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circle.asm(file created)

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1 + ; boot2.asm
2 + ; nasm -f bin boot2.asm -o boot2.img
3 + ; qemu-system-i386 -drive format=raw,file=boot2.img
4 +
5 + [org 0x7c00]
6 + bits 16
7 +
8 + start:
9 + cli
10 + xor ax, ax
11 + mov ds, ax
12 + mov es, ax
13 +
14 + ; --- 1) 切换到 320×200 256 色模式 (mode 13h) ---
15 + mov ah, 0x00
16 + mov al, 0x13
17 + int 0x10
18 +
19 + ; --- 2) 清屏（黑色） ---
20 + mov ax, 0xA000
21 + mov es, ax
22 + xor di, di
23 + mov al, 0
24 + mov cx, 320*200
25 + rep stosb
26 +
27 + ; --- 3) 绘制灰色圆 ---
28 + call draw_circle
29 +
30 + jmp $
31 +
32 + ; -------------------------------------------------------
33 + ; draw_circle: 在 (160,100) 处以半径 80 画灰色圆
34 + ; -------------------------------------------------------
35 + draw_circle:
36 + mov cx, 80 ; radius = 80
37 + xor dx, dx ; x = 0
38 + mov bx, cx ; y = radius
39 + mov ax, 1
40 + sub ax, cx ; d = 1 - radius
41 +
42 + circ_loop:
43 + ; 画出 8 个对称点
44 + call plot_octants
45 +
46 + ; 更新决策变量 d
47 + ; temp = 4*x
48 + mov si, dx
49 + add si, si
50 + add si, si
51 +
52 + cmp ax, 0
53 + jl d_negative
54 + ; d >= 0
55 + mov di, bx
56 + add di, di
57 + add di, di ; di = 4*y
58 + sub si, di ; si = 4*(x - y)
59 + add si, 10
60 + dec bx ; y--
61 + jmp d_update
62 +
63 + d_negative:
64 + ; d < 0
65 + add si, 6
66 +
67 + d_update:
68 + add ax, si ; d += si
69 + inc dx ; x++
70 + cmp dx, bx
71 + jle circ_loop
72 + ret
73 +
74 + ; -------------------------------------------------------
75 + ; plot_octants: 将 (dx, bx) 对称的 8 点传入 plot_pixel
76 + ; 入口：DX=x, BX=y
77 + ; -------------------------------------------------------
78 + plot_octants:
79 + ; 保存通用寄存
80 + push ax
81 + push bx
82 + push cx
83 + push dx
84 + push si
85 + push di
86 +
87 + ; ( x, y)
88 + mov si, dx
89 + mov di, bx
90 + call plot_pixel
91 +
92 + ; ( y, x)
93 + mov si, bx
94 + mov di, dx
95 + call plot_pixel
96 +
97 + ; (-x, y)
98 + mov si, dx
99 + neg si
100 + mov di, bx
101 + call plot_pixel
102 +
103 + ; (-y, x)
104 + mov si, bx
105 + neg si
106 + mov di, dx
107 + call plot_pixel
108 +
109 + ; (-x, -y)
110 + mov si, dx
111 + neg si
112 + mov di, bx
113 + neg di
114 + call plot_pixel
115 +
116 + ; (-y, -x)
117 + mov si, bx
118 + neg si
119 + mov di, dx
120 + neg di
121 + call plot_pixel
122 +
123 + ; ( x, -y)
124 + mov si, dx
125 + mov di, bx
126 + neg di
127 + call plot_pixel
128 +
129 + ; ( y, -x)
130 + mov si, bx
131 + mov di, dx
132 + neg di
133 + call plot_pixel
134 +
135 + ; 恢复寄存
136 + pop di
137 + pop si
138 + pop dx
139 + pop cx
140 + pop bx
141 + pop ax
142 + ret
143 +
144 + ; -------------------------------------------------------
145 + ; plot_pixel:
146 + ; SI = x 偏移 (signed), DI = y 偏移 (signed)
147 + ; 在中心 (160,100) 上绘制一个灰色像素 (palette index = 7)
148 + ; -------------------------------------------------------
149 + plot_pixel:
150 + push ax
151 + push bx
152 + push cx
153 + push dx
154 +
155 + ; 计算 px = 160 + SI, py = 100 + DI
156 + mov ax, 160
157 + add ax, si ; px
158 + mov cx, 100
159 + add cx, di ; py
160 +
161 + ; offset = py*320 + px = (py<<8)+(py<<6)+px
162 + mov bx, cx
163 + shl bx, 8
164 + mov di, bx
165 + mov bx, cx
166 + shl bx, 6
167 + add di, bx
168 + add di, ax
169 +
170 + mov al, 7 ; gray
171 + mov [es:di], al
172 +
173 + pop dx
174 + pop cx
175 + pop bx
176 + pop ax
177 + ret
178 +
179 + times 510-($-$$) db 0
180 + dw 0xAA55
181 +
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