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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_p2_table:
; map ecx-th P2 entry to a huge page that starts at address 2MiB*ecx ; map ecx-th P2 entry to a huge page that starts at address 2MiB*ecx
mov eax, 0x200000 ; 2MiB 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 or eax, 0b10000011 ; present + writable + huge
mov [p2_table + ecx * 8], eax ; map ecx-th entry 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. 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. Now the first gigabyte of our kernel is identity mapped and thus accessible through the same physical and virtual addresses.