pc-timer 0.1.8

Intel 8253 programming-based timer.
Documentation 
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
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163

bits 16
cpu 386
org 0x100

freq equ 120 ; ticks per second

    mov ax, cs
    mov ds, ax
    cli
    mov word [low_level_ticks_per_tick], 0x1234DD / freq
    mov al, 8
    mov ah, 0x35
    int 0x21
    mov [bios_handler], bx
    mov [bios_handler+2], es
    mov ah, 0x25
    mov dx, handler_entry
    int 0x21
    mov al, 0x34
    out 0x43, al
    mov al, (0x1234DD / freq) % 256
    out 0x40, al
    mov al, (0x1234DD / freq) / 256
    out 0x40, al
    sti
wait_init:
    mov al, [init]
    test al, al
    jz wait_init
    jmp timer
prev_ticks:
    dq 0
next_ticks:
    dq 0
ticks_delta:
    dq 0
msg:
    db "000000", 0x0D, 0x0A, "$"
timer:
    cli
    mov ax, [ticks]
    mov [prev_ticks], ax
    mov ax, [ticks+2]
    mov [prev_ticks+2], ax
    mov ax, [ticks+4]
    mov [prev_ticks+4], ax
    mov ax, [ticks+6]
    mov [prev_ticks+6], ax
    sti
timer_loop:
    mov cx, 128
timer_nops:
    nop
    loop timer_nops
    cli
    mov ax, [ticks]
    mov [next_ticks], ax
    mov [ticks_delta], ax
    mov ax, [ticks+2]
    mov [next_ticks+2], ax
    mov [ticks_delta+2], ax
    mov ax, [ticks+4]
    mov [next_ticks+4], ax
    mov [ticks_delta+4], ax
    mov ax, [ticks+6]
    mov [next_ticks+6], ax
    mov [ticks_delta+6], ax
    sti
    mov bx, ticks_delta
    mov ax, [prev_ticks]
    sub [bx], ax
    mov ax, [prev_ticks+2]
    sbb [bx+2], ax
    mov ax, [prev_ticks+4]
    sbb [bx+4], ax
    mov ax, [prev_ticks+6]
    sbb [bx+6], ax
    cmp word [bx+6], 0
    jg print
    cmp word [bx+4], 0
    jg print
    cmp word [bx+2], 0
    jg print
    cmp word [bx], freq
    jl timer_loop
print:
    add byte [msg+5], 1
    cmp byte [msg+5], '9'
    jle print_msg
    mov byte [msg+5], '0'
    add byte [msg+4], 1
    cmp byte [msg+4], '9'
    jle print_msg
    mov byte [msg+4], '0'
    add byte [msg+3], 1
    cmp byte [msg+3], '9'
    jle print_msg
    mov byte [msg+3], '0'
    add byte [msg+2], 1
    cmp byte [msg+2], '9'
    jle print_msg
    mov byte [msg+2], '0'
    add byte [msg+1], 1
    cmp byte [msg+1], '9'
    jle print_msg
    mov byte [msg+1], '0'
    add byte [msg], 1
    cmp byte [msg], '9'
    jle print_msg
    mov byte [msg], '0'
print_msg:
    mov dx, msg
    mov ah, 0x09
    int 0x21
    mov ax, [next_ticks]
    mov [prev_ticks], ax
    mov ax, [next_ticks+2]
    mov [prev_ticks+2], ax
    mov ax, [next_ticks+4]
    mov [prev_ticks+4], ax
    mov ax, [next_ticks+6]
    mov [prev_ticks+6], ax
    jmp timer_loop
    db 0
ticks:
    dq 0
low_level_ticks_per_tick:
    dw 0
low_level_ticks_mod_10000h:
    dw 0
init:
    db 0
handler_entry:
    pushf
    push ds
    push bx
    push ax
    mov ax, cs
    mov ds, ax
    mov byte [init], 1
    mov bx, ticks
    add word [bx], 1
    adc word [bx+2], 0
    adc word [bx+4], 0
    adc word [bx+6], 0
    mov ax, [low_level_ticks_per_tick]
    add [low_level_ticks_mod_10000h], ax
    jnc skip_bios_handler
    pop ax
    pop bx
    pop ds
    popf
    db 0xEA
bios_handler:
    dd 0
skip_bios_handler:
    mov al, 0x20
    out 0x20, al
    pop ax
    pop bx
    pop ds
    popf
    iret