1 ; org: all instructions from now on start in 7c00h
2 ; 7c00h is where the bios loads the 1st sector
3 ; assembler formatting:
7 ; at boot: real mode (like it's 8086)
8 ; we have to tell assembler that code is 16bit mode
12 %macro SETPAL 4 ; 4 = num arguments of macro (index rgb)
13 mov dx, 3c8h ; index register
21 mov al, %4 >> 2 ; shift to give the value from 0 - 255
28 ; start of BPB at offset 3
29 db "BSPL 0.1" ; 03h: OEM ident, 8 bytes
30 dw 512 ; 0bh: bytes per sector
31 db 1 ; 0dh: sectors per cluster
32 dw 1 ; 0eh: reserved sectors (including boot record)
33 db 2 ; 10h: number of FATs
34 dw 224 ; 11h: number of dir entries
35 dw 2880 ; 13h: number of sectors in volume
36 db 0fh ; 15h: media descriptor type (f = 3.5" HD floppy)
37 dw 9 ; 16h: number of sectors per FAT
38 dw 18 ; 18h: number of sectors per track
39 dw 2 ; 1ah: number of heads
40 dd 0 ; 1ch: number of hidden sectors
41 dd 0 ; 20h: high bits of sector count
42 db 0 ; 24h: drive number
43 db 0 ; 25h: winnt flags
44 db 28h ; 26h: signature(?)
45 dd 0 ; 27h: volume serial number
46 db "BOOTBOOT "; 2bh: volume label, 11 bytes
47 db "FAT12 " ; 36h: filesystem id, 8 bytes
52 ; initialize segment registers
59 ; bios sets in dl the num of the drive that
60 ; loaded you so if you overwrite dl you can reuse it
61 mov [saved_drive_num], dl
63 ; service 0: set videomode ah
64 ; param: which video mode (13h: 320x200, 256 colors) al
65 ; ax: ah (high 8 bits), al (low)
66 ; call 0 = set_videomode it expects the video mode in al
67 ; ah : 0, al: 13, ax:ahal
71 ; software interrupt 10h
72 ; what's the value of ah? al=video mode when ah=0
73 ; which set which video mode
78 ; load 2nd sector from boot device and jump to it
79 ; bios helpers, 13h = disk io
82 mov ah, 02h ; call 2: read sectors into memory
83 mov al, 3 ; number of sectors to read
84 mov ch, 0 ; low 8 bits of cylinder number
85 mov cl, 2 ; sector number that starts from 1
86 mov dh, 0 ; head number
87 mov dl, [saved_drive_num] ; 8bits
90 ; error check: if carry flag isn't set jump to loaded code
97 ; video ram is mapped in a0000
98 ; first 64000 bytes appear immediately on screen
99 ; mem addresses in real mode have 2 parts: segment and offset
100 ; bits overlap: segment is shifted by 4 and is added to the
101 ; overlapping offset (20 bits number = x86's addressable space = 1MB)
102 ; segment register for the segment: es, ds, cs, ss (and: fs, gs)
103 ; default register = ds (data segment), es = extra segment
104 ; cs = code segment cs:ip (it points where to read instructions and is
105 ; automatically set to 7c0 (7c00h)
106 ; offset can be paired with any register
117 ; dereferrence[] address
121 ; when cx is 0, 0 flag is set
128 ; assembler trick: write as many 0 needed to fill 510 bytes
130 times 510-($-$$) db 0
135 mov ax, 0a000h ; video segment points to video memory
137 mov bx, 0 ; video offset
143 test ax, 0ff00h ; lower 8 bits 0, highest 1, Z flag is set while < 256, test = and but doesnt change ax
147 mov [bx], al ; pixel written on screen
156 ; setup grayscale palette (64 first colors because ramdac uses 6 bits / color so 2^6)
157 ; ram dac (digital to analog converter) tis vga
158 ; in a ^ register which index we want to write
159 ; 3 writes in another ramdac register (data)
160 ; ramdac feature: when you need to set a palette
161 mov al, 0 ; first index
162 mov dx, 3c8h ; ramdac index registe
163 out dx, al ; because io port (address) > 255
164 inc dx ; ramdac data register: 3c9h
170 test al, 3fh ; test with 3fh to keep the lowest 6 bits of al
173 ; setup lovebug palette
174 SETPAL 64, 0, 255, 0 ; chr(64) = @
