Linux的16-bit to 32-bit跳转
Linux kernel的setup.S中向0x100000,__BOOT_CS跳转用的是手写的机器码。《自己动手写操作系统》里面解释说：这是因为当前代码处在.code16中，如果用普通写法汇编器会将__BOOT_CS截断。我查看了一下GRUB的代码，那里面用.code16和.code32的配合做得很好，无需手写机器码。Linux kernel setup.S中的做法是不是有些不合时宜了？
[B]Linux的做法： [/B]
# jump to startup_32 in arch/i386/boot/compressed/head.S
#
# NOTE: For high loaded big kernels we need a
# jmpi 0x100000,__BOOT_CS
#
# but we yet haven't reloaded the CS register, so the default size
# of the target offset still is 16 bit.
# However, using an operand prefix (0x66), the CPU will properly
# take our 48 bit far pointer. (INTeL 80386 Programmer's Reference
# Manual, Mixing 16-bit and 32-bit code, page 16-6)
.byte 0x66, 0xea # prefix + jmpi-opcode
code32: .long 0x1000 # will be set to 0x100000
# for big kernels
.word __BOOT_CS
[B]GRUB的做法： [/B]
DATA32 ljmp $GRUB_MEMORY_MACHINE_PROT_MODE_CSEG, $protcseg
.code32
protcseg:
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