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TUCoPS :: General Information :: tihack2.txt

Hacking the TI-85 Calculator





+============================================================================
|
|                       Hacking the TI-85 Calculator
|                                    by
|                     David Boozer (adb2y@virginia.edu)
|
|                           September 1, 1994
|
+============================================================================

This document explains how to do various spiffy things with your TI-85, such
as dumping the TI-85's ROM and programming the TI-85 in machine language.  I
have included several programs I wrote which are useful for TI hacking, and
an .85B file containing the first (to my knowledge) machine language program
ever written for the TI-85 (well, except for the ROM, of course :)).  The
information provided here has been obtained by diddling around with .85B
produced using the LINK85 package - the basic method is to:

   (1)  Make a backup of the calculator

   (2)  Study the backup

   (3)  Patch the backup

   (4)  Send it back to the calculator

Much of the information I have learned about the TI-85 has not made it into
this document, but I have tried to include the essentials.  I am working on a
more comprehensive version which should be ready in a few weeks.  Classes
have just started, and I figured I should write something up while I still
have lots of free time on my hands :).  Well, have fun tinkering around and
send questions/comments to adb2y@virginia.edu.

*  All numbers are in hexadecimal, unless otherwise stated

** The procedures outlined here could cause your TI to crash.  If this
   happens, remove one of the batteries, hold down the ON key for a few
   seconds, and replace the battery.  Also, remember to turn up the contrast
   by doing 2nd up-arrow a few times after you turn the calculator on.  If
   you are at all squeamish about crashing your TI, then don't try these
   procedures (but you still might like to read about them).

+----------------------------------------------------------------------------
| The Structure of a .85B File
+----------------------------------------------------------------------------

An .85B file has the following format:

       File Offset             Contents

   0000    -    0034        Header
   0035    -    0036        Size word (Number of data bytes)
   0037    -    XXXX-2      Data bytes (obtained from TI-85)
   XXXX-1  -    XXXX        Checksum word

The header contains the string "**TI85**", and the file comment that you see
when using LINK85.  The size word at offset 0035 is the number of bytes that
contain data obtained from the TI-85, and is equal to:

     (Size of .85B file in bytes)
   - (35 bytes for the header)
   - (2 bytes for the size word)
   - (2 bytes for the checksum)
   ------------------------------
     (# of data bytes)

Thus:

   (# of data bytes) = (Size of .85B file in bytes) - 39

The checksum word is calculated by adding together all the data bytes (but NOT
bytes from the header, size word, or checksum word) MOD 10000.  I wrote a
little assembly program called FIX.COM to automagically adjust the checksum
word of a .85B file to the correct value:

begin 644 fix.com
MOX``B@6*R#+M@_D`='E)1XH%/"!T\T&+][^2`?.DN`(]NI(!S2&+V')4M#^Y
M`("Z,0+-(:.0`7)%OV8"BPTSP#/2B_F!QV@"3XH5`\+B^8L^D`&!QR\"B06X
M`$(SR3/2S2%R&K1`BPZ0`;HQ`LTA<@VT";H4`LTAM#[-(<T@NN(!M`G-(>OQ
MNO0!M`G-(<T@````````````````````````````````````````````````
M````````````````````````````````````````````````````````````
M`$9I;&4@22]/($5R<F]R+@T*)$EN=F%L:60@;G5M8F5R(&]F('!A<F%M971E
C<G,N#0HD0VAE8VMS=6T@:&%S(&)E96X@=7!D871E9"X-"B0`
`
end

To use the program, simply type:

   FIX [filename]

FIX will also work with .85I files, etc.

The first 80 bytes of a typical .85B file are shown below:

0000  2A 2A 54 49 38 35 2A 2A-1A 0C 00 42 61 63 6B 75   **TI85**...Backu
0010  70 20 66 69 6C 65 20 64-61 74 65 64 20 30 39 2F   p file dated 09/
0020  30 31 2F 39 34 2C 20 31-39 3A 31 39 00 2E 00 55   01/94, 19:19...U
0030  27 F5 5D AE 92 0A 11 09-00 B1 08 1D 10 08 38 00   '.]...........8.
0040  F7 8B B1 08 20 00 10 00-20 00 01 01 0C 08 00 00   .... ... .......
0050  32 12 00 00 00 00 05 01-00 00 00 00 00 00 00 00   2...............
0060  00 00 00 20 3F 20 3F 63-84 6D 84 77 84 81 84 8B   ... ? ?c.m.w....
0070  84 95 84 15 00 00 C0 01-FC 10 00 00 00 00 00 00   ................

