[lkrnprefix] Function as a bzImage kernel
The .lkrn prefix currently provides a zImage kernel with unused setup sectors and the whole iPXE binary placed within the "protected mode kernel" portion of the zImage. The work carried out years ago to create the .mrom format provides a mechanism allowing the iPXE binary to be split into a small real-mode header and a larger payload. This neatly matches the way that a bzImage is loaded: the "setup sectors" can contain the header and the "protected mode kernel" can contain the payload. This removes the size restrictions on an iPXE .lkrn image (and hence on derived image formats such as .iso). Also remove obsolete copyright information, since none of the original code or functionality now remains. Signed-off-by: Michael Brown <mcb30@ipxe.org>
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Michael Brown
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@ -1,123 +1,59 @@
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/*
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Copyright (C) 2000, Entity Cyber, Inc.
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Authors: Gary Byers (gb@thinguin.org)
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Marty Connor (mdc@thinguin.org)
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This software may be used and distributed according to the terms
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of the GNU Public License (GPL), incorporated herein by reference.
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Description:
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This is just a little bit of code and data that can get prepended
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to a ROM image in order to allow bootloaders to load the result
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as if it were a Linux kernel image.
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A real Linux kernel image consists of a one-sector boot loader
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(to load the image from a floppy disk), followed a few sectors
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of setup code, followed by the kernel code itself. There's
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a table in the first sector (starting at offset 497) that indicates
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how many sectors of setup code follow the first sector and which
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contains some other parameters that aren't interesting in this
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case.
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When a bootloader loads the sectors that comprise a kernel image,
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it doesn't execute the code in the first sector (since that code
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would try to load the image from a floppy disk.) The code in the
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first sector below doesn't expect to get executed (and prints an
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error message if it ever -is- executed.)
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We don't require much in the way of setup code. Historically, the
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Linux kernel required at least 4 sectors of setup code.
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Therefore, at least 4 sectors must be present even though we don't
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use them.
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*/
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FILE_LICENCE ( GPL_ANY )
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#define SETUPSECS 4 /* Minimal nr of setup-sectors */
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#define PREFIXSIZE ((SETUPSECS+1)*512)
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#define PREFIXPGH (PREFIXSIZE / 16 )
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#define BOOTSEG 0x07C0 /* original address of boot-sector */
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#define INITSEG 0x9000 /* we move boot here - out of the way */
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#define SETUPSEG 0x9020 /* setup starts here */
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#define SYSSEG 0x1000 /* system loaded at 0x10000 (65536). */
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#define BZI_RM_SEGMENT 0x1000
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#define BZI_LOAD_HIGH_ADDR 0x100000
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.text
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.code16
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.arch i386
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.org 0
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.code16
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.section ".prefix", "ax", @progbits
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.globl _lkrn_start
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_lkrn_start:
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/*
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This is a minimal boot sector. If anyone tries to execute it (e.g., if
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a .lilo file is dd'ed to a floppy), print an error message.
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*/
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bootsector:
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jmp $BOOTSEG, $1f /* reload cs:ip to match relocation addr */
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1:
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movw $0x2000, %di /* 0x2000 is arbitrary value >= length
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of bootsect + room for stack */
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movw $BOOTSEG, %ax
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movw %ax,%ds
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movw %ax,%es
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cli
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movw %ax, %ss /* put stack at BOOTSEG:0x2000. */
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movw %di,%sp
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sti
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movw $why_end-why, %cx
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movw $why, %si
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movw $0x0007, %bx /* page 0, attribute 7 (normal) */
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movb $0x0e, %ah /* write char, tty mode */
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prloop:
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lodsb
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int $0x10
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loop prloop
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freeze: jmp freeze
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why: .ascii "This image cannot be loaded from a floppy disk.\r\n"
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why_end:
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/*
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The following header is documented in the Linux source code at
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Documentation/x86/boot.txt
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*/
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.org 497
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setup_sects:
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.byte SETUPSECS
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root_flags:
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.word 0
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syssize:
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.long -PREFIXPGH
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/*****************************************************************************
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*
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* Kernel header
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*
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* We place our prefix (i.e. our .prefix and .text16.early sections)
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* within the bzImage real-mode portion which gets loaded at
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* 1000:0000, and our payload (i.e. everything else) within the
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* bzImage protected-mode portion which gets loaded at 0x100000
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* upwards.
