refactor + wip bios call from protected mode

boot1/asm/gdt.s

@@ -0,0 +1,95 @@
+section .rodata
+
+gdt_flat_slice:
+  dw GDT_FLAT_LEN
+  dd gdt_flat
+
+global gdt_flat_slice
+
+; Segment descriptor layout
+; | Range (bits) | Field         |
+; |--------------|---------------|
+; |         0-16 | limit         |
+; |        16-32 | base          |
+; |        32-40 | base cont.    |
+; |        40-48 | access        |
+; |        48-52 | limit cont.   |
+; |        52-56 | flags         |
+; |        56-64 | base cont.    |
+;
+; Flags
+; - 0: reserved
+; - 1: long-mode code segment
+; - 2: size
+;   - unset: 16-bit
+;   - set: 32-bit
+; - 3: granularity
+;   - unset: limit is measured in bytes
+;   - set: limit is measured in 4KiB pages
+;
+; Access
+; - 0: accessed
+;   - unset: CPU will set it when the segment is accessed
+; - 1: readable / writable
+;   - data segments: is segment writable (data segments are always readable)
+;   - code segments: is segment readable (code segments are never writable)
+; - 2: direction / conforming
+;   - data segments: whether segment grows down
+;   - code segments: whether this can be executed from a lower-privilege ring
+; - 3: executable
+;   - unset: this is a data segment
+;   - set: this is a code segment
+; - 4: descriptor type
+;   - unset: this is a task state segment
+;   - set: this is a data or code segment
+; - 5-6: privilege level (ring number)
+; - 7: present (must be set)
+;
+align 8
+gdt_flat:
+  ; First GDT entry must be 0.
+  dq 0
+
+; 32-bit code segment.
+; Bytes 0x0000 - 0xffff.
+.segment_code_32:
+  db 0xff, 0xff, \
+     0x00, 0x00, \
+     0x00,       \
+     10011011b,  \
+     01000000b,  \
+     0x00
+
+; 16-bit code segment, to use if we want to switch back to real mode.
+; Bytes 0x0000 - 0xffff.
+.segment_code_16:
+  db 0xff, 0xff, \
+     0x00, 0x00, \
+     0x00,       \
+     10011011b,  \
+     00000000b,  \
+     0x00
+
+; Data segment.
+; Pages 0x000000 - 0x0fffff, which covers the entire 32-bit address space (start of 0xfffff-th page
+; is 0xfffff * 4096 = 0xfffff000, end of page exclusive is 0xfffff000 + 4096 = 0x100000000).
+.segment_data:
+  db 0xff, 0xff, \
+     0x00, 0x00, \
+     0x00,       \
+     10010011b,  \
+     11001111b,  \
+     0x00
+
+global gdt_flat
+
+GDT_FLAT_LEN equ ($ - gdt_flat)
+
+GDT_FLAT_IDX_CODE_32 equ (gdt_flat.segment_code_32 - gdt_flat)
+global GDT_FLAT_IDX_CODE_32
+
+GDT_FLAT_IDX_CODE_16 equ (gdt_flat.segment_code_16 - gdt_flat)
+global GDT_FLAT_IDX_CODE_16
+
+GDT_FLAT_IDX_DATA equ (gdt_flat.segment_data - gdt_flat)
+global GDT_FLAT_IDX_DATA