Classic Computer Magazine Archive ANTIC VOL. 3, NO. 9 / JANUARY 1985

assembly language


Simplifying machine code analysis


Opcode Finder is a chart that conveniently brings together the symbols for all three formats of representing machine language subroutines in BASIC programs.  This information makes it a lot simpler to hand-disassemble short subroutines for study or modification.

Once you understand the fundamentals of assembly language programming, a good way to improve your skills is by analyzing existing programs.  You can find machine language subroutines in many of the BASIC programs published in Antic.  To learn how the machine language is being used in the program, you can disassemble these subroutines (convert them back to assembly language).
   This job becomes a little harder because there are three possible formats that can be used for representing machine language data in BASIC programs.  These formats are:

Decimal Numbers
- numbers between 0 and 255 represent specific bytes of machine code.

Hexadecimal Numbers
- two characters stand for one byte of machine code.

ATASCII Characters
- the ATASCII numerical value of each character in a string represents a byte of machine code.

   I put together the following chart so it would be easier for me to hand-disassemble short subroutines, or modify them slightly, without having to load an assembler into my Atari.  The chart lists all opcodes (assembly language operating codes) in numerical order-alongside the corresponding hexadecimal values, ATASCII characters and assembly language formats (Alforms).

Donny Cherf is a computer science major from Merced, California.  One of his earlier projects was an Atari version of the Yahtzee game,

OP Codes