changed the segment register used for video memory access to es

to avoid interferring with rand which was using ds to access its state

diff --git a/bb.asm b/bb.asm
index d9d1fe6..a3f6011 100644 (file)
--- a/bb.asm
+++ b/bb.asm
@@ -135,14 +135,23 @@ sector_2:
 ; disable all interrupts (for bios to not read the keyboard)
        cli
 
-main_loop:
-       mov ax, 0a000h
+       ; 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 [bx], al
+       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