changed the segment register used for video memory access to es

[bootboot] / bb.asm

diff --git a/bb.asm b/bb.asm
index 397bb30..a3f6011 100644 (file)
--- a/bb.asm
+++ b/bb.asm
@@ -37,17 +37,6 @@ start:
        mov ax, 3
        call clearscreen
 
-; waiting for keypress
-; x86 has 2 special instructions to read/write from I/O
-; devices: in and out (because some processors have different
-; address spaces for devices and for memory, arm not)
-.key_wait:
-       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 .key_wait
-
-       in al, 60h      ; reads the keyb that was pressed to reset the flag
-
 ; load 2nd sector from boot device and jump to it
 ; bios helpers, 13h = disk io
        mov ax, 0
@@ -126,7 +115,68 @@ sector_2:
        cmp cx, 200
        jnz .y_loop
 
-; fix palette (256 colors)
-       
+; 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