We see that there are 110A data bytes.  The size of the .85B file is 1143;
note that 110A = 1143 - 39.

The data bytes from offset 0900 up to the checksum word are a memory image of
the TI-85's RAM (well, not the entire RAM, but some of it).  A byte at offset
XXXX in the .85B file corresponds to a byte at memory address:

   8C00 + (XXXX - 0900)

in the TI-85.  For example, a byte at offset 090B in the .85B file corresponds
to a byte at address 8C0B in the TI-85.

A hex dump of the bytes at offset 0900 from a .85B file is shown below:

0900  00 00 00 00 00 0B 00 44-35 00 0B 38 42 6C 65 74   .......D5..8Blet
0910  63 68 00 00 00 02 FC 12-30 00 00 00 00 00 00 00   ch......0.......
0920  FC 50 00 00 00 00 00 00-34 00 68 63 74 65 6C 42   .P......4.hctelB
0930  06 8C 1E 00 6F 6F 46 03-8C 14 00 21 01 8C 12 12   ....ooF....!....
0940  23 01 8C 05 12 73 6E 41-03 8B FB 00 74 61 74 53   #....snA....tatS
0950  79 05 8B F9 04 74 61 74-53 78 05 8B F7 04 32 BC   y....tatSx....2.

When this backup was made, the calculator had two variables defined:

   123->Foo
   5->Bletch

Notice how these variables appear in the RAM image - each variable has an
entry in a symbol table.  This is what the entry for the "Foo" variable means:


            +--- The number of characters in the variable name (3)
            |
            |  +--+--- The address of the variable in memory (8C14)
            |  |  |
   6F 6F 46 03-8C 14 00
    |  |  |           |
    |  |  |           +--- The type of the variable (00 => REAL)
    |  |  |
    +--+--+--- The name of the variable, backwards (Foo)

Note that the address of the variable in memory is stored high order byte
first!

Thus, the symbol table from the above hex dump gives us the following
information:

   Variable Name       Address of Variable      Type of Variable

      xStat                   8BF7                 04 => LIST
      yStat                   8BF9                 04 => LIST
      Ans                     8BFB                 00 => REAL
      Foo                     8C14                 00 => REAL
      Bletch                  8C1E                 00 => REAL

Variables "xStat", "yStat", and "Ans" were created automatically by the
calculator.

The bytes for different types of variables are given below:

       Type Byte         Variable Type

          00                REAL
          01                CMPLX
          02                VECTR
          03                VECTR
          04                LIST
          05                LIST
          06                MATRX
          07                MATRX
          08                CONS
          09                CONS
          0A                EQU
          0B                <blank>
          0C                STRNG
          0D                GDB
          0E                GDB
          0F                GDB
          10                GDB
          11                PIC
          12                PRGM

Now, remember that a byte at offset XXXX in the .85B file corresponds to
a byte at address 8C00 + (XXXX - 0900) in the TI-85's RAM.  Thus, since
the variable "Foo" is located at address 8C14 in RAM, it must be located
at offset

   (8C14 - 8C00) + 0900 = 0914

in our .85B file.  If we look at the ten bytes at 0914 in the above hex dump,
we see the following:

   00 02 FC 12 30 00 00 00 00 00

What it means is this:

         +--- <Range> (one byte)
         |
         |  +--+--+--+--+--+--+--- <Number> (in BCD, 7 bytes)
         |  |  |  |  |  |  |  |
   00 02 FC 12 30 00 00 00 00 00
   |  |
   +--+--- <Exponent> (in hex, 2 bytes)

Thus, for the variable "Foo":

   <Range>     =  FC
   <Exponent>  =  2
   <Number>    =  1.23

The value of the variable is given by:

   Value = <Number>E[<Exponent> + (100)(<Range> - FC)]

So for "Foo":

   Value = (1.23)E[2 - (100)(FC - FC)]
         = (1.23)E(2)
         = 123

Note that numbers which are less than one may be represented by using a <Range>
which is less than FC.

We have seen how data in the .85B file corresponds to data in the TI-85.  The
key points are:

   -  We can edit the .85B file

   -  Data in the .85B file is put into RAM when the .85B file is loaded

   -  Hence, by editing the .85B file, we can diddle around with bytes in
      the TI-85's RAM

   -  In particular, we can edit the memory address of a variable - make it
      point to something OTHER than the data it was given when it was
      created.  This brings us to...

