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fix #14 issue

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Alexander Kuleshov 2017-07-16 23:10:48 +06:00
parent 6ed0589b83
commit 0ce272fc6a
1 changed files with 5 additions and 5 deletions
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10
Booting/linux-bootstrap-1.md
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@ -335,7 +335,7 @@ gs = fs = es = ds = ss = 0x1000
cs = 0x1020 cs = 0x1020
``` ```
In my case, the kernel is loaded at `0x10000`. In my case, the kernel is loaded at `0x10000` address.
After the jump to `start_of_setup`, the kernel needs to do the following: After the jump to `start_of_setup`, the kernel needs to do the following:
@ -365,7 +365,7 @@ _start:
.byte start_of_setup-1f .byte start_of_setup-1f
``` ```
jump, which is at a `512` byte offset from [4d 5a](https://github.com/torvalds/linux/blob/master/arch/x86/boot/header.S#L46). It also needs to align `cs` from `0x10200` to `0x10000`, as well as all other segment registers. After that, we set up the stack: jump, which is at a `512` byte offset from [4d 5a](https://github.com/torvalds/linux/blob/master/arch/x86/boot/header.S#L46). It also needs to align `cs` from `0x1020` to `0x1000`, as well as all other segment registers. After that, we set up the stack:
```assembly ```assembly
pushw %ds pushw %ds
@ -389,13 +389,13 @@ Almost all of the setup code is in preparation for the C language environment in
This can lead to 3 different scenarios: This can lead to 3 different scenarios:
* `ss` has valid value `0x10000` (as do all other segment registers beside `cs`) * `ss` has valid value `0x1000` (as do all other segment registers beside `cs`)
* `ss` is invalid and `CAN_USE_HEAP` flag is set (see below) * `ss` is invalid and `CAN_USE_HEAP` flag is set (see below)
* `ss` is invalid and `CAN_USE_HEAP` flag is not set (see below) * `ss` is invalid and `CAN_USE_HEAP` flag is not set (see below)
Let's look at all three of these scenarios in turn: Let's look at all three of these scenarios in turn:
* `ss` has a correct address (`0x10000`). In this case, we go to label [2](https://github.com/torvalds/linux/blob/master/arch/x86/boot/header.S#L584): * `ss` has a correct address (`0x1000`). In this case, we go to label [2](https://github.com/torvalds/linux/blob/master/arch/x86/boot/header.S#L584):
```assembly ```assembly
2: andw $~3, %dx 2: andw $~3, %dx
@ -406,7 +406,7 @@ Let's look at all three of these scenarios in turn:
sti sti
``` ```
Here we can see the alignment of `dx` (contains `sp` given by bootloader) to `4` bytes and a check for whether or not it is zero. If it is zero, we put `0xfffc` (4 byte aligned address before the maximum segment size of 64 KB) in `dx`. If it is not zero, we continue to use `sp`, given by the bootloader (0xf7f4 in my case). After this, we put the `ax` value into `ss`, which stores the correct segment address of `0x10000` and sets up a correct `sp`. We now have a correct stack: Here we can see the alignment of `dx` (contains `sp` given by bootloader) to `4` bytes and a check for whether or not it is zero. If it is zero, we put `0xfffc` (4 byte aligned address before the maximum segment size of 64 KB) in `dx`. If it is not zero, we continue to use `sp`, given by the bootloader (0xf7f4 in my case). After this, we put the `ax` value into `ss`, which stores the correct segment address of `0x1000` and sets up a correct `sp`. We now have a correct stack:
![stack](http://oi58.tinypic.com/16iwcis.jpg) ![stack](http://oi58.tinypic.com/16iwcis.jpg)