175 SETPAL 65, 0, 0, 0 ; A
176 SETPAL 66, 50, 50, 50 ; B
177 SETPAL 67, 57, 4, 4 ; C
178 SETPAL 68, 108, 9, 9 ; D
179 SETPAL 69, 164, 4, 4 ; E
181 ; disable all interrupts (for bios to not read the keyboard)
184 ; ds on 0 because rand uses it for accesses to mem
188 shr ax, 4 ; to use it as segment! (shift right)
191 ; draw to back buffer
192 xor bx, bx ; mov bx, 0 to clear the register
195 and ax, 3fh ; last six bits of eax (see rand)
196 mov [es:bx], al ; [] -> bx offset and default segment ds we change it to es
198 cmp bx, 64000 ; num pixels
202 mov di, (200 - 32) * 320 + 160 - 16
213 and ax, 3 ; random value in 0, 2
214 sub ax, 1 ; random value in -1, 1
227 and al, 8 ; the 4th bit is 1 when we are in the vertical blanking period
230 ; copy backbuffer to video memory
234 mov ds, ax ; ds points to backbuffer now (was in es)
238 xor di, di ; write in es:di
239 mov ecx, 16000 ; 16000 double words (dwords) = 64000 bytes
244 inc word [num_frames]
246 ; cmp word [num_frames], 200 - 32
249 in al, 64h ; 60h = keyb data port, 64h = keyb status port
250 and al, 1 ; 1 = OUTBUF_FULL = the keyb controller out buf is full
251 jz main_loop ; no key pressed, loop back
252 in al, 60h ; reads the keyb that was pressed to reset the flag
255 ; re-enable all interrupts
259 mov ax, 3 ; text mode
270 mov dx, 80h ; default to load from drive 80h
271 cmp byte [saved_drive_num], 80h
275 ; load hard disk boot sector
289 ; reads from ds:si writes to es:di (bug top left corner)
290 ; knows how many pixels on screen and how many in the lovebug
292 mov cx, 1024 ; 32 x 32 lovebug
302 dec cx ; dec the num_pixels we want to write
303 jz .end ; all pixels have been written
304 test cx, 1fh ; keep 5 least significant bits, 3 most significant bits 0
305 jnz .loop ; when this is 0 we are in a multiple of 32
306 add di, 320 - 32 ; screen width - bug width
325 ; converts digits to ascii - print
336 div cx ; ax contains cx / 10
354 ; random number generator
365 randmul dd 1103515245
369 str_frames db 'frames: ', 0
370 str_newline db 13, 10, 0 ; crlf carriage return line feed newline
372 ; db to write pixels transparent 255
380 db '@@@@BAAAB@@@@@BAAB@@@@@BAAAB@@@@'
381 db '@@@@@@@@AAAB@BAAAAB@BAAA@@@@@@@@'
382 db '@@@@@@@@@@@AAAAAAAAAA@@@@@@@@@@@'
383 db '@@@@@@@@@@@@AAAAAAAA@@@@@@@@@@@@'
384 db '@@@@@@@@@@@@AAAAAAAA@@@@@@@@@@@@'
385 db '@@@@@@@@@@@@BAAAAAAB@@@@@@@@@@@@'
386 db '@@@@@@@@@@@@@AAACAA@@@@@@@@@@@@@'
387 db '@@@@@@@@@@CDDEECDEEDDC@@@@@@@@@@'
388 db '@@@@@@@CDDEEEEEDCEEEEEDDC@@@@@@@'
389 db '@@@@@CDEEEEDCDECDEDCDEEEEDC@@@@@'
390 db '@@@@CDDCDEECACEDCECACEEDCDDC@@@@'
391 db '@@@CDECACEEDCDECDEDCDEECACEDC@@@'
392 db '@@@DEEDCDEEEEEEDCEEEEEEDCDEED@@@'
393 db '@@CEEEEEEEEEEEDCDDEEEEEEEEEEEC@@'
394 db '@@DEDCDEEEDCDECACCEDCDEEEDCDED@@'
395 db '@@EECACEEECACEDCDDECACEEECACEE@@'
396 db '@CEEDCDEEEDCDEEDCEEDCDEEEDCDEEC@'
397 db '@DEEEEEEEEEEEEDCCDEEEEEEEEEEEED@'
398 db 'CEEEEEEDCDEEEDCBACDEEEDCDEEEEEEC'
399 db 'CEDCDEECACEEDCAABACDEECACEEDCDEC'
400 db 'DECACEEDCDEDCAABAAACDEDCDEECACED'
401 db 'DEDCDEEEEEDCAAAABAAACDEEEEEDCDED'
402 db 'CEEEEEEEEECAAAABAAAAACEEEEEEEEEC'
403 db 'CEEEEEEEEDAAAAAABAAAAADEEEEEEEEC'
404 db '@DEDCDEEDCAAAAABAAAAAACDEEDCDED@'
405 db '@DECACEDCAAAAAAABAAAAAACDECACED@'
406 db '@DEDCDDCAAAAAAABAAAAAAAACDDCDED@'
407 db '@CEEEECAAAAAAAAABAAAAAAAACEEEEC@'
408 db '@@EEED@AAAAAAAABAAAAAAAAA@DEEE@@'
409 db '@@EEDC@@AAAAAAAABAAAAAAA@@CDEE@@'
410 db '@@DEC@@@@@AAAAABAAAAAA@@@@@CED@@'
411 db '@@@C@@@@@@@@AAAABAAA@@@@@@@@C@@@'
413 align 16 ; if the address is not a multiple of 16 add some 0 to become one (because we'll use backbuffer as a segment)