8.13. DEMOS
Let’s get back to the demoThe demo, although very tiny (just 64 bytes or 30 instructions), implements the common algorithm de-
scribed here, but using some coding tricks.The source code is easily downloadable, so here is it, but let’s also add comments:Listing 8.26: 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