= 46 47 This preserves compatibility across newer and older version of the header. 48 The current version is defined as 0.2. 49 50- The "magic" field is deprecated as of version 0.2. In a future 51 release, it may be removed. This originally should have matched up 52 with the ARM64 header "magic" field, but unfortunately does not. 53 The "magic2" field replaces it, matching up with the ARM64 header. 54 55- In current header, the flags field has only one field. 56 57 ===== ==================================== 58 Bit 0 Kernel endianness. 1 if BE, 0 if LE. 59 ===== ==================================== 60 61- Image size is mandatory for boot loader to load kernel image. Booting will 62 fail otherwise.