asm/a20.asm at master · LoopZ/asm · GitHub

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
; --------------------------------------- A20 line routines ---------------------------------------
USE16
WaitKBC:
	mov cx,0ffffh
A20L:
	in al,64h
	test al,2
	loopnz A20L
ret

EnableA20:
	call WaitKBC
	mov al,0d1h
	out 64h,al
	call WaitKBC
	mov al,0dfh
	out 60h,al
ret


EnableA20f:
	call WaitKBC
	mov al,0d1h
	out 64h,al
	call WaitKBC
	mov al,0dfh
	out 60h,al
retf


DisableA20:
	call WaitKBC
	mov al,0d1h
	out 64h,al
	call WaitKBC
	mov al,0ddh
	out 60h,al
ret

CheckA20:
    PUSH ax
    PUSH ds
    PUSH es

    XOR ax,ax
    MOV ds,ax
    NOT ax
    MOV es,ax
    MOV ah,[ds:0]
    CMP ah,[es:10h]
    JNZ A20_ON

    CLI
    INC ah
    MOV [ds:0],ah
    CMP [es:10h],ah
    PUSHF
    DEC ah
    MOV [ds:0],ah
    STI
    POPF
    JNZ A20_ON

    CLC
    POP es
    POP ds
    POP ax
    RET

A20_ON:
    STC
    POP es
    POP ds
    POP ax
RET