Revamp MBR

New features:
- ES:DI (potential PnP structure) is preserved
- FAT detection now works by checking for BPB values
- Uses booterr procedure

boot/mbr.asm

@@ -1,51 +1,65 @@
-; Memory layout:
-%define self      0x00600 ; 1 sector
-%define prog      0x07C00 ; 1 sector
-
 ; FDC fields in VBR
-%define spt       (prog + 0x18)
-%define nos       (prog + 0x1A)
-%define po        (prog + 0x1C)
+%define bps	(bs + 0x0B)
+%define md	(bs + 0x15)
+%define spt	(bs + 0x18)
+%define nos	(bs + 0x1A)
+%define po	(bs + 0x1C)
 
-org self
-cpu 8086
+; blah dont judge me, i miss 8080
+%macro retnz 0
+	jz short ($+3)
+	ret
+%endmacro
+%macro callc 1
+	jnc short ($+5)
+	call %1
+%endmacro
 
-init:
-  cli
+	cpu 8086
+	org 0x0600
+bs:	equ 0x7C00
 
-  ; Stack grows down from PSP + 64k
+init:	cli
 	xor ax, ax
 	mov ss, ax
-  mov sp, ax
+	xor sp, sp
 
+	; save driver number
 	push dx
 
-  ; Relocate from [prog] to [self]
+	; save PnP ptr
+	push es
+	push di
+
+	; Relocate away from bootsector loading area
 	mov ds, ax
 	mov es, ax
-  mov si, prog
-  mov di, self
+	mov si, bs
+	mov di, $$
 	mov cx, 0x100
 	rep movsw
 
 	jmp 0:main
 
-main:
-  mov bp, 0x3335
-  mov si, part1
+	%include "inc/booterr.asm"
+
+	; find active partition or bail out
+	; result in si
+findp:	mov si, part1
 	mov cx, 4
-.loop:
-  test BYTE [si], 0x80
-  jnz loadpart
+.loop:	test BYTE [si], 0x80
+	retnz
 	add si, 0x10
 	loop .loop
-  jmp error
+	call errcll
+
+main:	; find active partition
+	call findp
 
-loadpart:
 	; transfer starting address into DAP
 	push si
 	add si, 0x08
-  mov di, dap.blocknum
+	mov di, dap.num
 	movsw
 	movsw
 	pop si
@@ -53,24 +67,30 @@ loadpart:
 	; load sector
 	push si
 	mov si, dap
-  mov bp, 0x3336
 	mov ah, 0x42
 	stc
 	int 0x13
-  jc error
+	callc errcll
 	pop si
 
-  cmp BYTE [si+4], 0x01
-  jne jump
+	; FAT bpb has "bytes per sector" at this place
+	; we support only 512 byte sectors anyways
+	cmp word [bps], 512
+	jne chain
 
-adjust:
-  push dx
+	; Media descriptor byte is never smaller than 0xF0
+	cmp byte [md], 0xF0
+	jc chain
 
-  mov bp, 0x3337
+	; ask BIOS what our disk geometry is
 	mov ah, 0x08
 	stc
 	int 0x13
-  jc error
+	jc chain
+
+	; ES might be messed up from int 13h, fix it
+	push cs
+	pop es
 
 	; update sectors per track
 	xor ax, ax
@@ -90,47 +110,33 @@ adjust:
 	movsw
 	pop si
 
+chain:	; restore PnP structure
+	pop di
+	pop es
 	pop dx
-
-jump:
-  jmp 0:prog
-
-error:
-  mov ax, bp
-  mov ah, 0x0e
-  mov bx, 7
-  int 0x10
-  mov al, 0x21
-  int 0x10
-  xor ax, ax
-  int 0x16
-  int 0x19
+	xor cx, cx
+	mov bp, si
+	; chain into partition boot sector
+	jmp 0:bs
 
 dap:
-.size:
+.size:	db 0
 	db 0
-  db 0
-.count:
-  dw 1
-.buffer:
-  dw prog
+.count:	dw 1
+.buf:	dw bs
 	dw 0
-.blocknum:
-  dq 0
+.num:	dq 0
 
-times (0x1BE - ($-$$)) db 0
-part1:
-  db 0x80
-.chs_start:
+	times (0x1BE - ($-$$)) db 0
+
+part1:	db 0x80
 	db 0xFF, 0xFF, 0xFF
-.type:
 	db 0x01
-.chs_end:
 	db 0xFF, 0xFF, 0xFF
-.begin:
 	dd 1
-.end:
 	dd (FLOPPY * 2)
 
-times (0x1FE - ($-$$)) db 0
-dw 0xAA55
+	times (0x1FE - ($-$$)) db 0
+
+	; Boot signature
+	dw 0xAA55