Java Bytecode Subroutines - Cannot load return address

Question

I have been trying to write some Java bytecode and assemble it using Jasmin.

I am trying to get my head around subroutines, and am not sure why I obtain the following error message when running my program:

>java -jar jasmin.jar test.j
Generated: test.class

>java test
Exception in thread "main" java.lang.VerifyError: (class: test,
method: main signature: ([Ljava/lang/String;)V)
Cannot load return address from register 0
Could not find the main class: test. Program will exit.

Here's the bytecode in test.j:

.class public test
.super java/lang/Object
.method public static main([Ljava/lang/String;)V
.limit stack 6
.limit locals 5

jsr a     ;Jump to subroutine 'a', pushing return address on operand stack
return    ;Exit the program

a:
astore_0  ;Store the return address in variable 0
aload_0   ;Save the address onto the stack as address will be overwritten in 'b'
jsr b     ;Jump to subroutine 'b', pushing return address on operand stack
astore_0  ;Store the address that was on the stack back into variable 0
ret 0     ;Return to just after "jsr a"

b:
astore_0  ;Store return address in variable 0
ret 0     ;Return to address stored in 0 (ie back to just after the jump in 'a')

.end method

I haven't had any problems with jumping to a single subroutine, but it seems as though something is going wrong when jumping to a subroutine from within a subroutine.

Any insight as to why this is failing would be much appreciated!

Answer 1

You can't load an address type value into any register, you can only store it and then ret instruction can retrieve it from there.

Java Virtual Machine Specification:

• ret
• jsr