load lba 1

boot0.s

@@ -46,22 +46,48 @@ bits 16
   mov di, 0x00
   mov ah, 0x08
   int 0x13
-  jb panic
-  and cl, 0x3f
+  jc panic
+  and cl, 0x3f     ; sectors per track
   mov [bp + 1], cl
-  movzx ax, dh
-  add ax, 1
-  mov [bp + 2], ax
+  movzx bx, dh
+  inc bx           ; number of heads
+  mov [bp + 2], bx
 
+  ; Load LBA 1.
+  cmp cl, 1  ; division of 1 by nonzero can be done with cmp
+  sete al    ; temp = LBA / (sectors per track)
+  setne cl   ; sector - 1 = LBA % (sectors per track)
+  inc cl
+  xor ah, ah ; zero-extend temp
+  xor dx, dx ; TODO: explain why this is needed for div
+  div bx     ; ah = mod (head), al = div (cylinder)
   
+  mov dh, ah ; head
+  mov ch, al ; cylinder
+             ; we already have sector in cl
+  mov ah, 2  ; read disk
+  mov al, 1  ; load one sector
+  mov bx, 0x7e00
+  mov dl, [bp] ; drive number
+  int 0x13
+  jc panic
+
+  ; head = temp % number of heads
+  ; cylinder = temp / number of heads
 
   ; Set VGA mode
   ; https://mendelson.org/wpdos/videomodes.txt
   mov ax, 0x0003
   int 0x10
 
-  mov word fs:[0x0000], 0xc048
-  mov word fs:[0x0002], 0xc069
+  ; mov word fs:[0x0000], 0xc048
+  ; mov word fs:[0x0002], 0xc069
+  mov al, [0x7e00]
+  mov byte fs:[0x0000], al
+  mov al, [0x7e01]
+  mov byte fs:[0x0002], al
+  mov al, [0x7e02]
+  mov byte fs:[0x0004], al
   hlt
 
 panic: