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Path: utzoo!utgpu!water!watmath!clyde!rutgers!seismo!sundc!netxcom!jallen
From: jallen@netxcom.UUCP
Newsgroups: comp.sys.ibm.pc
Subject: Re: IBM ROM Format???
Message-ID: <13@netxcom.UUCP>
Date: Thu, 23-Apr-87 12:25:20 EST
Article-I.D.: netxcom.13
Posted: Thu Apr 23 12:25:20 1987
Date-Received: Sat, 25-Apr-87 06:44:31 EST
References: <4342@beta.UUCP>
Reply-To: jallen@netxcom.UUCP (John Allen)
Lines: 33
Keywords: Help!

In article <4342@beta.UUCP> myxm@beta.UUCP (Mike Mitchell) writes:
>Hello,
>
>Can anyone explain the format of a ROM which lives in the address
>space C8000 to F4000? I am aware that the first two bytes of one
>of any 2K increments would need to be 0x55 and 0xAA. The third byte
>is the length of the ROM software / 512 bytes. The fourth byte is
>the beginning of the actual code.
>
>Mike Mitchell
>myxm@lanl.gov OR
>...!{ucbvax,decvax,cmcl2,sun,...}!lanl!myxm

The ROM checksum is the simple sum of ALL bytes in the device
modulo 256, and forced to zero.  This includes the checksum byte
itself.  So, what you have to do is:

1)  Choose a location for your checksum.  Anyplace other
    than the first three + the initial JMP is OK.

2)  Take the sum of all other bytes on the device, subtract
    that from 256, and write the result into the checksum location.

3)  Test your end product by making sure that the checksum for the
    entire device is zero.


John Allen
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