User`s manual

Start Address Record (MCS-86 hex format)
Byte Number
1 Colon (:)
2—3 Record length, “04"
4—7 “0000"
8—9 Record type, “03"
10—13 8086 CS value
14—17 8086 IP value
18—19 Check sum
20—21 CR, LF
The checksum is the two’s compliment of the 8-bit sum, without carry,
of all the data bytes, the two bytes in the load address, and the byte
count.
Example:
:03012300010203D3
In the above example add 3 + 1 + 23h + 0 + 1 + 2 + 3 = 2Dh. The
total of the above bytes is 2Dh. If you do a two’s compliment on the
number the result is D3h which, you will notice is the checksum. A
simple visual way of doing this is to write the number in binary, then
invert each bit. After you do that, add 1 to it and that is the checksum:
00101101 = 2D
11010010 = D2 (inversion or negation)
+1 = add 1 for 2’s compliment
11010011 = 2’s compliment checksum.
Chapter 7 Hex Formats Model 9000
Page 26