Support smaller number of frames than pages

Hi,

I've written a FIFO (First-In First-Out) Algorithm function, and I am trying to provide support for smaller number of frames than pages.

Code:

---

private void pageFaultFIFO(int pageNumber){
        pageFaults++;
       
        for(int i = 0; i<pageTable.length; i++){
            if(pageTable[i] == freePos)
                pageTable[i] = -1;
        }
       
        try {
            pageFile.seek(pageNumber*PageSize);
            for(int b=0;b<PageSize;b++)
                RAM[freePos*PageSize+b]=pageFile.readByte();
        } catch (IOException ex) {
            Logger.getLogger(MemoryManager.class.getName()).log(Level.SEVERE, null, ex);
        }
       
        pageTable[pageNumber] = freePos;
       
        if(pageTable.length == pageNumber){
            freePos = 0;
        }
        else{
            freePos++;
        }
    }

---

I made it work, when I pass 255, 255, 255, all equal sizes. But if the last number of frames is e.g. 128, I get ArrayIndexOutOfBoundsException: 32768. I am not sure, how can I modify a small change to support all types of sizes dynamically?

The problem in particularly occurs at RAM[freePos*PageSize+b]=pageFile.readByte();

Can you explain the code's logic?

On What statement does the exception happen? What are the values of the variables used to compute the index?