noxos (docs): merged 'platform' docs from .wiki
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title: "cpu.h"
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summary: "stuff related directly with the cpu"
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---
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OSDev Wiki: [x86 CPU Registers](https://wiki.osdev.org/CPU_Registers_x86)
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# `cpu_state_T` - struct
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- **cr3** - Control register 3, holds the current page table
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- **rax** - General purpose register
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- **rbx** - General purpose register
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- **rcx** - General purpose register
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- **rdx** - General purpose register
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- **rsi** - General purpose register
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- **rdi** - General purpose register
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- **rbp** - The Bottom of the current stack frame
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- **interrupt_id** - The ID of the interrupt, that captured the cpu state
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- **error_code** - Some exceptions such as the Page fault push more detailed information into here
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- **rip** - The current instruction address
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- **cs** - Segment selector of the associated IDT descriptor
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- **flags** - The CPU's FLAGS register, a status bitmap -> [cpu_flags_E](https://nerdcult.net/projects/noxos/docs/codebase/platform/cpu.h/#cpu_flags_e---enum)
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- **rsp** - The Top of the current stack frame
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- **ss** - Not totally sure, what this does, but it has to do with security rings
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This struct defines a *complete* CPU state, that can be saved and restored.
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Such a state is saved when the CPU fires an interrupt and restored by the interrupt handler when it's finished.
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This allows multithreading and exception analysis.
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# `cpu_flags_E` - enum
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- **CPU_FLAG_CARRY**
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- **CPU_FLAG_PARITY**
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- **CPU_FLAG_AUXILIARY**
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- **CPU_FLAG_ZERO**
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- **CPU_FLAG_SIGN**
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- **CPU_FLAG_TRAP**
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- **CPU_FLAG_INTERRUPT_ENABLE**
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- **CPU_FLAG_DIRECTION**
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- **CPU_FLAG_OVERFLOW**
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- **CPU_FLAG_IO_PRIVILEGE_0**
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- **CPU_FLAG_IO_PRIVILEGE_1**
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- **CPU_FLAG_NESTED_TASK**
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- **CPU_FLAG_RESUME**
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- **CPU_FLAG_VIRTUAL_8086**
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- **CPU_FLAG_ALIGNMENT_CHECK**
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- **CPU_FLAG_VIRTUAL_INTERRUPT**
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- **CPU_FLAG_VIRTUAL_INTERRUPT_PENDING**
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- **CPU_FLAG_CPUID**
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---
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title: "exceptions.h"
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summary: "CPU-exception handling"
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---
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OSDev Wiki: [Exceptions](https://wiki.osdev.org/Exceptions)
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# `exception_type_E` - enum
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These are just the definitions of the CPU-exception interrupt IDs.
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# `g_exception_type_strings` - global variable
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This array of strings defines the names of the CPU-exceptions.
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# `exception_handle(cpu_state)` - function (cpu_state_T*)
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If an interrupt is an exception, the interrupt handler will call this function to handle the exception.
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At the moment it will just panic, but in far future this could get expanded for page swapping, etc.
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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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---
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title: "interrupts.h"
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summary: "the infrastructure to handle x86 interrupts"
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---
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# `idt_register_T` - struct [packed]
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This struct is very similar to the [GDT descriptor](https://nerdcult.net/projects/noxos/docs/codebase/platform/gdt.h/#gdt_descriptor_t---struct-packed).
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It holds the size and address of the Table, where the interrupt handlers are looked up.
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# `idt_descriptor_entry_T` - struct
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This struct stores information about one interrupt handler.
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The osdev wiki explains this more detailed.
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# `g_idt_register` - global variable
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The default IDT configuration loaded when the IDT gets initialized.
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# `g_handling_interrupt` - global variable
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When the system isn't handling an interrupt this is set to 0.
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If this is greater than 0 the system is currently handling an interrupt,
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# `idt_init()` - function (void)
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This function fills all the interrupt gates (handlers) into the IDT and loads it.
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