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From: bill@hpcvlo.HP.COM (Bill Frolik)
Newsgroups: comp.sys.ibm.pc
Subject: Re: IBM ROM Format???
Message-ID: <1610012@hpcvlo.HP.COM>
Date: Thu, 23-Apr-87 11:16:44 EST
Article-I.D.: hpcvlo.1610012
Posted: Thu Apr 23 11:16:44 1987
Date-Received: Sat, 25-Apr-87 23:59:22 EST
References: <4342@beta.UUCP>
Organization: Hewlett-Packard Co., Corvallis, OR, USA
Lines: 30


The checksum is the one-byte sum of the bytes in however many 512-byte
blocks the ROM claims to contain.  A valid ROM has a zero checksum,
which can be tested as follows:

	mov	ax,<ROM segment>;
	mov	ds,ax		;DS:SI ->
	xor	si,si		; start of ROM
	xor	ah,ah		;Clear checksum accumulator
	mov	dl,ds:[si+2]	;DL = Number of 512-byte blocks

Sum1:	mov	cx,512		;In AH, sum up
Sum2:	lodsb			; all the bytes in
	add	ah,al		; this 512-byte
	loop	Sum2		; block.

	dec	dl		;Sum up as many 512-byte blocks
	jnz	Sum1		; as the ROM claims it contains
	test	ah,ah		;Is the sum zero?
	jz	GoodROM		;If yes, checksum passes

You can get the checksum to work out to zero on a new ROM 
by just setting any unused byte (within the region of however
many 512-byte blocks you're using) to whatever value it takes
to get the total to become zero.

Bill Frolik
hp-pcd!bill
Hewlett-Packard Portable Computer Division
Corvallis, Oregon