changed the segment register used for video memory access to es

[bootboot] / bb.asm

diff --git a/bb.asm b/bb.asm
index b3ef814..a3f6011 100644 (file)
--- a/bb.asm
+++ b/bb.asm
@@ -17,16 +17,22 @@ start:
        mov es, ax
        mov ss, ax
 
-; save dx value
+; 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
@@ -79,7 +85,7 @@ clearscreen:
        ret
 
 saved_drive_num:
-       db 0 ; declare byte 0
+       db 0 ; define byte 0
 
 ; assembler trick: write as many 0 needed to fill 510 bytes
 ; $ <- means here
@@ -87,7 +93,90 @@ saved_drive_num:
        dw 0aa55h
 
 sector_2:
-       mov ax, 5
-       call clearscreen
+; 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