New bootsect program to dump bios data area, aid for debugging hw

boot/dumpbda.asm

@@ -0,0 +1,71 @@
+	org 0x7C00
+
+	xor ax, ax
+	mov ds, ax
+	mov es, ax
+	mov ss, ax
+	mov sp, 0x7C00
+
+	mov si, 0x400
+
+loop:	mov dx, si
+	call dmpdx
+	mov al, 0x20
+	int 0x10
+	int 0x10
+
+	push si
+	mov cx, 8
+wloop:	lodsw
+	xchg al, ah
+	mov dx, ax
+	push cx
+	call dmpdx
+	mov al, 0x20
+	int 0x10
+	pop cx
+	loop wloop
+	pop si
+	mov al, 0x20
+	int 0x10
+
+	mov cx, 16
+aloop:	lodsb
+	cmp al, 0x20
+	jnc noadj
+	mov al, '.'
+noadj:	int 0x10
+	loop aloop
+	mov al, 0x0A
+	int 0x10
+	mov al, 0x0D
+	int 0x10
+
+	cmp si, 0x500
+	jc loop
+
+hlt:	hlt
+	jmp hlt
+
+dmpdx:	; setup bx and ah for int 10h call
+	xor bx, bx
+	mov ah, 0x0e
+	mov cl, 4
+	; this double-call is essentially a 4 times repeating loop
+	call dmpdx1
+dmpdx1:	call dmpdx2
+dmpdx2:	; grab highest nibble from dx
+	mov al, dh
+	; remove highest nibble from dx
+	shl dx, cl
+	; shift away second-highest nibble that we accidentally copied
+	shr al, cl
+	; map 0-9 to ascii codes for '0' to '9'
+	add al, 0x30
+	; if result is larger than '9', ...
+	cmp al, 0x3a
+	jl dmpdx3
+	; ... add 7 so we continue at 'A'
+	add al, 7
+dmpdx3:	int 0x10
+	ret