Mux ALU

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Elements of Computing Systems, Nisan & Schocken, MIT Press, www.nand2tetris.org, Chapter 4: Machine Language slide 1

www.nand2tetris.org

Building a Modern Computer From First Principles

Machine (Assembly) Language

Where we are at:

Assembler Chapter 6 H.L. Language

&

Operating Sys.

abstract interface

Compiler

Chapters 10 - 11

VM Translator Chapters 7 - 8

Computer Architecture

Chapters 4 - 5

Gate Logic

Chapters 1 - 3 Electrical

Engineering

Physics Virtual

Machine abstract interface

Software hierarchy

Assembly Language abstract interface

Hardware hierarchy Machine

Language abstract interface

Hardware Platform abstract interface

Chips &

Logic Gates abstract interface Human

Thought

Abstract design Chapters 9, 12

Machine language

Abstraction – implementation duality:



Machine language ( = instruction set) can be viewed as a programmer- oriented abstraction of the hardware platform



The hardware platform can be viewed as a physical means for realizing the machine language abstraction

Another duality:



Binary version



Symbolic version Loose definition:



Machine language = an agreed-upon formalism for manipulating a memory using a processor and a set of registers



Same spirit but different syntax across different hardware platforms.

Lecture plan



Machine languages at a glance



The Hack machine language:



Symbolic version



Binary version



Perspective

(The assembler will be covered in lecture 6).

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Typical machine language commands

 ALU operations

 Memory access operations



Immediate addressing, LDA R1, 67 // R1=67



Direct addressing, LD R1, 67 // R1=M[67]



Indirect addressing, LDI R1, R2 // R1=M[R2]

 Flow control operations

Typical machine language commands

(a small sample)

// In what follows R1,R2,R3 are registers, PC is program counter, // and addr is some value.

ADD R1,R2,R3 // R1  R2 + R3 ADDI R1,R2,addr // R1  R2 + addr

AND R1,R1,R2 // R1  R1 and R2 (bit-wise) JMP addr // PC  addr

JEQ R1,R2,addr // IF R1 == R2 THEN PC  addr ELSE PC++

LOAD R1, addr // R1  RAM[addr]

STORE R1, addr // RAM[addr]  R1 NOP // Do nothing

// Etc. – some 50-300 command variants

The Hack computer

A 16-bit machine consisting of the following elements:

Data memory: RAM – an addressable sequence of registers Instruction memory: ROM – an addressable sequence of registers Registers: D, A, M, where M stands for RAM[A]

Processing: ALU, capable of computing various functions Program counter: PC, holding an address

Control: The ROM is loaded with a sequence of 16-bit instructions, one per memory location, beginning at address 0. Fetch-execute cycle: later

Instruction set: Two instructions: A-instruction, C-instruction.

The A-instruction

@value

^{// A} value

Where value is either a number or a symbol referring to some number.

Used for:



Entering a constant value

( A = value) @17 // A = 17

D = A // D = 17

Coding example:

@17 // A = 17 D = M // D = RAM[17]



Selecting a

RAM

location

( register = RAM[A])

@17 // A = 17

JMP // fetch the instruction // stored in ROM[17]



Selecting a

^ROM

location

( PC = A )

Why A-instruction? It is impossible to pack both addr and instr into 16 bits.

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The C-instruction

(first approximation)

Exercise: Implement the following tasks using Hack commands:

 Set D to A-1

 Set both A and D to A + 1

 Set D to 19

 Set both A and D to A + D

 Set RAM[5034] to D - 1

 Set RAM[53] to 171

 Add ¹to RAM[7], and store the result in D^.

dest = x + y

dest = x - y dest = x dest = 0 dest = 1 dest = -1

x =

^{A

,

^D

,

^M}

y =

{A

,

D

,

M

,

1}

dest =

{A

,

^D

,

^M

,

^MD

,

^A

,

^AM

,

^AD

,

^AMD

,

^null}

The C-instruction

(first approximation)

j 3012 sum 4500 q 3812 arr 20561 Symbol table:

(All symbols and values are arbitrary examples)

Exercise: Implement the following tasks using Hack commands:

 sum = 0

 j = j + 1

 q = sum + 12 – j

 arr[3] = -1

 arr[j] = 0

 arr[j] = 17

 etc.

dest = x + y dest = x - y dest = x dest = 0 dest = 1 dest = -1

x =

^{A

,

^D

,

^M}

y =

{A

,

D

,

M

,

1}

dest =

{A

,

^D

,

^M

,

^MD

,

^A

,

^AM

,

^AD

,

^AMD

,

^null}

Control

(focus on the yellow chips only)

ALU

Mux

D

A/M a-bit D register

A register A

RAM M (selected register)

ROM (selected register)

PC Instruction

In the Hack architecture:

 ROM = instruction memory

 Program = sequence of 16-bit numbers, starting at ROM[0]

 Current instruction = ROM[PC]