+----------------------------------------------------------------------------
| Dumping the TI-85's ROM
+----------------------------------------------------------------------------

The method I developed to dump the TI's ROM is the following:

  (1)  Clear the calculator, then use STPIC to make a PIC file - lets call it
       "PIC1"

  (2)  Back up the calculator - we then get a file "BACKUP.85B"

  (3)  Edit "BACKUP.85B" - change the memory address of "PIC1" to point to a
       ROM location

  (4)  Fix up the checksum of "BACKUP.85B"

  (5)  Transfer the hacked version of "BACKUP.85B" to the TI-85

  (6)  Use RCPIC to load "PIC1" into the graphics screen - the stuff you see
       on the screen is ROM data

  (7)  Use STPIC to store this ROM data in a new PIC file - lets call it "Dump"

  (8)  Transfer "Dump" from the TI-85 to your computer

  (9)  Strip of header & other junk from "Dump"

You now have a 3F0 byte core dump of the TI-85's ROM.  I wrote an assembly
program called DUMP.COM to automate most of this:

begin 644 dump.com
MM`FZ"0+-(3/;OX``B@TR[8/Y`'0Z24>*!3P@=/-!,N2#^0!T*DD\87P&/'I_
M`@3@+#`\"GP"+`<*P'P3/`]_#]'CT>/1X]'C`]A'B@7KT8D>Z@&+P^B'`+0)
MNDT"S2&_%A"AZ@%(2(;@B06_/P.+#3/`,]*+^8''00-/BA4#PN+YOSD-@<<(
M`XD%M#PSR;KL`<TA<A2+V+1`N3D-N@H#S2%R!K0^S2'-(+KW`;0)S2'K\5!2
MBM"T`LTA6EC#4%(E#P`%D``G%$`GZ.;_6EC#4;D$`-+(Z.7_TLCHX/]9PX;@
MZ.O_AN#HYO_#``!"04-+55`N.#5"`$9I;&4@22]/($5R<F]R+@T*)%1H92!B
M86-K=7`@9FEL92!"04-+55`N.#5"(&ES(&)E:6YG(&-R96%T960@=&\@9'5M
M<"!M96UO<GD@861D<F5S<R`D+@T*5&\@;V)T86EN('1H92!D=6UP(&9I;&4Z
M#0H@("`H,2D@5')A;G-F97(@=&AE(&9I;&4@0D%#2U50+C@U0B!T;R!T:&4@
M5$DM.#4-"B`@("@R*2!';R!T;R!T:&4@1U)!4$@@;65N=2P@86YD(%)#4$E#
M(")024,Q(@T*("`@*#,I(%-44$E#(")$=6UP(@T*("`@*#0I(%1R86YS9F5R
M(")$=6UP(B!T;R!Y;W5R(&-O;7!U=&5R#0HD*BI423@U*BH:#`!-96UO<GD@
M1'5M<"!"86-K=7`@("`@("`@("`@("`@("`@("`@("`@("``#0D`L0@=#@0P
M`/>+L0@``!``(`````P(```P$@`````%````````````````(#\@/V.$;81W
MA(&$BX25A!4``(`!_!```````````/P`````````@@```/P```````````#\
M8H,84P<8``#_^Q,(F6DXF5<``/P```````````#\````````````_&*#&%,'
M&```__L3")EI.)E7@`'\$``````````!_!```````````/P0````````@`'\
M$``````````!_!```````````/P0`````````/W[$`````````#[^Q``````
M`````/Q```````````#\0`````````#_^Q6',!6',!8`__LR)8!D46$I``#\
M8P``````````_#$```````!1``#``?P0````````0`'\$````````$``_!``
M``````#``?P0````````0`'\$````````$``_!````````"``?P0````````
M``'\$````````%L`````_````````````/QB@QA3!Q@``/_[$PB9:3B95X`!
M_!```````````?P0``````````#\$````````(`!_!```````````?P0````
M``````#\$````````%L`````_````````````/QB@QA3!Q@``/_[$PB9:3B9
M5X`!_!```````````?P0``````````#\$````````(`!_!```````````?P0
M``````````#\$````````'$```#]^Q```````````/P```````````#\````
M````````_&*#&%,'&```__L3")EI.)E7@`'\$``````````!_!``````````
M`/P0````````@`'\$``````````!_!```````````/P0`````````!``]_\`
M`$SZ"8SW_P``````````````````````````````````````````````````
M````````````````````````````````````````````````````````````
M````````````````````````````````````````````````````````````
M````````````````````````````````````````````````````````````
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M````````````````````````````````````````````````````````````
M```````````````````````````````````````````````````$_!(T4```
M````!/QGB0````````````````````````````4"`````"`@("`@("`@("`@
M("`@("`@("`@("`@("`@("`@("`@("`@("`@("`@("`@("`@("`@("`@("`@
M("`@("`@("`@("`@("`@("`@("`@("`@("`@("`@("`@("`@("`@("`@("`@
M("`@("`@("`@("`@("`@("`@("`@("`@("`@("`@("`@("`@("`@("`@("`@
M("`@("`@("`@("`@("`@("`@("`@(`````````F,"8Q"]D+V`````/?_``#F
M9PX+H6@`!P```````````/\`````````````````````````````````````
M````````````````````````````````````````````````````````````
M`````````````````````/?_```%D```````````````````````````````
M"0\`````````````````````````````````````````.VH`]__W_T?Z]_\%
MD`60!9`_^C_Z`````#_Z``````````````X$````````_```````````````
M``#\`````````/`#````````````````````````````````````````````
M````````````````````````````````````````````````````````````
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E(0&,!Q(C`8P%$G-N00.+^P!T8713>06+^01T8713>`6+]P3FNP``
`
end

