52 lines
2.4 KiB
Markdown
52 lines
2.4 KiB
Markdown
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---
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title: "gdt.h"
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summary: "definitions to handle segments (_Global Descriptor Table_)"
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---
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Segmentation is deprecated and is only needed for backwards compatibility.
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OSDev Wiki: [Global Descriptor Table](https://wiki.osdev.org/GDT)
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# `gdt_selector_E` - enum
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- **Null**
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- **Kernel Code** - Readable
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- **Kernel Data** - Readable + Writable
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- **User Null**
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- **User Code** - Readable
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- **User Data** - Readable + Writable
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# `gdt_descriptor_T` - struct [packed]
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| Name | Type | Description |
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|--------|----------|-----------------------------------------------------|
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| size | uint16_t | The tables size in bytes (-1) |
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| offset | uint64_t | The virtual address, where the table lies in memory |
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# `gdt_entry_T` - struct [packed]
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| Name | Type | Description |
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|--------------|----------|--------------------------------------------------------------|
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| limit0 | uint16_t | Can be ignored in long mode |
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| base0 | uint16_t | Can be ignored in long mode |
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| base1 | uint8_t | Can be ignored in long mode |
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| access | uint8_t | Specifies permissions (details in osdev wiki) |
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| limit1_flags | uint8_t | The first 4 bits can be ignored and the last 4 specify flags |
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| base2 | uint8_t | Can be ignored in long mode |
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# `gdt_T` - struct [packed / page aligned]
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| Name | Type | Description |
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|-------------|-------------|------------------------------------------|
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| null | gdt_entry_T | The entry for `GDT_SELECTOR_NULL` |
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| kernel_code | gdt_entry_T | The entry for `GDT_SELECTOR_KERNEL_CODE` |
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| kernel_data | gdt_entry_T | The entry for `GDT_SELECTOR_KERNEL_DATA` |
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| user_null | gdt_entry_T | The entry for `GDT_SELECTOR_USER_NULL` |
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| user_code | gdt_entry_T | The entry for `GDT_SELECTOR_USER_CODE` |
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| user_data | gdt_entry_T | The entry for `GDT_SELECTOR_USER_DATA` |
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# `g_default_gdt` - global variable
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The systems GDT.
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# `gdt_init` - function (void)
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Populates and loads [g_default_gdt](https://nerdcult.net/projects/noxos/docs/codebase/platform/gdt.h/#g_default_gdt---global-variable).
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This will also set all the data segment registers to 0x10 (Kernel Data) and `cs` to 0x08 (Kernel Code).
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