eceg431/12/Memory.jack

// This file is part of www.nand2tetris.org
// and the book "The Elements of Computing Systems"
// by Nisan and Schocken, MIT Press.
// File name: projects/12/Memory.jack
/**
 * This library provides two services: direct access to the computer's main
 * memory (RAM), and allocation and recycling of memory blocks. The Hack RAM
 * consists of 32,768 words, each holding a 16-bit binary number.
 */ 
class Memory {
    static Array ram;
    static Array heap;
    static int freeList;
    static int heapBottom;

    /** Initializes the class. */
    function void init() {
        let ram = 0;
        let heap = 2048; // heapBase
        let heapBottom = 16384;
        let freeList = heap;
        let heap[0] = heapBottom - heap; // length of first block
        let heap[1] = 0; // next pointer
        return;
    }

    /** Returns the RAM value at the given address. */
    function int peek(int address) {
        return ram[address];
    }

    /** Sets the RAM value at the given address to the given value. */
    function void poke(int address, int value) {
        let ram[address] = value;
        return;
    }

    /** Finds an available RAM block of the given size and returns
     *  a reference to its base address. */
    function int alloc(int size) {
        var int curr, next, len;
        var int block;
        
        let curr = freeList;
        // Search for a block with enough space (size + 1 for header)
        while (~(curr = 0)) {
            let len = ram[curr];
            
            // Check if block is large enough (size + 1 for header)
            // Ideally we also want to leave at least 2 words for the free block itself to remain valid
            if (len > (size + 2)) {
                 // Carve from the end of the block
                 let ram[curr] = len - (size + 1); // Reduce free block size
                 
                 let block = curr + ram[curr]; // Base of new block (including header)
                 let ram[block] = size + 1; // Set allocated block size (including header)
                 
                 return block + 1; // Return user pointer
            }
            
            let curr = ram[curr + 1]; // Next block
        }
        return 0; // Out of memory
    }

    /** De-allocates the given object (cast as an array) by making
     *  it available for future allocations. */
    function void deAlloc(Array o) {
        var int block;
        let block = o - 1; // Get header
        
        // Append to head of freeList
        let ram[block + 1] = freeList;
        let freeList = block;
        return;
    }    
}