CHAPTER 83. DEMOS CHAPTER 83. DEMOS83.2.2 Let’s get back to the demo.
The demo, although very tiny (just 64 bytes or 30 instructions), implements the common algorithm described here, but using
some coding tricks.The source code is easily downloadable, so here is it, but let’s also add comments:Listing 83.5: Commented source code
1 ; X is column on screen
2 ; Y is row on screen
3
4
5 ; X=0, Y=0 X=319, Y=0
6 ; +------------------------------->
7 ; |
8 ; |
9 ; |
10 ; |
11 ; |
12 ; |
13 ; v
14 ; X=0, Y=199 X=319, Y=199
15
16
17 ; switch to VGA 320200256 graphics mode
18 mov al,13h
19 int 10h
20 ; initial BX is 0
21 ; initial DI is 0xFFFE
22 ; DS:BX (or DS:0) is pointing to Program Segment Prefix at this moment
23 ; ... first 4 bytes of which are CD 20 FF 9F
24 les ax,[bx]
25 ; ES:AX=9FFF:20CD
26
27 FillLoop:
28 ; set DX to 0. CWD works as: DX:AX = sign_extend(AX).
29 ; AX here 0x20CD (at startup) or less then 320 (when getting back after loop),
30 ; so DX will always be 0.
31 cwd
32 mov ax,di
33 ; AX is current pointer within VGA buffer
34 ; divide current pointer by 320
35 mov cx,320
36 div cx
37 ; DX (start_X) - remainder (column: 0..319); AX - result (row: 0..199)
38 sub ax,100
39 ; AX=AX-100, so AX (start_Y) now is in range -100..99
40 ; DX is in range 0..319 or 0x0000..0x013F
41 dec dh
42 ; DX now is in range 0xFF00..0x003F (-256..63)
43
44 xor bx,bx
45 xor si,si
46 ; BX (temp_X)=0; SI (temp_Y)=0
47
48 ; get maximal number of iterations
49 ; CX is still 320 here, so this is also maximal number of iteration
50 MandelLoop:
51 mov bp,si ; BP = temp_Y
52 imul si,bx ; SI = temp_Xtemp_Y
53 add si,si ; SI = SI2 = (temp_Xtemp_Y)2
54 imul bx,bx ; BX = BX^2 = temp_X^2
55 jo MandelBreak ; overflow?
56 imul bp,bp ; BP = BP^2 = temp_Y^2
57 jo MandelBreak ; overflow?
58 add bx,bp ; BX = BX+BP = temp_X^2 + temp_Y^2
59 jo MandelBreak ; overflow?
60 sub bx,bp ; BX = BX-BP = temp_X^2 + temp_Y^2 - temp_Y^2 = temp_X^2
61 sub bx,bp ; BX = BX-BP = temp_X^2 - temp_Y^2
62