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alignment fixed

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Signed-off-by: Alexander Kuleshov <kuleshovmail@gmail.com>
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Alexander Kuleshov 2017-12-30 00:41:20 +06:00
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commit 04f10187e0
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Booting/linux-bootstrap-4.md
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@ -186,10 +186,10 @@ label: pop %reg
After this, a `%reg` register will contain the address of a label. Let's look at the similar code which searches address of the `startup_32` in the Linux kernel: After this, a `%reg` register will contain the address of a label. Let's look at the similar code which searches address of the `startup_32` in the Linux kernel:
```assembly ```assembly
leal (BP_scratch+4)(%esi), %esp leal (BP_scratch+4)(%esi), %esp
call 1f call 1f
1: popl %ebp 1: popl %ebp
subl $1b, %ebp subl $1b, %ebp
``` ```
As you remember from the previous part, the `esi` register contains the address of the [boot_params](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/uapi/asm/bootparam.h#L113) structure which was filled before we moved to the protected mode. The `boot_params` structure contains a special field `scratch` with offset `0x1e4`. These four bytes field will be temporary stack for `call` instruction. We are getting the address of the `scratch` field + `4` bytes and putting it in the `esp` register. We add `4` bytes to the base of the `BP_scratch` field because, as just described, it will be a temporary stack and the stack grows from top to down in `x86_64` architecture. So our stack pointer will point to the top of the stack. Next, we can see the pattern that I've described above. We make a call to the `1f` label and put the address of this label to the `ebp` register because we have return address on the top of stack after the `call` instruction will be executed. So, for now we have an address of the `1f` label and now it is easy to get address of the `startup_32`. We just need to subtract address of label from the address which we got from the stack: As you remember from the previous part, the `esi` register contains the address of the [boot_params](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/uapi/asm/bootparam.h#L113) structure which was filled before we moved to the protected mode. The `boot_params` structure contains a special field `scratch` with offset `0x1e4`. These four bytes field will be temporary stack for `call` instruction. We are getting the address of the `scratch` field + `4` bytes and putting it in the `esp` register. We add `4` bytes to the base of the `BP_scratch` field because, as just described, it will be a temporary stack and the stack grows from top to down in `x86_64` architecture. So our stack pointer will point to the top of the stack. Next, we can see the pattern that I've described above. We make a call to the `1f` label and put the address of this label to the `ebp` register because we have return address on the top of stack after the `call` instruction will be executed. So, for now we have an address of the `1f` label and now it is easy to get address of the `startup_32`. We just need to subtract address of label from the address which we got from the stack: