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Volume Number: 12
Issue Number: 9
Column Tag: Programmer’s Challenge
Programmer’s Challenge 
By Bob Boonstra
Note: Source code files accompanying article are located on MacTech CD-ROM orsource code disks.
Byte Code Interpreter
September is Assembly Language month at the Programmer’s Challenge, and this year
we will be accepting solutions in PowerPC Assembly for the first time. This month’s
Challenge, suggested by Xan Gregg, is to write an interpreter for a subset of the byte
code language used by the Java Virtual Machine.
The prototype for the code you should write is:
void JavaMiniVM(
void *constant_pool, /* pointer to cp_info array */
void *fields, /* pointer to field_info array */
void *methods, /* pointer to method_info array */
void *classFile, /* pointer to class file */
long methodToExecute, /* index of method to start executing */
void *heapSpace, /* preallocated storage for your use */
void *returnStack /* stack where return values are stored */
);
Your Challenge is to write an efficient interpreter for a subset of the Java byte
code instruction set. A Java instruction consists of a single-byte opcode specifying the
operation to be performed, followed by zero or more operand bytes. So, for example,
in the byte sequence 0x10 0xFF, the opcode 0x10 (bipush) indicates that the operand
byte 0xFF is to be pushed onto the operand stack. The instruction 0x60 (iadd)
indicates that two integers are to be popped off the operand stack, added, and the result
pushed back onto the stack. The virtual machine operates by repeatedly fetching an
opcode and performing the indicated action on the operands.
To participate in this Challenge, you don’t need to know anything about Java
itself, but you do need to understand the Java Virtual Machine. A Java Virtual Machine
executes a .class file, the format of which is too complicated to provide here; it is
described in the Java Virtual Machine Specification (release 1.0 Beta) available at
http://java.sun.com/java.sun.com/newdocs.html.
The first three parameters passed to your JavaMiniVM routine are pointers to
the constants, fields, and methods contained in the .class file that your interpreter is to
execute. These parameters are taken directly from the classFile described in Section
2 of the VM specification. A pointer to the classFile is provided as the fourth
parameter for those who feel they need direct access to the .class file. The parameter
methodToExecute indicates which of the methods your VM is to start executing.
Stack space and execution frames should be established by your virtual machine
in the memory provided in heapSpace. Adequate heap space will be allocated by the
caller. Your code may include static data that might be needed for lookup tables, etc., to
efficiently implement the virtual machine. The parameter returnStack is provided
as the stack for the execution environment of the calling routine. It is to be used when
To simplify the Challenge, your code need not implement the long, float, and
double data types supported by the Java Virtual Machine. You also do not need to
process exceptions, breakpoints, monitored code regions, or the wide modifier for
Load and Store instructions. Your interpreter should be robust enough to determine
the operand size of these unimplemented instructions in order to skip any that are
encountered. All methods invoked will be in the single .class file provided to your
routine.
Sample test .class files will be provided via the Programmer’s Challenge mailing
list, and are also available by writing me at bob_boonstra@mactech.com. If there are
any questions about what needs to be implemented, please send me a note at the same
email address.
Your code may be written in PowerPC Assembly, 68K Assembly, C, or C++.
Testing will be performed on an 8500 using the latest CodeWarrior environment.
Because this is a more difficult Challenge than usual, it has been sent to the mailing
list earlier than normal to provide additional solution time. For those of you who
haven’t had a chance to investigate Java in detail, it is a good opportunity to find out
what all the excitement (hype?) is about. I hope you find this Challenge enjoyable and
educational.
Two Months Ago Winner
Once again, congratulations go to Ernst Munter (Kanata, Ontario), this time for
submitting the fastest entry to the Connect IV Challenge. Recall that the Challenge was
to compete in a round-robin tournament against the other solutions in a generalized
version of the well-known Connect 4 game. Pieces are inserted into the top of a
column in the vertically oriented board, with the winner being the first player to
arrange four or more pieces into a vertical, horizontal, or diagonal line.