To use the program, type:

   DUMP [address, in hex]

This will create a file called "BACKUP.85B" with all the necessary
modifications already made (ie. in effect, it performs steps 1 to 4).  Just
send this to your TI as outlined in steps 5 to 9 in the above list, and you
will end up with a file called DUMP.85I.  Bytes 0000 to 0044 of DUMP.85I are
the header & can be ignored; bytes from 0045 onward contain the ROM data.
Here is a dump I made of memory address 33D9:

33D0                             E3 F5 D5 5E 23 56 23            ...^#V#
33E0  7E D3 05 EB D1 F1 E3 C9-CD AC 33 38 7D 06 CD AC   ~.........38}...
33F0  33 7B 7D 06 CD AC 33 81-43 07 CD AC 33 12 49 07   3{}...3.C...3.I.
3400  CD AC 33 65 67 06 CD AC-33 12 69 06 CD AC 33 5C   ..3eg...3.i...3\
3410  69 06 CD AC 33 26 6D 06-CD AC 33 D6 41 07 CD AC   i...3&m...3.A...
3420  33 8B 68 07 CD AC 33 6F-7D 06 CD AC 33 16 52 07   3.h...3o}...3.R.
3430  CD AC 33 54 54 07 CD AC-33 59 77 06 CD AC 33 1F   ..3TT...3Yw...3.
3440  4B 07 CD AC 33 D6 5D 06-CD AC 33 09 55 06 CD AC   K...3.]...3.U...
3450  33 F9 56 06 CD AC 33 D3-57                        3.V...3.W

I ran this through a Z80 disassembler to get the following code:

33D9: E3                   EX    (SP),HL
33DA: F5                   PUSH  AF
33DB: D5                   PUSH  DE
33DC: 5E                   LD    E,(HL)
33DD: 23                   INC   HL
33DE: 56                   LD    D,(HL)
33DF: 23                   INC   HL
33E0: 7E                   LD    A,(HL)
33E1: D3 05                OUT   (PORT05H),A
33E3: EB                   EX    DE,HL
33E4: D1                   POP   DE
33E5: F1                   POP   AF
33E6: E3                   EX    (SP),HL
33E7: C9                   RET
33E8: CD AC 33             CALL  L33AC
33EB: 38 7D                JR    C,L346A
33ED: 06 CD                LD    B,205
33EF: AC                   XOR   H
33F0: 33                   INC   SP
33F1: 7B                   LD    A,E
33F2: 7D                   LD    A,L
33F3: 06 CD                LD    B,205
33F5: AC                   XOR   H
33F6: 33                   INC   SP
33F7: 81                   ADD   A,C
33F8: 43                   LD    B,E
etc.

+----------------------------------------------------------------------------
| A Few Technical Tidbits about the TI-85
+----------------------------------------------------------------------------

In the next section I will discuss how to write your own machine language
programs, but before I do, we will need a few facts about the TI-85.  First
of all, we need some information about the video display:

   Mode       Address     Dimensions        Size (in bytes)

   Text        80DD         8 x 21       8*21 = 168 bytes
   Graphics    8641         63 x 128     (63*128)/8 = 1008 bytes

Next, we need to understand the CUSTOM menu.  At offset 08B1 in a .85B file,
you will see a sequence of 1C bytes, as shown:

08B0     9C 59 00 00 A1 59 00-00 00 00 00 00 00 00 00
08C0  00 00 00 00 00 00 00 00-00 00 00 00 00

Each pair of bytes (word) corresponds to menu item in the CUSTOM menu.  In the
above example, we have

   Item #        Name         Word

   Item #1       "abs"        599C
   Item #2       <blank>      0000
   Item #3       "and"        59A1
   Items #4      <blank>      0000
      :                        :
      :                        :
   Item #15      <blank>      0000

The word associated with a menu item is a memory address - it points to a
data structure, such as:

   41 00 07 C3 40 3F 45 44 49 54 00

There are actually several possible data structures, but this is the most
useful.

