anduin / circle.asm

; boot2.asm ; nasm -f bin boot2.asm -o boot2.img ; qemu-system-i386 -drive format=raw,file=boot2.img [org 0x7c00] bits 16 start: cli xor ax, ax mov ds, ax mov es, ax ; --- 1) 切换到 320×200 256 色模式 (mode 13h) --- mov ah, 0x00 mov al, 0x13 int 0x10 ; --- 2) 清屏（黑色） --- mov ax, 0xA000 mov es, ax xor di, di mov al, 0 mov cx, 320*200 rep stosb ; --- 3) 绘制灰色圆 --- call draw_circle jmp $ ; ------------------------------------------------------- ; draw_circle: 在 (160,100) 处以半径 80 画灰色圆 ; ------------------------------------------------------- draw_circle: mov cx, 80 ; radius = 80 xor dx, dx ; x = 0 mov bx, cx ; y = radius mov ax, 1 sub ax, cx ; d = 1 - radius circ_loop: ; 画出 8 个对称点 call plot_octants ; 更新决策变量 d ; temp = 4*x mov si, dx add si, si add si, si cmp ax, 0 jl d_negative ; d >= 0 mov di, bx add di, di add di, di ; di = 4*y sub si, di ; si = 4*(x - y) add si, 10 dec bx ; y-- jmp d_update d_negative: ; d < 0 add si, 6 d_update: add ax, si ; d += si inc dx ; x++ cmp dx, bx jle circ_loop ret ; ------------------------------------------------------- ; plot_octants: 将 (dx, bx) 对称的 8 点传入 plot_pixel ; 入口：DX=x, BX=y ; ------------------------------------------------------- plot_octants: ; 保存通用寄存 push ax push bx push cx push dx push si push di ; ( x, y) mov si, dx mov di, bx call plot_pixel ; ( y, x) mov si, bx mov di, dx call plot_pixel ; (-x, y) mov si, dx neg si mov di, bx call plot_pixel ; (-y, x) mov si, bx neg si mov di, dx call plot_pixel ; (-x, -y) mov si, dx neg si mov di, bx neg di call plot_pixel ; (-y, -x) mov si, bx neg si mov di, dx neg di call plot_pixel ; ( x, -y) mov si, dx mov di, bx neg di call plot_pixel ; ( y, -x) mov si, bx mov di, dx neg di call plot_pixel ; 恢复寄存 pop di pop si pop dx pop cx pop bx pop ax ret ; ------------------------------------------------------- ; plot_pixel: ; SI = x 偏移 (signed), DI = y 偏移 (signed) ; 在中心 (160,100) 上绘制一个灰色像素 (palette index = 7) ; ------------------------------------------------------- plot_pixel: push ax push bx push cx push dx ; 计算 px = 160 + SI, py = 100 + DI mov ax, 160 add ax, si ; px mov cx, 100 add cx, di ; py ; offset = py*320 + px = (py<<8)+(py<<6)+px mov bx, cx shl bx, 8 mov di, bx mov bx, cx shl bx, 6 add di, bx add di, ax mov al, 7 ; gray mov [es:di], al pop dx pop cx pop bx pop ax ret times 510-($-$$) db 0 dw 0xAA55