mbr: make use of BPB includes

boot/mbr.asm

@@ -1,20 +1,17 @@
-; FDC fields in VBR
-%define bps	(bs + 0x0B)
-%define md	(bs + 0x15)
-%define spt	(bs + 0x18)
-%define nos	(bs + 0x1A)
-%define po	(bs + 0x1C)
-
 	; blah dont judge me, i miss 8080
 	%macro retnz 0
-	jz short ($+3)
+	jz short ($+2+1)
 	ret
 	%endmacro
 	%macro callc 1
-	jnc short ($+5)
+	jnc short ($+2+3)
 	call %1
 	%endmacro
 
+	%include "inc/bpb.asm"
+
+	%define bpb	bs+0x0B
+
 	cpu 8086
 	org 0x0600
 bs:	equ 0x7C00
@@ -75,11 +72,11 @@ main:	; find active partition
 
 	; FAT bpb has "bytes per sector" at this place
 	; we support only 512 byte sectors anyways
-	cmp word [bps], 512
+	cmp word [bpb+bpb_ss], 512
 	jne chain
 
 	; Media descriptor byte is never smaller than 0xF0
-	cmp byte [md], 0xF0
+	cmp byte [bpb+bpb_md], 0xF0
 	jc chain
 
 	; ask BIOS what our disk geometry is
@@ -95,17 +92,17 @@ main:	; find active partition
 	; update sectors per track
 	xor ax, ax
 	mov al, cl
-	mov [spt], ax
+	mov [bpb+bpb_spt], ax
 
 	; update number of sides
 	xor ax, ax
 	mov al, dh
-	mov [nos], ax
+	mov [bpb+bpb_nos], ax
 
 	; update partition offset
 	push si
 	add si, 0x08
-	mov di, po
+	mov di, bpb+bpb_po
 	movsw
 	movsw
 	pop si