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, freeList;

    /** Initializes the class. */
    function void init() {
	let RAM = 0;
	let freeList = 2048;
	let freeList[0] = 16383 - 2048 + 1;
	let freeList[1] = -1;
	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 Array block;
	var int next, blockSize;
	let blockSize = freeList[0];
	let next = freeList[1];
	if (blockSize > (size + 2)) {
	    let block = freeList;
	    if (~(next = -1)) {
		let freeList = next;
	    } else {
		let freeList = block + size + 2;
		let freeList[0] = blockSize - size - 2;
		let freeList[1] = -1;
	    }
	    // let block[0] = blockSize - size - 1;
	    // let block[1] = freeList;
	    // let block[block[0]] = size + 1;
	    // return block + block[0] + 1;
	    let block[0] = size;
	    let block[1] = -1;
	    return block + 2;
	}
	// if (~(next = -1)) {
	//     let freeList = next;
	//     return Memory.alloc(size);
	// }
	while (~(next = -1)) {
	    let block = next;

	    let next = block[1];
	    if (blockSize > (size + 2)) {
		let blockSize = blockSize - size - 2;
		let block[0] = blockSize;
		let block[blockSize] = size;
		let block[blockSize + 1] = -1;
		return block + blockSize + 2;
	    }
	}
	return -1;
    }

    /** De-allocates the given object (cast as an array) by making
     *  it available for future allocations. */
    function void deAlloc(Array o) {
	var Array seg, next;
	let seg = freeList;
	let next = seg[1];
	while (~(next = -1)) {
	    let seg = next;
	    let next = seg[1];
	}
	let next = o;
	return;
    }
}