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Machine Language

Catalogue
  1. 1. Overview (Unit 4.1)
  2. 2. How the Instructions look in a computer
  3. 3. Memory Hierarchy
  4. 4. Hack Computer
    1. 4.1. Hardware
    2. 4.2. Software
    3. 4.3. How does the program run? (Control)
    4. 4.4. Registers
  5. 5. The A-instruction Syntax
    1. 5.1. Binary Syntax
  6. 6. The C-instruction Syntax
    1. 6.1. Binary Syntax

Overview (Unit 4.1)

Alan Turing raised the idea of Universality and considered the theoretical model.

This idea was turned into practice by Von Neumann, who built the first serious architecture of a general computing machine.

How the Instructions look in a computer

0100010 0011 0010

ADD R3 R2 : Add R3 to R2

Memory Hierarchy

Accessing memory directly could be costly, because the addresses are long, and getting a value from a large bulk of memory is very slow compared to the CPU speed.

Therefore, instead of just having a large block of memory, we do it in a whole sequence of memories that get bigger and bigger.

Register (Data Registers, Address Registers) -> Cache -> Main Memory -> Disk

Hack Computer

Hardware

Hardware Structure

Say, this is a 16-bit computer:

  • Data memory (RAM): a sequence of 16-bit registers
    • RAM[0], RAM[1], RAM[2],…
  • Instruction memory (ROM, read-only): a sequence of 16-bit registers:
    • ROM[0], ROM[1], ROM[2],…
  • Central Processing Unit (CPU): performs 16-bit instructions
  • Buses: Instruction bus / data bus / address buses

Software

Hack program is sequence of instructions written by the Hack machine language.

Hack Machine Language:

  • 16-bit A-instructions
  • 16-bit C-instructions

How does the program run? (Control)

  1. The Hack program loads in the ROM;
  2. reset button is pushed;
  3. The program start running.

Registers

The Hack computer recognizes 3 registers:

  • D: used to store data
  • A: used to store data / address the memory
  • M: represents the currently addressed memory register: M = RAM[A]

The A-instruction Syntax

@value means (1) set the register to the value value; (2) select the register reg[value], or the M register above.

Example:

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# Set RAM[100] to -1
@100  # A = 100
M = -1  #RAM[100] = -1

Because A is for “addressing”.

Binary Syntax

The number after ampersand “@” is a small non-negative number ($0 ~ 2^15 - 1$).

So, the “@” becomes a leading zero in Binary syntax.

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# @21 would be
0 00000 00000 10101

The C-instruction Syntax

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dest = comp; jump

![C-instruction Syntax]/images/Nand2Tetris/C-instruction%20Syntax.png)

Note:

  1. Compute the value of comp, and store it into dest;
  2. jump represents comparing with zero;
  3. JMP means unconditional jump. A convention is do 0; jmp, and this is an unconditional jump;
  4. If the Boolean Expression (comp jump 0) is true, then jump to execute the next instruction stored in RAM[A];
  5. Remember to “address” the register first.

Example:

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# Sets RAM[300] to the value of the D register plus 1
@300 # A = 300
M=D+1 # RAM[300] = D + 1
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# If (D-1 == 0) jumps to execute the instruction stored in ROM[56]
@56 # A = 56
D-1;JEQ # if (D-1 == 0) goto to instruction ROM[A]

Binary Syntax

C-instruction Binary Syntax

The complete specification is:

C-instruction Specification