A20 enable code added

1 files changed, 147 insertions, 0 deletions

diff --git a/src/boot/boot.stage2.a20.asm b/src/boot/boot.stage2.a20.asm
new file mode 100644
index 0000000..6a9c250
--- /dev/null
+++ b/src/boot/boot.stage2.a20.asm
@@ -0,0 +1,147 @@
+    ;;  boot.stage2.a20.asm
+    
+_enable_a20:    
+    call _check_a20
+    test ax, ax
+    jnz .end
+
+    call _enable_a20_bios
+    call _check_a20
+    test ax, ax
+    jnz .end
+    
+    call _enable_a20_keyb
+    call _check_a20
+    test ax, ax
+    jnz .end
+
+    call _enable_a20_io92
+    call _check_a20
+    test ax, ax
+    jnz .end
+
+    .halt: hlt
+    jmp .halt
+    .end
+    ret
+
+_check_a20:     
+    pushf
+    push ds
+    push es
+    push di
+    push si
+    cli                         ; Clear Interrupts
+    
+    xor ax, ax
+    mov es, ax
+    not ax
+    mov ds, ax
+    ;; 0500 and 0510 are guaranteed to be free
+    mov di, 0x0500
+    mov si, 0x0510
+    
+    ;; Save the original values
+    mov dl, byte [es:di]
+    push dx
+    mov dl, byte [ds:si]
+    push dx
+
+    mov byte [es:di], 0x00
+    mov byte [ds:si], 0xFF
+    cmp byte [es:di], 0xFF
+
+    mov ax, 0                   ; The A20 is disabled
+    je .a20_disabled
+    mov ax, 1                   ; The A20 is enabled
+    .a20_disabled:
+    pop dx
+    mov byte [ds:si], dl
+    pop dx
+    mov byte [es:di], dl
+    
+    pop si
+    pop di
+    pop es
+    pop ds
+    popf
+    sti                         ; Enable interrupts again
+    ret
+
+_enable_a20_bios:   
+    mov ax, 0x2401
+    int 0x15
+    jc .fast_gate
+    test ah, ah
+    jnz .failed
+    call _check_a20
+    test ax, ax
+    jnz .done
+    
+    .fast_gate:
+    in al, 0x92
+    test al, 2
+    jnz .done
+    
+    or al, 2
+    and al, 0xfe
+    out 0x92, al
+   
+    call _check_a20
+    test ax, ax
+    jnz .done
+
+    .done:
+    mov ax, 1
+    ret
+
+    .failed:
+    mov ax, 0
+    ret
+
+_enable_a20_keyb:   
+    cli 
+    call .wait_io1
+    mov al, 0xad
+    out 0x64, al
+
+    call .wait_io1
+    mov al, 0x0d0
+    out 0x64, al
+
+    call .wait_io2
+    in al, 0x60
+    push eax
+
+    call .wait_io1
+    mov al, 0xd1
+    out 0x64, al
+
+    call .wait_io1
+    mov al, 0xae
+    out 0x64, al
+    
+    sti 
+    ret
+
+    .wait_io1:
+    in al, 0x64
+    test al, 2
+    jnz .wait_io1
+    ret
+    
+    .wait_io2:  
+    in al, 0x64
+    test al, 1
+    jz .wait_io2
+    ret
+
+_enable_a20_io92:   
+    in al, 0x92                 ; Read from port 0x92
+    test al, 2                  ; Check if bit 1
+    jnz .end
+    or al, 2
+    and al, 0xfe
+    out 0x92, al
+    .end
+    ret