start:
mov sp, 7c00h
+; initialize segment registers
+ mov ax, 0
+ mov ds, ax
+ mov es, ax
+ mov ss, ax
+
+; save dl value
+; bios sets in dl the num of the drive that
+; loaded you so if you overwrite dl you can reuse it
+ mov [saved_drive_num], dl
+
; service 0: set videomode ah
; param: which video mode (13h: 320x200, 256 colors) al
; ax: ah (high 8 bits), al (low)
+; call 0 = set_videomode it expects the video mode in al
+; ah : 0, al: 13, ax:ahal
mov ax, 13h
; calling video bios
; software interrupt 10h
+; what's the value of ah? al=video mode when ah=0
+; which set which video mode
int 10h
+ mov ax, 3
call clearscreen
-; infinite loop
-end:
- jmp end
+; load 2nd sector from boot device and jump to it
+; bios helpers, 13h = disk io
+ mov ax, 0
+ mov ds, ax
+ mov ah, 02h ; call 2: read sectors into memory
+ mov al, 1 ; number of sectors to read
+ mov ch, 0 ; low 8 bits of cylinder number
+ mov cl, 2 ; sector number that starts from 1
+ mov dh, 0 ; head number
+ mov dl, [saved_drive_num] ; 8bits
+ mov bx, sector_2
+ int 13h
+; error check: if carry flag isn't set jump to loaded code
+ jnc sector_2
+.inf_loop:
+ jmp .inf_loop
+
clearscreen:
; video ram is mapped in a0000
; automatically set to 7c0 (7c00h)
; offset can be paired with any register
; pair the registers
+ push ax
mov ax, 0a000h
mov ds, ax
mov di, 0
+ pop ax
; counter cx
mov cx, 64000
- mov ax, 3
+
.loop_begin:
; dereferrence[] address
mov [di], al
jnz .loop_begin
ret
+saved_drive_num:
+ db 0 ; define byte 0
+
; assembler trick: write as many 0 needed to fill 510 bytes
; $ <- means here
times 510-($-$$) db 0
dw 0aa55h
+
+sector_2:
+; disp palette
+ mov ax, 0a000h ; video segment points to video memory
+ mov ds, ax
+ mov bx, 0 ; video offset
+ mov cx, 0 ; y
+.y_loop:
+ mov dx, 0 ; x
+.x_loop:
+ mov ax, dx
+ test ax, 0ff00h ; lower 8 bits 0, highest 1, Z flag is set while < 256, test = and but doesnt change ax
+ jz .skip_clear_ax
+ mov al, 0
+.skip_clear_ax:
+ mov [bx], al ; pixel written on screen
+ inc bx ; increment
+ inc dx
+ cmp dx, 320
+ jnz .x_loop
+ inc cx
+ cmp cx, 200
+ jnz .y_loop
+
+; setup grayscale palette (64 first colors because ramdac uses 6 bits / color so 2^6)
+; ram dac (digital to analog converter) tis vga
+; in a ^ register which index we want to write
+; 3 writes in another ramdac register (data)
+; ramdac feature: when you need to set a palette
+ mov al, 0 ; first index
+ mov dx, 3c8h ; ramdac index registe
+ out dx, al ; because io port (address) > 255
+ inc dx ; ramdac data register: 3c9h
+.pal_loop:
+ out dx, al ; r
+ out dx, al ; g
+ out dx, al ; b
+ inc al
+ test al, 3fh ; test with 3fh to keep the lowest 6 bits of al
+ jnz .pal_loop
+
+; disable all interrupts (for bios to not read the keyboard)
+ cli
+
+ ; ds on 0 because rand uses it for accesses to mem
+ xor ax, ax
+ mov ds, ax
+ mov ax, 0a000h
+ mov es, ax
+main_loop:
+; wait for vblank
+ mov dx, 3dah
+ in al, dx
+ and al, 8 ; the 4th bit is 1 when we are in the vertical blanking period
+ jz main_loop
+
+ xor bx, bx ; mov bx, 0 to clear the register
+.static_loop:
+ call rand
+ and ax, 3fh ; last six bits of eax (see rand)
+ mov [es:bx], al ; [] -> bx offset and default segment ds we change it to es
+ inc bx
+ cmp bx, 64000 ; num pixels
+ jnz .static_loop
+
+ in al, 64h ; 60h = keyb data port, 64h = keyb status port
+ and al, 1 ; 1 = OUTBUF_FULL = the keyb controller out buf is full
+ jz main_loop ; no key pressed, loop back
+ in al, 60h ; reads the keyb that was pressed to reset the flag
+
+; re-enable all interrupts
+ sti
+
+.inf_loop:
+ jmp .inf_loop
+
+; random number generator
+; by nuclear
+rand:
+ mov eax, [randval]
+ mul dword [randmul]
+ add eax, 12345
+ and eax, 0x7fffffff
+ mov [randval], eax
+ shr eax, 16
+ ret
+
+randmul dd 1103515245
+randval dd 0ace1h
projects / bootboot / blobdiff