diff --git a/_posts/2015-08-25-entering-longmode.md b/_posts/2015-08-25-entering-longmode.md index 00fdd18f..127e97c5 100644 --- a/_posts/2015-08-25-entering-longmode.md +++ b/_posts/2015-08-25-entering-longmode.md @@ -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.