Fix A20 by copying others' implementation

The implementation of the A20 wraparound check was off in some way,
which is why the new implementation was copied from osdev.org, just
using minor modifications to the label naming.

i386/loader/src-asm/memory/a20.asm

@@ -1,95 +1,48 @@
 
+; A20 Wraparound check as gotten from osdev.org
 check_a20_wraparound:
-.prolog:
-    push dword esi
-    sub esp, 64
-    mov esi, esp
+    pushf
+    push ds
+    push es
+    push di
+    push si
 
-    ; EAX is used for the return value,
-    ; so it doesn't have to be preserved.
-    mov [esi + (64 - 8)], ebx
-    mov [esi + (64 - 12)], edi
+    xor ax, ax ; ax = 0
+    mov es, ax
 
-.test_a20:
-    ; Save the values which are going to be overwritten
-    mov edi, 0x0800
-    mov eax, [edi]
-    mov edi, 0x00100800
-    mov ebx, [edi]
+    not ax ; ax = 0xFFFF
+    mov ds, ax
 
-    ; Write to two locations which, if A20 wraps around,
-    ; will both be written to by the second statement.
-    ; Write 0 to 0x08000 to avoid the unlikely case that
-    ; those bytes are there coincidentally and A20 is on.
-    mov edi, 0x0800
-    mov [edi], dword 0x00000000
-    mov edi, 0x00100800
-    mov [edi], dword 0xffaa00dd
+    mov di, 0x0500
+    mov si, 0x0510
 
-    ; Check the lower address (which should NOT contain)
-    ; this value for the value. If it does contain it, the
-    ; memory space wraps around to 0 every 2^19 bytes.
-    cmp [0x0800], dword 0xffaa00dd
-    je .return_a20_not_enabled      ; If the value at 0x0800
-                                    ; was written to instead
-                                    ; (or at the same time) of 
-                                    ; writing to 0x100800,
-                                    ; the A20 wasn't enabled.
-    ; Reset the values at the addresses to which the test
-    ; values were written znd return that A20 is enabled.
-    mov edi, 0x0800
-    mov [edi], eax
-    mov edi, 0x00100800
-    mov [edi], ebx
+    mov al, byte [es:di]
+    push ax
 
-.return_a20_enabled:
+    mov al, byte [ds:si]
+    push ax
 
-    mov eax, 1
+    mov byte [es:di], 0x00
+    mov byte [ds:si], 0xFF
 
-    ; Restore the one used register which
-    ; doesn't contain the return value.
-    mov ebx, [esi + (64 - 8)]
+    cmp byte [es:di], 0xFF
+
+    pop ax
+    mov byte [ds:si], al
+
+    pop ax
+    mov byte [es:di], al
+
+    mov ax, 0
+    je .exit
+
+    mov ax, 1
+
+.exit:
+    pop si
+    pop di
+    pop es
+    pop ds
+    popf
 
-    add esp, 64
-    pop dword esi
-    ret
-
-.return_a20_not_enabled:
-    ; It's only necessary to write to one of the addresses
-    ; as both of them are equal either way.
-    mov edi, 0x0800
-    mov [edi], eax
-
-    mov eax, 0
-
-    ; Restore the one used register which
-    ; doesn't contain the return value.
-    mov ebx, [esi + (64 - 12)]
-    mov edi, [esi + (64 - 8)]
-
-    add esp, 64
-    pop dword esi
-    ret
-
-
-
-enable_a20_through_keyboard_controller:
-.prolog:
-    push dword esi
-    sub esp, 64
-    mov esi, esp
-
-    mov [esi + (64 - 8)], ebx
-    mov [esi + (64 - 12)], ecx
-    mov [esi + (64 - 16)], edx
-    mov [esi + (64 - 20)], edi
-
-.epilog:
-    mov ebx, [esi + (64 - 8)]
-    mov ecx, [esi + (64 - 12)]
-    mov edx, [esi + (64 - 16)]
-    mov edi, [esi + (64 - 20)]
-    
-    add esp, 64
-    pop dword esi
     ret

i386/loader/src-asm/memory/gdt.asm

@@ -2,6 +2,8 @@
 GDT_MEMORY_AREA equ 0x7e00
 GDT_ENTRY_COUNT equ 3
 
+
+
 ; Arguments (note: all arguments are 4-byte padded):
 ;   [Furthest from EBP]
 ;
@@ -109,114 +111,6 @@ make_regular_data_segment_in_gdt:
 
 
 
