What does the second-to-the-last byte in a program?

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amihart
Member (Posts: 119)

What does the second-to-the-last byte in a program?
06 Nov 2013 06:06:38 pm

Here I compiled two basic "Hello, World!" programs. The only difference between them is that one has a capital "W" and the other has a lowercase "w".

This is the hex output of the two:

Code:

2A 2A 54 49 38 33 46 2A 1A 0A 00 43 6F 6D 70 69 6C 65 64 20 77 69 74 68 20 4F 52 47 3A 20 68 74 74 70 3A 2F 2F 63 6C 72 68 6F 6D 65 2E 6F 72 67 2F 61 73 6D 2F 35 00 0D 00 24 00 06 48 45 4C 4C 4F 00 00 00 00 00 24 00 22 00 BB 6D EF 40 45 21 00 00 22 4B 84 21 A8 9D EF 0A 45 EF 2E 45 C9 48 65 6C 6C 6F 20 77 6F 72 6C 64 21 00 CB 0E





**TI83F*


Compiled with ORG: http://clrhome.org/asm/5


$HELLO$"�m�@E!"K�!���


E�.E�Hello world!�

Code:

2A 2A 54 49 38 33 46 2A 1A 0A 00 43 6F 6D 70 69 6C 65 64 20 77 69 74 68 20 4F 52 47 3A 20 68 74 74 70 3A 2F 2F 63 6C 72 68 6F 6D 65 2E 6F 72 67 2F 61 73 6D 2F 35 00 0D 00 24 00 06 48 45 4C 4C 4F 00 00 00 00 00 24 00 22 00 BB 6D EF 40 45 21 00 00 22 4B 84 21 A8 9D EF 0A 45 EF 2E 45 C9 48 65 6C 6C 6F 20 57 6F 72 6C 64 21 00 AB 0E





**TI83F*


Compiled with ORG: http://clrhome.org/asm/5


$HELLO$"�m�@E!"K�!���


E�.E�Hello World!�

If you look, all the bytes are exactly identical except for two.
The first one is " 6F 20 77 " and " 6F 20 57 ". That's because "57" is a "W" while "77" is a "w". That's not what's confusing me.

What I want to know about is the second-to-the-last byte.

In the first file, it says "CB". In the next file, it says "AB".
This number changes even if I change the value of even one byte in the file (such as, "w" to "W").

And if I change that value at all, the program becomes "Invalid" and won't run.

What is this byte? How do I generate the correct byte so that the program will run on the calculator?

If that byte isn't exactly the right value, the assembled program would refuse to run.

BlackOnyx
New Member (Posts: 87)

06 Nov 2013 06:26:03 pm

My immediate guess would be that its part of the checksum or file version. But for only one byte, that seems really small. Im not really sure. Anyone else? Kerm?

tr1p1ea
1
Cemetech Expert (Posts: 1744)

06 Nov 2013 06:49:46 pm

Yes it is the checksum. You can tell since the difference between 'W' and 'w' is 32, which is $20 in hex and thus the difference between AB and CB.

TI uses the lower 16 bits of the sum of all bytes in the data section for a checksum. If this is incorrect your linking software may reject the file.

Remember that z80 is little-endian, so the Checksum is $0ECB/AB.

BlackOnyx
New Member (Posts: 87)

06 Nov 2013 06:56:56 pm

Is there a way to calculate the sum manually, and write it in (if you modified a file in hex)

tr1p1ea
1
Cemetech Expert (Posts: 1744)

06 Nov 2013 07:06:37 pm

Yes there is, the checksum is the lower 16-bits of all bytes in the data section. The hex above is a complete file so the data section begins at offset 55 ($37). So:

First 55 bytes:

Code:

2A   2A   54   49   38   33   46   2A   1A   0A   00   43   6F   6D   70   69   6C   65   64   20   77   69   74   68   20   4F   52   47   3A   20   68   74   74   70   3A   2F   2F   63   6C   72   68   6F   6D   65   2E   6F   72   67   2F   61   73   6D   2F   35   00

Data in data section (excluding final 2 bytes (checksum):

Code:

0D   00   24   00   6   48   45   4C   4C   4F   00   00   00   00   00   24   00   22   00   BB   6D   EF   40   45   21   00   00   22   4B   84   21   A8   9D   EF   0A   45   EF   2E   45   C9   48   65   6C   6C   6F   20   57   6F   72   6C   64   21   00

If you add those together you get 3755, or 0EAB in hex. If you change any bytes you can add these up again.

The TI Link Protocol Guide can assist, it is available at ticalc.org

BlackOnyx
New Member (Posts: 87)

06 Nov 2013 08:04:53 pm

Gotcha. Very helpful info. Thank you!

amihart
Member (Posts: 119)

06 Nov 2013 10:57:05 pm

tr1p1ea wrote:

Yes there is, the checksum is the lower 16-bits of all bytes in the data section. The hex above is a complete file so the data section begins at offset 55 ($37). So:

First 55 bytes:

Code:

2A   2A   54   49   38   33   46   2A   1A   0A   00   43   6F   6D   70   69   6C   65   64   20   77   69   74   68   20   4F   52   47   3A   20   68   74   74   70   3A   2F   2F   63   6C   72   68   6F   6D   65   2E   6F   72   67   2F   61   73   6D   2F   35   00

Data in data section (excluding final 2 bytes (checksum):

Code:

0D   00   24   00   6   48   45   4C   4C   4F   00   00   00   00   00   24   00   22   00   BB   6D   EF   40   45   21   00   00   22   4B   84   21   A8   9D   EF   0A   45   EF   2E   45   C9   48   65   6C   6C   6F   20   57   6F   72   6C   64   21   00

If you add those together you get 3755, or 0EAB in hex. If you change any bytes you can add these up again.

The TI Link Protocol Guide can assist, it is available at ticalc.org

Thank you! This is very helpful.

merthsoft
1
Administrator Emeritus (Posts: 3820)

07 Nov 2013 07:13:28 am

The linkguide is very handy for understanding these things:
http://merthsoft.com/linkguide

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