 To select instruction nfrom the ROM, we set A to n, using the instruction @n

address input

Coding examples

(practice) Exercise: Implement the following

tasks using Hack commands:

 goto 50

 if D==0 goto 112

 if D<9 goto 507

 if RAM[12] > 0 goto 50

 if sum>0 goto END

 if x[i]<=0 goto NEXT.

sum 2200 x 4000 i 6151 END 50 NEXT 120 Symbol table:

(All symbols and values in are arbitrary examples) Hack commands:

A-command: @value // set A to value C-command: dest =comp ;jump // dest =and ;jump

// are optional Where:

comp= 0, 1, ‐1, D, A, !D, !A, ‐D, ‐A, D+1, A+1, D‐1, A‐1, D+A, D‐A, A‐D, D&A, D|A, M, !M, ‐M,M+1, M‐1, D+M, D‐M, M‐D, D&M, D|M

dest= M, D, MD, A, AM, AD, AMD, or null jump= JGT, JEQ, JGE, JLT, JNE, JLE, JMP, or null

In the command dest = comp; jump, the jump materialzes if (comp jump 0) is true. For example, in D=D+1,JLT, we jump if D+1 < 0.

Hack convention:

True is represented by -1

False is represented by 0

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if condition { code block 1}

else {

code block 2}

code block 3 High level:

D  not condition

@IF_TRUE D;JEQ code block 2

@END 0;JMP (IF_TRUE)

code block 1 (END)

code block 3 Hack:

IF logic – Hack style

Hack convention:

WHILE logic – Hack style

while condition { code block 1 }

Code block 2 High level:

(LOOP)

D  not condition)

@END D;JEQ code block 1

@LOOP 0;JMP (END)

code block 2 Hack:

Hack convention:

Side note (focus on the yellow chip parts only)

ALU

Mux

D

A/M a-bit D register

A register A

M RAM

(selected register)

ROM (selected register)

PC Instruction

address input

In the Hack architecture, the A register addresses both the RAM and the ROM, simultaneously. Therefore:

 Command pairs like @addr followed by D=M;someJumpDirective make no sense

 Best practice: in well-written Hack programs, a C-instruction should contain

 either a reference to M, or

 a jump directive,

 but not both.

Complete program example

// Adds 1+...+100.

int i = 1;

int sum = 0;

while (i <= 100){

sum += i;

i++;

}

C language code:

// Adds 1+...+100.

@i // i refers to some RAM location M=1 // i=1

@sum // sum refers to some RAM location M=0 // sum=0

(LOOP)

@i

D=M // D = i

@100

D=D-A // D = i - 100

@END

D;JGT // If (i-100) > 0 goto END

@i

D=M // D = i

@sum

M=D+M // sum += i

@i

M=M+1 // i++

@LOOP

0;JMP // Got LOOP (END)

@END

0;JMP // Infinite loop Hack assembly code:

Demo CPU emulator Hack assembly convention:

Variables: lower-case

Labels: upper-case

Commands: upper-case

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Symbols in Hack assembly programs

Symbols created by Hack programmers and code generators:

 Label symbols: Used to label destinations of goto commands. Declared by the pseudo command (XXX). This directive defines the symbol XXXto refer to the instruction memory location holding the next command in the program (within the program, XXXis called “label”)

 Variable symbols:Any user-defined symbol xxxappearing in an assembly program that is not defined elsewhere using the (xxx) directive is treated as a variable, and is “automatically” assigned a unique RAM address, starting at RAM address 16

 By convention, Hack programmers use lower-case and upper-case letters for variable names and labels, respectively.

Predefined symbols:

 I/O pointers:The symbols SCREENand KBDare “automatically”

predefined to refer to RAM addresses 16384 and 24576, respectively (base addresses of the Hack platform’s screen and keyboard memory maps)

 Virtual registers:covered in future lectures.

 VM control registers:covered in future lectures.

// Typical symbolic // Hack code, meaning // not important

@R0 D=M

@INFINITE_LOOP D;JLE

@counter M=D

@SCREEN D=A

@addr M=D (LOOP)

@addr A=M M=-1

@addr D=M

@32 D=D+A

@addr M=D

@counter MD=M-1

@LOOP D;JGT (INFINITE_LOOP)

@INFINITE_LOOP 0;JMP

Q: Who does all the “automatic” assignments of symbols to RAM addresses?

A: The assembler, which is the program that translates symbolic Hack programs into binary Hack program. As part of the translation process, the symbols are resolved to RAM addresses.(more about this in future lectures)

Perspective



Hack is a simple machine language



User friendly syntax:

D=D+A

instead of

ADD D,D,A



Hack is a “½-address machine”: any operation that needs to operate on the RAM must be specified using two commands: an A-command to address the RAM, and a subsequent C-command to operate on it



A Macro-language can be easily developed



D=D+M[XXX] => @XXX followed by D=D+M



GOTO YYY => @YYY followed by 0; JMP