-; Arguments (note: all arguments are 4-byte padded):
-;   [Furthest from EBP]
-;
-;    4. uint32       privilege_level
-;       - Only 0 to 3; 0 being kernel and 3 being userland.
-;         Everything above 3 is an error.
-;       
-;    3  uint32      num_pages
-;       - Only 20 bits are usable. Everything above will
-;         be cut off to be written to the segment descriptor.
-;
-;    2. ptr32       base_address
-;
-;    1. uint32      segment_index
-;       Note:
-;       - Zero is NOT usable and will throw an error.
-;         the highest theoretical value for this is 8191,
-;         but generally, one should stick to GDT_ENTRY_COUNT.
-;
-;   [Nearest to EBP]
-; COMMENTED OUT DUE TO BEING BUGGY!
-; make_stack_segment_in_gdt:
-; .prolog:
-;     push esi
-;     add esp, 64
-;     mov esi, esp
-
-;     mov [esi + (64 - 4)], eax
-;     mov [esi + (64 - 8)], ebx
-;     mov [esi + (64 - 12)], ecx
-;     mov [esi + (64 - 16)], edx
-;     mov [esi + (64 - 20)], edi
-
-; .get_arguments:
-;     mov eax, [ebp - 4]  ; segment_index
-;     mov ebx, [ebp - 8]  ; base_address
-;     mov ecx, [ebp - 12] ; num_pages
-;     mov edx, [ebp - 16] ; privilege_level
-
-;     mov [esi + 16], eax ; segment_index
-;     mov [esi + 20], ebx ; base_address
-;     mov [esi + 24], ecx ; num_pages
-;     mov [esi + 28], edx ; privilege_level
-
-; .validate_arguments:
-;     cmp eax, 8191
-;     ja .segment_index_too_tall
-;     cmp eax, 0
-;     je .segment_index_is_zero
-
-;     cmp edx, 3
-;     ja .privilege_level_too_large
-
-; .calculate_work_address:
-;     shl eax, 3 ; Entry offset from table start
-;     add eax, GDT_MEMORY_AREA ; Entry address
-;     mov [esi + 12], eax
-
-; .write_segment_limit:
-;     mov [eax], cx
-;     shr ecx, 16
-;     and ecx, 0x0f
-;     mov [eax + 6], cl
-
-; .write_base_address:
-;     mov [eax + 2], bx
-;     shr bx, 16
-;     mov [eax + 4], bl
-;     mov [eax + 7], bh
-
-; .set_default_flags:
-;     or [eax + 6], word (0b1100 << 4)
-
-; .set_stack_specific_access_byte:
-;     mov al, 0b1001
-;     ; OR the privilege level into the access byte
-;     mov dl, [ebp - 16]
-;     shl dl, 1
-;     and dl, 0b0110
-;     or al, dl
-;     shl al, 4 ; Move the 4 bits of general flags to their place
-
-;     ; Stack specific part
-;     or al, 0b0110 ; DATA | STACK | READ_WRITE | NOT_ACCESSED
-
-;     mov ebx, [esi + 12]
-;     mov [ebx + 5], al ; Write to the final destination
-
-; .epilog:
-;     mov edi, [esi + (64 - 20)]
-;     mov edx, [esi + (64 - 16)]
-;     mov ecx, [esi + (64 - 12)]
-;     mov ebx, [esi + (64 - 8)]
-;     mov eax, [esi + (64 - 4)]
-
-;     sub esp, 64
-;     pop esi
-;     ret
-
-; .segment_index_too_tall:
-; .segment_index_is_zero:
-; .privilege_level_too_large:
-;     ; todo(logs): Write debug message.
-;     cli
-;     hlt
-
-
-
 ; Arguments (note: all arguments are 4-byte padded):
 ;   [Furthest from EBP]
 ;
@@ -323,6 +217,7 @@ make_code_segment_in_gdt:
     hlt
 
 
+
 ; [Furthest to EBP]
 ;   1)  Return Address
 ; [Nearest to EBP]
@@ -344,73 +239,59 @@ load_flat_gdt:
 .set_segments:
     push ebp
     mov ebp, esp
-    push dword 2        ; Segment Index
-    push dword 0        ; Start Offset
-    push dword 0xfffff  ; Page Count
-    push dword 0        ; Required privilege level
+    push dword 2            ; Segment Index
+    push dword 0            ; Start Offset
+    push dword 0x000fffff   ; Page Count
+    push dword 0            ; Required privilege level
     call make_regular_data_segment_in_gdt
     mov esp, ebp
     pop ebp
 
-    ; COMMENTED OUT DUE TO BEING BUGGY
-    ; push ebp
-    ; mov ebp, esp
-    ; push dword 3                    ; Segment Index
-    ; push dword LOWER_STACK_POINTER  ; Start Offset
-    ; push dword 0xfffff              ; Page Count
-    ; push dword 0                    ; Required privilege level
-    ; call make_stack_segment_in_gdt
-    ; mov esp, ebp
-    ; pop ebp
-
     push ebp
     mov ebp, esp
-    push dword 1        ; Segment Index
-    push dword 0x0000   ; Start Offset
-    push dword 0xfffff  ; Page Count
-    push dword 0        ; Required privilege level
+    push dword 1            ; Segment Index
+    push dword 0x0000       ; Start Offset
+    push dword 0x000fffff   ; Page Count
+    push dword 0            ; Required privilege level
     call make_code_segment_in_gdt
     mov esp, ebp
     pop ebp
 
     mov bx, 0
     mov ds, bx
-    lgdt [.gdtr]
-
-    mov bx, (2 << 3)
-    mov ds, bx
     mov es, bx
     mov fs, bx
     mov gs, bx
-    ; mov bx, (3 << 3)
     mov ss, bx
+    lgdt [.gdtr]
 
     mov ebx, cr0
     or ebx, 1
     mov cr0, ebx
 
-    xor bx, bx
-    mov ds, bx
-
-    jmp dword 0x08:.epilog
+    jmp dword (1 << 3):.epilog
 
 bits 32
 .epilog:
+    mov bx, (2 << 3)
+    mov ds, bx
+    mov es, bx
+    mov fs, bx
+    mov gs, bx
+    mov ss, bx
+
     mov edi, [esi + (64 - 20)]
     mov edx, [esi + (64 - 16)]
     mov ecx, [esi + (64 - 12)]
     mov ebx, [esi + (64 - 8)]
     mov eax, [esi + (64 - 4)]
 
-    add esp, 64
-    pop esi
-    jmp stage2_true_entry
-    ; jmp after_gdt_loaded
+    jmp dword stage2_true_entry
 
 align 16
 .gdtr:
 .gdtr.length:
-    dw 15
+    dw 23
 .gdtr.pointer:
     dd GDT_MEMORY_AREA