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@ -252,6 +252,8 @@ main:
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call print_memmap
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call unreal_enable
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.hlt_loop:
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hlt
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jmp .hlt_loop
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@ -746,6 +748,42 @@ enable_a20_intel_8042:
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ret
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unreal_enable:
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fnstart
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push ds
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; Load GDT
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lgdt [unreal_setup_gdt_slice]
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; Switch to protected mode.
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mov eax, cr0
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or al, 0x01
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mov cr0, eax
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; Set the code segment to the code segment in our GDT (offset 0x08)
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jmp (GDT_DESCRIPTOR_SIZE):.protected_mode
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.protected_mode:
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; In protected mode, the value in a segment register refers to an offset into the GDT. Setting
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; a segment register updates the segment descriptor cache with the selected GDT descriptor. The
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; limit of our second GDT descriptor is 0xfffff pages, which covers the entire 32-bit address
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; space. This limit is unchanged when we return to real mode, so we'll be able to address the
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; whole 32-bit address space.
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mov ax, (2 * GDT_DESCRIPTOR_SIZE)
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mov ds, ax
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; Switch back to real mode.
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mov eax, cr0
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and eax, 0xfe
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mov cr0, eax
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; Reset code segment back to 0 since we're using real-mode addressing again
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; (0x08 would now offset us by 16 * 8 = 128 bytes if we left it there)
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jmp 0x00:.unreal_mode
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.unreal_mode:
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pop ds
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fnret
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; FIXME:
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; - We want this to work in a streaming fashion, so we can see all the reserved regions even if we
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; don't have enough space to store all the regions. We can always rerun with a different buffer
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@ -921,6 +959,75 @@ print_memmap:
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fnret
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unreal_setup_gdt_slice:
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dw UNREAL_SETUP_GDT_LEN
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dd unreal_setup_gdt
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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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unreal_setup_gdt:
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dq 0
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; Code segment for low memory, bytes 0x0000 - 0xffff
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.segment_code:
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db 0xff, 0xff, \
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0x00, 0x00, \
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0x00, \
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010011011b, \
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00000000b, \
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0x00
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; Data segment for pages 0x000000 - 0x0fffff, which covers the entire 32-bit address space
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; (start of 0xfffff-th page is 0xfffff * 4096 = 0xfffff000, end of page exclusive is
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; 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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UNREAL_SETUP_GDT_LEN equ ($ - unreal_setup_gdt)
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msg_boot1_loaded db "boot1 loaded. hello!", 0
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msg_a20_enabled db "a20 enabled", 0
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msg_a20_disabled db "a20 not enabled", 0
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pantonshire
1 year ago