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96 lines
2.4 KiB
ArmAsm
96 lines
2.4 KiB
ArmAsm
section .rodata
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gdt_flat_slice:
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dw GDT_FLAT_LEN
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dd gdt_flat
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global gdt_flat_slice
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; Segment descriptor layout
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; | Range (bits) | Field |
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; |--------------|---------------|
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; | 0-16 | limit |
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; | 16-32 | base |
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; | 32-40 | base cont. |
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; | 40-48 | access |
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; | 48-52 | limit cont. |
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; | 52-56 | flags |
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; | 56-64 | base cont. |
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;
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; Flags
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; - 0: reserved
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; - 1: long-mode code segment
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; - 2: size
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; - unset: 16-bit
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; - set: 32-bit
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; - 3: granularity
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; - unset: limit is measured in bytes
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; - set: limit is measured in 4KiB pages
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;
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; Access
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; - 0: accessed
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; - unset: CPU will set it when the segment is accessed
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; - 1: readable / writable
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; - data segments: is segment writable (data segments are always readable)
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; - code segments: is segment readable (code segments are never writable)
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; - 2: direction / conforming
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; - data segments: whether segment grows down
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; - code segments: whether this can be executed from a lower-privilege ring
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; - 3: executable
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; - unset: this is a data segment
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; - set: this is a code segment
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; - 4: descriptor type
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; - unset: this is a task state segment
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; - set: this is a data or code segment
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; - 5-6: privilege level (ring number)
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; - 7: present (must be set)
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;
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align 8
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gdt_flat:
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; First GDT entry must be 0.
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dq 0
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; 32-bit code segment.
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; Bytes 0x0000 - 0xffff.
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.segment_code_32:
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db 0xff, 0xff, \
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0x00, 0x00, \
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0x00, \
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10011011b, \
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01000000b, \
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0x00
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; 16-bit code segment, to use if we want to switch back to real mode.
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; Bytes 0x0000 - 0xffff.
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.segment_code_16:
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db 0xff, 0xff, \
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0x00, 0x00, \
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0x00, \
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10011011b, \
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00000000b, \
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0x00
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; Data segment.
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; Pages 0x000000 - 0x0fffff, which covers the entire 32-bit address space (start of 0xfffff-th page
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; is 0xfffff * 4096 = 0xfffff000, end of page exclusive is 0xfffff000 + 4096 = 0x100000000).
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.segment_data:
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db 0xff, 0xff, \
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0x00, 0x00, \
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0x00, \
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10010011b, \
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11001111b, \
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0x00
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global gdt_flat
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GDT_FLAT_LEN equ ($ - gdt_flat)
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GDT_FLAT_IDX_CODE_32 equ (gdt_flat.segment_code_32 - gdt_flat)
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global GDT_FLAT_IDX_CODE_32
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GDT_FLAT_IDX_CODE_16 equ (gdt_flat.segment_code_16 - gdt_flat)
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global GDT_FLAT_IDX_CODE_16
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GDT_FLAT_IDX_DATA equ (gdt_flat.segment_data - gdt_flat)
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global GDT_FLAT_IDX_DATA
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