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*
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*/
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.org 0x1f1
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setup_sects:
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.byte -1 /* Allow for initial "boot sector" */
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.section ".zinfo.fixup", "a", @progbits /* Compressor fixups */
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.ascii "ADDL"
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.ascii "ADHL"
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.long setup_sects
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.long 512
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.long 0
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.previous
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root_flags:
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.word 0
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syssize:
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.long 0
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.section ".zinfo.fixup", "a", @progbits /* Compressor fixups */
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.ascii "ADPL"
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.long syssize
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.long 16
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.long 0
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.previous
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ram_size:
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ram_size:
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.word 0
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vid_mode:
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vid_mode:
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.word 0
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root_dev:
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root_dev:
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.word 0
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boot_flag:
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.word 0xAA55
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boot_flag:
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.word 0xaa55
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jump:
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/* Manually specify a two-byte jmp instruction here rather
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* than leaving it up to the assembler. */
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.byte 0xeb
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.byte setup_code - header
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* than leaving it up to the assembler.
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*/
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.byte 0xeb, ( setup - header )
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header:
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.byte 'H', 'd', 'r', 'S'
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version:
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@ -125,13 +61,13 @@ version:
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realmode_swtch:
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.long 0
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start_sys:
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.word 0
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.word BZI_RM_SEGMENT
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kernel_version:
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.word version_string - 0x200
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type_of_loader:
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.byte 0
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loadflags:
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.byte 0
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.byte 0x01 /* LOADED_HIGH */
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setup_move_size:
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.word 0
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code32_start:
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@ -144,21 +80,22 @@ bootsect_kludge:
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.long 0
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heap_end_ptr:
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.word 0
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pad1:
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.word 0
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ext_loader_ver:
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.byte 0
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ext_loader_type:
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.byte 0
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cmd_line_ptr:
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.long 0
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initrd_addr_max:
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/* We don't use an initrd but some bootloaders (e.g. SYSLINUX) have
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* been known to require this field. Set the value to 2 GB. This
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* value is also used by the Linux kernel. */
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.long 0x7fffffff
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.long 0xffffffff
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kernel_alignment:
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.long 0
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relocatable_kernel:
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.byte 0
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pad2:
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.byte 0, 0, 0
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min_alignment:
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.byte 0
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xloadflags:
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.word 0
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cmdline_size:
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.long 0x7ff
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hardware_subarch:
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@ -169,28 +106,16 @@ hardware_subarch_data:
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version_string:
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.asciz VERSION
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/*
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We don't need to do too much setup.
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This code gets loaded at SETUPSEG:0. It wants to start
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executing the image that's loaded at SYSSEG:0 and
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whose entry point is SYSSEG:0.
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*/
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setup_code:
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/* We expect to be contiguous in memory once loaded. The Linux image
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* boot process requires that setup code is loaded separately from
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* "non-real code". Since we don't need any information that's left
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* in the prefix, it doesn't matter: we just have to ensure that
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* %cs:0000 is where the start of the image *would* be.
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*/
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ljmp $(SYSSEG-(PREFIXSIZE/16)), $run_ipxe
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.org PREFIXSIZE
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/*
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We're now at the beginning of the kernel proper.
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/*****************************************************************************
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*
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* Setup code
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*
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*/
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run_ipxe:
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setup:
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/* Fix up code segment */
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ljmp $BZI_RM_SEGMENT, $1f
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1:
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/* Set up stack just below 0x7c00 and clear direction flag */
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xorw %ax, %ax
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movw %ax, %ss
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@ -198,7 +123,7 @@ run_ipxe:
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cld
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/* Retrieve command-line pointer */
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movl %ds:cmd_line_ptr, %edx
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movl cmd_line_ptr, %edx
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testl %edx, %edx
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jz no_cmd_line
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jnz 1f
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orl $0xffffffff, %ebp /* Allow arbitrary relocation if no initrd */
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1:
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/* Install iPXE */
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call alloc_basemem
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xorl %esi, %esi
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@ -282,3 +206,30 @@ no_cmd_line:
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/* Boot next device */
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int $0x18
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/*****************************************************************************
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*
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* Open payload (called by libprefix)
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*
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* Parameters:
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* %ds:0000 : Prefix
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* %esi : Buffer for copy of image source (or zero if no buffer available)
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* %ecx : Expected offset within buffer of first payload block
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* Returns:
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* %esi : Valid image source address (buffered or unbuffered)
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* %ecx : Actual offset within buffer of first payload block
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* CF set on error
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*/
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.section ".text16.early", "awx", @progbits
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.globl open_payload
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open_payload:
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/* Our payload will always end up at BZI_LOAD_HIGH_ADDR */
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movl $BZI_LOAD_HIGH_ADDR, %esi
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xorl %ecx, %ecx
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lret
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/* Payload must be aligned to a whole number of setup sectors */
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.globl _payload_align
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.equ _payload_align, 512
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