linux内核原代码boot.s部分的注释

　　boot.s
　　|
　　| boot.s is loaded at 0x7c00 by the bios-startup routines, and moves itself
　　| out of the way to address 0x90000, and jumps there.
　　|
　　| It then loads the system at 0x10000, using BIOS interrupts. Thereafter
　　| it disables all interrupts, moves the system down to 0x0000, changes
　　| to protected mode, and calls the start of system. System then must
　　| RE-initialize the protected mode in it's own tables, and enable
　　| interrupts as needed.
　　|
　　| NOTE! currently system is at most 8*65536 bytes long. This should be no
　　| problem, even in the future. I want to keep it simple. This 512 kB
　　| kernel size should be enough - in fact more would mean we'd have to move
　　| not just these start-up routines, but also do something about the cache-
　　| memory (block IO devices). The area left over in the lower 640 kB is meant
　　| for these. No other memory is assumed to be "physical", ie all memory
　　| over 1Mb is demand-paging. All addresses under 1Mb are guaranteed to match
　　| their physical addresses.
　　|
　　| NOTE1 abouve is no longer valid in it's entirety. cache-memory is allocated
　　| above the 1Mb mark as well as below. Otherwise it is mainly correct.
　　|
　　| NOTE 2! The boot disk type must be set at compile-time, by setting
　　| the following equ. Having the boot-up procedure hunt for the right
　　| disk type is severe brain-damage.
　　| The loader has been made as simple as possible (had to, to get it
　　| in 512 bytes with the code to move to protected mode), and continuos
　　| read errors will result in a unbreakable loop. Reboot by hand. It
　　| loads pretty fast by getting whole sectors at a time whenever possible.
　　| 1.44Mb disks: sectors = 18
　　| 1.2Mb disks:
　　| sectors = 15
　　| 720kB disks:
　　| sectors = 9
　　.globl begtext, begdata, begbss, endtext, enddata, endbss
　　.text
　　begtext:
　　.data
　　begdata:
　　.bss
　　begbss:
　　.text

BOOTSEG = 0x07c0
　　INITSEG = 0x9000
　　SYSSEG = 0x1000 | system loaded at 0x10000 (65536).
　　ENDSEG = SYSSEG + SYSSIZE | SYSSIZE在Makefile中定义的 ^_^

entry start
　　start:
　　 mov ax,#BOOTSEG | 现在应仍处在REAL MODE下.
　　 mov ds,ax | 移动自身从BOOTSEG:0000到INITSEG:0000
　　 mov ax,#INITSEG | 共512字节.
　　 mov es,ax | 那么BOOT.S处在0x90000-0x90200.
　　 mov cx,#256
　　 sub si,si
　　 sub di,di
　　 rep
　　 movw
　　 jmpi go,INITSEG
　　go: mov ax,cs
　　 mov ds,ax | 将DS,ES,SS均设为0x9000,所有数据都以