WIP on loading files from disk

kernel/drive.asm

@@ -0,0 +1,48 @@
+; Load a sector from drive.
+; IN    DX:AX   sector number
+;	BP	ptr to BPB
+drive_read:
+  push ax
+  push cx
+  push dx
+  push bx
+  push si
+
+  mov cx, dx
+  or cx, ax
+  jz .fast
+
+  div word [bp+13] ; ax:temp     = (lba / spt)
+  inc dx           ; dx:sector   = (lba % spt) + 1
+  mov cl, dl       ; sector number
+
+  div word [bp+15] ; ax:cylinder = (tmp / heads)
+                   ; dx:head     = (tmp % heads)
+  mov ch, al       ; cylinder number
+  mov dh, dl       ; head number
+  mov si, 5        ; retry count
+.try:
+  mov dl, [bp+(bpb_size-1)] ; drive number
+  mov bx, diskbuf
+  mov ax, 0x0201
+  int 0x13
+.giveup:
+  pop si
+  pop bx
+  pop dx
+  pop cx
+  pop ax
+  ret
+.fast: ; fast lane for boot sector
+  inc cl
+  jmp .try
+.fail:
+  ; reset disk
+  mov ah, 0x00
+  int 0x13
+  ; loop with si
+  dec si
+  test si, si
+  jnz .try
+  stc
+  jmp .giveup