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Use mul instead of imul

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See phil-opp/blog_os#21
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Philipp Oppermann
2015-10-31 14:00:13 +01:00
parent 44a8f72c28
commit 7cfa96b72d
1 changed files with 2 additions and 2 deletions
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_posts/2015-08-25-entering-longmode.md
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@@ -269,7 +269,7 @@ setup_page_tables:
.map_p2_table:
; map ecx-th P2 entry to a huge page that starts at address 2MiB*ecx
mov eax, 0x200000 ; 2MiB
imul eax, ecx ; start address of ecx-th page
mul ecx ; start address of ecx-th page
or eax, 0b10000011 ; present + writable + huge
mov [p2_table + ecx * 8], eax ; map ecx-th entry
@@ -281,7 +281,7 @@ setup_page_tables:
```
Maybe I first explain how an assembly loop works. We use the `ecx` register as a counter variable, just like `i` in a for loop. After mapping the `ecx-th` entry, we increase `ecx` by one and jump to `.map_p2_table` again if it's still smaller 512.
To map a P2 entry we first calculate the start address of its page in `eax`: The `ecx-th` entry needs to be mapped to `ecx * 2MiB`. Then we set the `present`, `writable`, and `huge page` bits and write it to the P2 entry. The address of the `ecx-th` entry in P2 is `p2_table + ecx * 8`, because each entry is 8 bytes large.
To map a P2 entry we first calculate the start address of its page in `eax`: The `ecx-th` entry needs to be mapped to `ecx * 2MiB`. We use the `mul` operation for that, which multiplies `eax` with the given register and stores the result in `eax`. Then we set the `present`, `writable`, and `huge page` bits and write it to the P2 entry. The address of the `ecx-th` entry in P2 is `p2_table + ecx * 8`, because each entry is 8 bytes large.
Now the first gigabyte of our kernel is identity mapped and thus accessible through the same physical and virtual addresses.