Classic Computer Magazine Archive COMPUTE! ISSUE 53 / OCTOBER 1984 / PAGE 159

TI Disassembler

James Dunn

Since information on the operating system and BASIC interpreter used by the TI-99 is scarce, "TI Disassembler" will come in handy if you want to try your hand at programming in TI-9900 machine language.

A disassembler converts the jumble of numbers that actually constitute a machine language program into a more readily understandable form. For each machine language instruction (called an opcode), TI has established a one- to four-letter representation called a mnemonic. This disassembler decodes the contents of memory into standard TI mnemonics, making ML programs less difficult to understand. However, this program will not teach you machine language programming. To use this program, you must have at least an elementary understanding of TI machine language and a familiarity with TIís standard format for ML assemblers. Refer to any of the several books on this subject for further information.

This Disassembler is written in Extended BASIC. However, it can be easily translated for the Mini Memory or Editor/Assembler cartridges. All that is necessary is to unstack the lines so that there is only one statement on a line. All the commands can be found in console BASIC except the PEEK command which is in Extended BASIC, and also available when the Mini Memory or Editor/Assembler cartridge is installed.

Printer Output

Depending upon your printer setup, you may have to modify line 110 or the subroutine starting on line 860, which prints to the screen. It might be wiser to leave that routine as is and just add the extra lines necessary to output to your printer.

Notice that all computations and input are in decimal. If you want hexadecimal numbers, you can modify the program to add conversions. Be warned, however, that this will slow down the program. When you are disassembling 16K blocks, that can be something to think about.

The Disassembler does an excellent job on machine language programs; however, it has one weakness. It cannot tell if the area of memory you ask it to disassemble contains data, text, or jump tables. It will attempt to disassemble these as if they were legitimate opcodes. To tell if this is happening, watch for the BYT output, which indicates that the area you are disassembling contains something other than machine language.

Where You Can't PEEK

The Disassembler can only look into the CPU address space. This is a fault of the architecture of the computer itself. Since the 16K RAM area used by console BASIC is not connected to the CPU, but rather to the VDP (Video Display Processor), the Disassembler cannot access it. Also unreadable are the GROMs which contain the GPL. If you have expansion memory, it is accessible, as are the command modules. Both the Mini Memory and the Editor/Assembler cartridges provide PEEK and POKE commands which can access these areas.

In order to be consistent with TI machine language conventions, the Disassembler uses the same field symbols and addressing mode symbols used in the TI Editor/Assembler package. In case you don't have that package, Tables 1 and 2 show the symbols.

Explanation Of Program

30–110	 	 Initialization and input.
120		 Start of main loop.
140 		 PEEK locations.
150–260 	 Determines the format of the opcode and sends
		 program to appropriate line number for decoding.
270–370	 Decodes Format VIII opcodes.
380–420   	 Decodes Format VI opcodes.
430–450	 Decodes Format V opcodes.
460–530	 Decodes Format II opcodes.
540–590	 Decodes Format IV opcodes.
600–680	 Decodes Format III and IX opcodes.
690–780	 Decodes Format I opcodes.
790–810	 Decodes Format VII opcodes.
820	If not one of the above, byte is not a valid opcode.
840	Optional sound signal and hold when no opcode found.
860	Print to screen routine.
900	Subroutine to READ DATA and pick out mnemonic.
930	Subroutine to decode the Ts address mode.
1000-	DATA statements which contain mnemonics listed according to their Format.

Variables Used

AStart address
A1Temporary variable to cover quirk of PEEK statement
A$Opcode
BEnd address
B$Source field
C$Destination field
HHigh byte of PEEK address
ITemporary loop variable
JBase to which value K is added
J$Want printout
KDisplacement variable for loop
LLow byte of PEEK address
NComputed total of H and L
01
02}next bytes in order after L
03
04
PRPrintout variable (0 = no, 1 = y)
Q$Temporary storage for txfr to A$
RRegister number
TRLoop indicator
ZNumber of opcodes in format type

Table 1: TI Opcode Field Symbols

COCount
DDestination operand
NUNumber
SSource operand
TdSpecific address mode of destination operand
TsSpecific address mode of source operand
WRWorkspace register

Table 2: TI Addressing Mode Symbols

*means Indirect address mode
(R)means Indexed address mode
+after * means Auto Increment address mode
#means Workspace Register address mode
@means Direct address mode