This data structure means the following:

   +--+--+--+------- ??? (I have some ideas, but no time to explain...)
   |  |  |  |
   |  |  |  |        +--+--+--+--- Name of menu item (EDIT)
   |  |  |  |        |  |  |  |
   41 00 07 C3 40 3F 45 44 49 54 00
               |  |              |
               |  |              +-- Zero termination byte
               |  |
               +--+--- Memory address (3F40)

When you select a menu item, the code at "Memory address" is executed - in
this case, the code ad 3F40 is executed.  Using my DUMP program, I was able
to dump this memory location to find:

   3F40  C3 D9 33

Or, in Z80-speak:

   C3 D9 33     CALL 33D9

With this information, we can move on to...

+----------------------------------------------------------------------------
| Writing Machine Language Programs
+----------------------------------------------------------------------------

The basic idea is this:

   (1)  Edit the BACKUP.85B file so that the item #1 of the custom menu
        points to 8641 (the first byte of the graphics display)

   (2)  By editing PIC files (or drawing the bits in manually :)), we can
        diddle around with bytes in the video memory.

   (3)  Put the following bytes in the video memory:

           41 00 07 C3 50 86 45 00

        Note that the memory address 8650 is also in the video memory.

   (4)  Put your machine language program at address 8650.

Now, when you select item #1 of the custom menu, your program will be
executed!  Just as a demonstration, I created the following .85I file, to
be loaded into video memory:

0000  2A 2A 54 49 38 35 2A 2A-1A 0C 00 50 69 63 74 75   **TI85**...Pictu
0010  72 65 20 66 69 6C 65 20-64 61 74 65 64 20 30 39   re file dated 09
0020  2F 30 31 2F 39 34 2C 20-32 31 3A 30 38 00 00 55   /01/94, 21:08..U
0030  27 F5 5D AE 92 FE 03 08-00 F2 03 11 04 44 75 6D   '.]..........Dum
0040  70 F2 03 F0 03 41 00 07-C3 50 86 45 00 00 00 00   p....A...P.E....
0050  00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00   ................
0060  00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00   ................
0070  00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00   ................
0080  00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00   ................
0090  00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00   ................
00A0  00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00   ................
00B0  00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00   ................
00C0  00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00   ................
00D0  00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00   ................
00E0  00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00   ................
00F0  C3 40 3F 00 00 00 00 00-00 00 00 00 00 00 00 00   .F?.............
<just more zeros>

The first 44 bytes are just header info - ignore them.  The stuff at offset
0045 is our menu data structure:

   41 00 07-C3 50 86 45 00
               |  |
               +--+--- Points to 8650

The three bytes at 00F0 represent a JP (jump) instruction on the Z80:

   C3 46 3F   JP 3F40

Address 3F40 is the code for the "Edit Matrix" menu item.  What about all
the zeros between 004C and 00F0?  Well, 00 is a NOP on the Z80, so they do
nothing.

The sequence of events is as follows:

   (1)  The user selects item #1 from the CUSTOM menu

   (2)  The TI looks at memory location 8641, which is the first byte of
        video memory (remember, we changed the CUSTOM menu so that item #1
        would point here)

   (3)  The TI finds the bytes:

           41 00 07 C3 50 86 45 00

        which we cleverly placed in the graphics display (and hence at memory
        address 8641)

   (4)  Thus, the calculator executes the instruction at 8650

   (5)  At 8650, it encounters a string of NOPs (zeros), leading up to

        C3 46 3F   JP 3F40

        The TI takes the jump, which leads to the matrix editor

Here is a backup of the TI-85 all set up to demonstrate the above program:

begin 644 backup.85b
M*BI423@U*BH:#`!"86-K=7`@9FEL92!D871E9"`P.2\P,2\Y-"P@,3DZ,3D`
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end

Just transfer it to your TI, go into the CUSTOM menu, and select menu item #1
(marked "E").  You might also want to take a look at the graphics display, or
transfer it to your computer to play with.  Some other interesting memory
locations to jump to are:

   Eqn. Solver     3F28
   Draw Program    3F52
   Program Editor  3F46

Well, the three byte program C3 46 3F is admittedly not very impressive, but
it does demonstrate that programming the TI-85 in machine language is
possible.  More advanced programs are in the works...

   TO BE CONTINUED



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