Java Card Local Fixed-Size Array Variable Memory Storage Location

Question

I wonder if anyone knows exactly where is temp array stored (EEPROM or RAM) in the following Java Card method (part of source code of WalletDemoApplet I have found inside JCIDE samples directory) . Please note that the array is not allocated by means of makeTransientByteArray. Also it is declared as a local variable inside a method.

private void calIntegral(byte [] buf,byte soff,short len)
{
    byte temp[]={0x00,0x00,0x00,0x00};
    short low=0;
    byte aa=0;
    if (len==2)
        Util.arrayCopy(buf, soff, temp,(short)2, len);
    else
        Util.arrayCopy(buf, soff, temp,(short)0, len);

    ...
}

Also, in the Java Card Applet Developer's Guide, you can find:

The Converter ensures that memory is allocated for the contents of static fields, namely, primitive data types and references to arrays. Memory is allocated for instances by using the new bytecode from the system heap and cannot be reclaimed (unless the smart card implements a garbage collector). Memory for method variables, locals, and parameters is allocated from the stack and is reclaimed when the method returns.

The demo is being looked at by the good people at JavaCardOS as we speak; there may be other bugs (I sent a link to the Q/A)... — Maarten Bodewes
– Maarten Bodewes, Commented Dec 11, 2018 at 9:31

Maarten Bodewes · Accepted Answer · 2018-12-10 16:44:40Z

4

This part of the WalletDemoApplet should certainly not be generated in a local variable:

byte temp[]={0x00,0x00,0x00,0x00};

That's EEPROM storage; the new byte[] part may be hidden, but the array creation is still performed, as specified by the JLS:

An array initializer creates an array and provides initial values for all its components.

here the array initializer is the braces and anything within it.

Please ignore at least that part of the example. Generally you would use a static method for that, and using a class field with makeTransientByteArray (called during instantiation of the Applet) as buffer. That is: if you require an array, simply defining 4 byte variables or - in this case - directly setting byte values could work as well.

edited Dec 10, 2018 at 16:44

answered Dec 9, 2018 at 14:10

Maarten Bodewes

94.6k15 gold badges169 silver badges289 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

milanbalazs · Accepted Answer · 2020-01-02 14:57:24Z

0

Any Array will be created in EEPROM, that is not created with makeTransientByteArray. You can check available and consumed memory in JCIDE tool. In this example memory will be assign again and again in EEPROM, when this methods will be called.

edited Jan 2, 2020 at 14:57

milanbalazs

5,3895 gold badges32 silver badges52 bronze badges

answered Jan 2, 2020 at 10:00

Deepanshu

416 bronze badges

Collectives™ on Stack Overflow

Java Card Local Fixed-Size Array Variable Memory Storage Location

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related