[Solved] The value of selector in the IDT table

[growlnx]

Why the value of "selector" is always 0x08 in the IDT table?

Code: Select all

struct IDT_entry{
	unsigned short int offset_lowerbits;
	unsigned short int selector;
	unsigned char zero;
	unsigned char type_attr;
	unsigned short int offset_higherbits;
};

Can the "gdt_code" label in the GDT table be used instead?

Code: Select all

	
	irq0_address = (unsigned long)irq0; 
	IDT[32].offset_lowerbits = irq0_address & 0xffff;
	IDT[32].selector = 0x08; /* KERNEL_CODE_SEGMENT_OFFSET */
	IDT[32].zero = 0;
	IDT[32].type_attr = 0x8e; /* INTERRUPT_GATE */
	IDT[32].offset_higherbits = (irq0_address & 0xffff0000) >> 16;

• https://wiki.osdev.org/Interrupts_tutorial
  https://wiki.osdev.org/Interrupt_Descriptor_Table

[Octocontrabass]

Why the value of "selector" is always 0x08 in the IDT table?

Because 0x08 is the kernel code segment selector in the example code.

Can the "gdt_code" label in the GDT table be used instead?

No. That label will translate into the address of the code segment descriptor in memory, but you need the code segment selector.

[growlnx]

Because 0x08 is the kernel code segment selector in the example code.

This offset is in relation to the beginning of the GDT table, right?

Code: Select all

gdt_start:
    dq 0x00 ; 8 null bytes here        

gdt_code:    ; so here's the "kernel code segment selector" ?
    dw 0xFFFF      
    dw 0x0000       
    db 0x00         
    db 10011010b   
    db 11001111b    
    db 0x00        

.gdt_data:    ; and the "kernel data segment selector" ?
    dw 0xFFFF       
    dw 0x0000       
    db 0x00         
    db 10010010b    
    db 11001111b   
    db 0x00         
gdt_end:

; GDT descriptor
gdt_descriptor:
    dw gdt_end - gdt_start - 1 
    dd gdt_start 

[Octocontrabass]

This offset is in relation to the beginning of the GDT table, right?

That's a selector. The offset is encoded using the upper 13 bits of the selector, and some other information goes in the lower 3 bits. The lower 3 bits are all zero for ring 0 GDT segments, so your selectors are equal to the offset, but that's not true for all selectors.

For example, if you add a ring 3 descriptor at offset 0x18 in your GDT, you'll probably use the selector 0x1B to access it.

[growlnx]

This offset is in relation to the beginning of the GDT table, right?

That's a selector. The offset is encoded using the upper 13 bits of the selector, and some other information goes in the lower 3 bits. The lower 3 bits are all zero for ring 0 GDT segments, so your selectors are equal to the offset, but that's not true for all selectors.

For example, if you add a ring 3 descriptor at offset 0x18 in your GDT, you'll probably use the selector 0x1B to access it.

thanks, i believe i understand now.

Some References i found about segment selector:

• https://stackoverflow.com/questions/911 ... 32/9170325
  https://www.cs.cmu.edu/~410/doc/segments/segments.html
  https://manybutfinite.com/post/memory-t ... mentation/