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refine statements about early page table build

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zhaoxiaoqiang 2015-12-30 15:36:39 +08:00
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commit 8deab213d9
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Booting/linux-bootstrap-4.md
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@ -389,7 +389,7 @@ Now we can build the top level page table - `PML4` - with:
Here we get the address stored in the `ebx` with `pgtable` offset and put it in `edi`. Next we put this address with offset `0x1007` in the `eax` register. `0x1007` is 4096 bytes (size of the PML4) + 7 (PML4 entry flags - `PRESENT+RW+USER`) and puts `eax` in `edi`. After this manipulation `edi` will contain the address of the first Page Directory Pointer Entry with flags - `PRESENT+RW+USER`. Here we get the address stored in the `ebx` with `pgtable` offset and put it in `edi`. Next we put this address with offset `0x1007` in the `eax` register. `0x1007` is 4096 bytes (size of the PML4) + 7 (PML4 entry flags - `PRESENT+RW+USER`) and puts `eax` in `edi`. After this manipulation `edi` will contain the address of the first Page Directory Pointer Entry with flags - `PRESENT+RW+USER`.
In the next step we build 4 Page Directory entries in the Page Directory Pointer table, where the first entry will be with `0x7` flags or present, write, userspace (`PRESENT WRITE | USER`): In the next step we build 4 Page Directory entries in the Page Directory Pointer table with `0x7` flags or present, write, userspace (`PRESENT WRITE | USER`):
```assembly ```assembly
leal pgtable + 0x1000(%ebx), %edi leal pgtable + 0x1000(%ebx), %edi
@ -406,7 +406,7 @@ We put the base address of the page directory pointer table in `edi` and the add
After this `edi` will contain the address of the first page directory pointer entry with flags `0x7`. Next we just calculate the address of following page directory pointer entries where each entry is 8 bytes, and write their addresses to `eax`. After this `edi` will contain the address of the first page directory pointer entry with flags `0x7`. Next we just calculate the address of following page directory pointer entries where each entry is 8 bytes, and write their addresses to `eax`.
The next step is building the `2048` page table entries by 2 megabytes: The next step is building the `2048` page table entries with 2-MByte page:
```assembly ```assembly
leal pgtable + 0x2000(%ebx), %edi leal pgtable + 0x2000(%ebx), %edi
@ -419,7 +419,7 @@ The next step is building the `2048` page table entries by 2 megabytes:
jnz 1b jnz 1b
``` ```
Here we do almost the same as in the previous example, except the first entry will be with flags - `$0x00000183` - `PRESENT + WRITE + MBZ` and all other entries. In the end we will have 2048 pages by 2 megabytes. Here we do almost the same as in the previous example, all entries will be with flags - `$0x00000183` - `PRESENT + WRITE + MBZ`. In the end we will have 2048 pages with 2-MByte page.
Our early page table structure are done, it maps 4 gigabytes of memory and now we can put the address of the high-level page table - `PML4` - in `cr3` control register: Our early page table structure are done, it maps 4 gigabytes of memory and now we can put the address of the high-level page table - `PML4` - in `cr3` control register: