Cpu Emulator

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Slide 1/40CPU Emulator Tutorial, www.idc.ac.il/tecs Tutorial Index

This program is part of the software suitethat accompanies the book

The Elements of Computing Systems

by Noam Nisan and Shimon Schocken

MIT Press

www.idc.ac.il/tecs

This software was developed by students at the

Efi Arazi School of Computer Science at IDC

Chief Software Architect: Yaron Ukrainitz

CPU Emulator Tutorial
http://www.idc.ac.il/tecshttp://www.idc.ac.il/tecs


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Background

The Elements of Computing Systemsevolves aroundthe construction of a complete computer system,done in the framework of a 1- or 2-semester course.

In the first part of the book/course, we build the

hardware platform of a simple yet powerfulcomputer, called Hack. In the second part, we buildthe computers software hierarchy, consisting of anassembler, a virtual machine, a simple Java-likelanguage called Jack, a compiler for it, and a mini

operating system, written in Jack.The book/course is completely self-contained,requiring only programming as a pre-requisite.

The books web site includes some 200 test

programs, test scripts, and all the softwaretools necessary for doing all the projects.


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The books software suite

This tutorial isabout theCPU emulator.

Translators (Assembl er ,J ackCompi l er ):

Used to translate from high-level to low-level; Developed by the students, using the books

specs; Executable solutions supplied by us.

Other

Bi n: simulators and translators software;

bui l t I n: executable versions of all the logic

gates and chips mentioned in the book;

OS: executable version of the J ack OS;

Text Compar er : a text comparison utility.

(All the supplied tools are dual-platform: Xxx. bat startsXxx in Windows, and Xxx. sh starts it in Unix)

Simulators(HardwareSi mul at or , CPUEmul at or , VMEmul at or ):

Used to build hardware platforms andexecute programs;

Supplied by us.


4/37Slide 4/40CPU Emulator Tutorial, www.idc.ac.il/tecs Tutorial Index

Tutorial Objective

Learn how to use the

CPU Emulator

for simulating the

execution of machine

language programs onthe Hack computer



The Hack computer

This CPU emulator simulates theoperations of the Hack computer,built in chapters 1-5 of the book.

Hack -- a 16-bit computer equippedwith a screen and a keyboard --resembles hand-held computerslike game machines, PDAs, andcellular telephones.

Before such devices are actuallybuilt in hardware, they are plannedand simulated in software.

The CPU emulator is one of

the software tools used forthis purpose.




I. Basic Platform

II. I/O devices

III. Interactive simulation

IV. Script-based simulation

V. Debugging

Relevant reading (from The Elements of Computing Systems):

Chapter 4: Machine Language

Chapter 5: Computer Architecture

Appendix B: Test Scripting Language




Part I:

Basic Platform


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instruction

memory

registers

keyboard

enabler

screen

datamemory

ALU

The Hack Computer Platform


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Instruction memory

Instruction memory

(32K): Holds a machine

language program

The loaded code can be viewed

either in binary, or in symbolic

notation (present view)

Program counter(PC) (16-bit):Selects the next instruction.

Next instruction

is highlighted


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Data memory (RAM)

Data memory (32K RAM), used for:

General-purpose data storage(variables, arrays, objects, etc.)

Screen memory map

Keyboard memory map

Address (A) register, used to:

Select the current RAM location

OR

Set the Program Counter (PC) forjumps (relevant only if the currentinstruction includes a jump directive).


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Registers

Registers (all 16-bit):

D: Data register

A: Address register

M: Stands for the memory register

whose address is the currentvalue of the Address register

M (=RAM[A])

D

A


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Arithmetic/Logic Unit

Arithmetic logic unit (ALU)

The ALU can compute various arithmeticand logical functions (lets call them f) onsubsets of the three registers {M,A,D}

All ALU instructions are of the form{M,A,D}= f ({M,A,D})

(e.g. M=M-1, MD=D+A , A=0, etc.)

The ALU operation (LHS destination,function, RHS operands) is specified bythe current instruction.

Current

instruction

M (=RAM[A])

D

A


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Part II:

I/O Devices


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I/O devices: screen and keyboard

Simulated screen: 256 columns by512 rows, black & white memory-mapped device. The pixels arecontinuously refreshed from respectivebits in an 8K memory-map, located at

RAM[16384] - RAM[24575].

Simulated keyboard:One click on this button causes theCPU emulator to intercept all thekeys subsequently pressed on thereal computers keyboard; anotherclick disengages the real keyboardfrom the emulator.


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Screen action demo

1. Select a word in the RAM regionthat serves as the screen memory

map, e.g. address 16384 (the firstword in the screen memory map).

2. Enter a value, say 1(1111111111111111 in binary)

3. Buil t-in Refresh action:

The emulator draws thecorresponding pixels on thescreen. In this case, 16 blackpixels, one for each binary 1.

Perspective:Thats how computer

programs put images (text, pictures,video) on the screen: they write bits intosome display-oriented memory device.

This is rather hard to do in machinelanguage programming, but quite easy

in high-level languages that write to thescreen indirectly, using OS routines likepr i nt St r i ng or dr awCi r cl e, as we

will see in chapters 9 and 12.

Since all high level programs and OS

routines are eventually translated intomachine language, they all end updoing something like this example.

K b d i d


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Keyboard action demo

1. Click the keyboard enabler

2. Press some key on the

real keyboard, say S

3. Watch here:

Keyboard memory

map

(a single 16-bit

memory location)

K b d ti d


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Keyboard action demo

Keyboard memory

map

(a single 16-bit

memory location)

As long a keyis pressed,

The emulator displaysIts character code in thekeyboard memory map

Perspective:Thats how computerprograms read from the keyboard: theypeek some keyboard-oriented memorydevice, one character at a time.

This is rather tedious in machine

language programming, but quite easy inhigh-level languages that handle thekeyboard indirectly, using OS routineslike r eadLi ne or r eadI nt , as we will see

in Chapters 9 and 12.

Since all high level programs and OSroutines are eventually translated intomachine language, they all end up doingsomething like this example.

Visual echo

(convenient GUIeffect, not part of thehardware platform)

CPU E l t T t i l


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Part III:

Interactive

Simulation

Loading a program


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Loading a program

Navigate to adirectory and selecta . hack or . asmfile.

Loading a program


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Loading a program

Can switchfrom binary tosymbolicrepresentation

Running a program


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Running a program

1. Enter anumber,say 50.

2. Click therunbutton.

4. Watch

here

3. To speed upexecution,use the speedcontrol slider

Programs description: Draws a rectangle at the

top left corner of the screen. The rectangles widthis 16 pixels, and its length is determined by thecurrent contents of RAM[0].

Note:There is no need to understand the programscode in order to understand whats going on.

Hack programming at a glance (optional)


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Hack programming at a glance (optional)

Program action:

Since RAM[0] happens to be 50,

the program draws a 16X50rectangle. In this example theuser paused execution whenthere are 14 more rows to draw.

Programs description: Draws a rectangle at the

top left corner of the screen. The rectangles widthis 16 pixels, and its length is determined by thecurrent contents of RAM[0].

Note:There is no need to understand the programscode in order to understand whats going on.

Next instruction is M=- 1.

Since presently A=17536, the next ALUinstruction will effect RAM[17536] =1111111111111111. The 17536 address,which falls in the screen memory map,corresponds to the row just below therectangles current bottom. In the next screen

refresh, a new row of 16 black pixels will bedrawn there.

Animation options


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Animation options

Animation control: Pr ogr am f l ow(default): highlights the

current instruction in the instruction memoryand the currently selected RAM location

Pr ogr am&dat a f l ow: animates all

program and data flow in the computer No ani mat i on: disables all animation

Usage tip:To execute any non-trivial programquickly, select no animation.The simulator can

animate both program

flow and data flow

Controls execution

(and animation)speed.



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Part IV:

Script-Based

Simulation

Interactive VS Script-Based Simulation


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Interactive VS Script Based Simulation

A program can be executed and debugged:

Interactively, by ad-hoc playing with the emulators GUI(as we have done so far in this tutorial)

Batch-ly, by running a pre-planned set of tests, specified in a script.

Script-based simulation enables planning and using tests that are:

Pro-active

Documented

Replicable

Complete (as much as possible)

Test scripts:

Are written in a Test Description Language (described in Appendix B)

Can cause the emulator to do anything that can be done interactively,

and quite a few things that cannot be done interactively.

The basic setting


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The basic setting

tested program

test script

Example: Max. asm


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Example: Max. asm

/ / Comput es M[ 2] =max( M[ 0] , M[ 1] ) where M st ands f or RAM

@0

D=M / / D = M[ 0]@1

D=D- M / / D = D - M[ 1]

@FI RST_I S_GREATER

D; J GT / / I f D>0 goto FI RST_I S_GREATER

@1

D=M / / D = M[ 1]

@SECOND_I S_GREATER

0; J MP / / Got o SECOND_I S_GREATER

( FI RST_I S_GREATER)

@0

D=M / / D=f i r st number

( SECOND_I S_GREATER)

@2

M=D / / M[ 2]=D ( great er number )

( I NFI NI TE_LOOP)

@I NFI NI TE_LOOP / / I nf i ni t e l oop ( our st andar d0; J MP / / way t o t er mi nat e pr ogr ams) .

/ / Comput es M[ 2] =max( M[ 0] , M[ 1] ) where M st ands f or RAM

@0

D=M / / D = M[ 0]

@1

D=D- M / / D = D - M[ 1]

@FI RST_I S_GREATER

D; J GT / / I f D>0 goto FI RST_I S_GREATER

@1

D=M / / D = M[ 1]@SECOND_I S_GREATER

0; J MP / / Got o SECOND_I S_GREATER

( FI RST_I S_GREATER)

@0

D=M / / D=f i r st number( SECOND_I S_GREATER)

@2

M=D / / M[ 2] =D ( great er number )

( I NFI NI TE_LOOP)

@I NFI NI TE_LOOP / / I nf i ni t e l oop ( our st andar d0; J MP / / way t o t er mi nat e pr ogr ams) .

Hack language at a glance:

( l abel ) // defines a label

@xxx // sets the A register

// to xxxs value

The other commands are self-explanatory; J ump directiveslikeJ GT andJ MP mean J ump

to the address currently storedin the A register

Before any command involvinga RAM location (M), the A

register must be set to the

desired RAM address(@addr ess)

Before any command involvinga jump, the A register must be

set to the desired ROMaddress (@l abel ).

Note: For now, it is not necessary to understand either the Hackmachine language or the Max program. It is only important to

grasp the programs logic. But if youre interested, we give alanguage overview on the right.

Sample test script: Max. t st


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p p

| RAM[ 0] | RAM[ 1] | RAM[ 2] || 15 | 32 | 32 |

| 47 | 22 | 47 |

| RAM[ 0] | RAM[ 1] | RAM[ 2] || 15 | 32 | 32 |

| 47 | 22 | 47 |

The scripting language

has commands for:

Loading programs

Setting up output and compare files

Writing values into RAM locations

Writing values into registers

Executing the next command (t i ckt ack)

Looping (r epeat )

And more (see Appendix B).

Notes: As it turns out, the Max program requires 14

cycles to complete its execution

All relevant files ( . asm, . t st , . cmp) must

be present in the same directory.

Output

/ / Load t he pr ogr am and set up:l oad Max. asm,out put - f i l e Max. out ,compare- t o Max. cmp,out put - l i st RAM[ 0] %D2. 6. 2

RAM[ 1] %D2. 6. 2RAM[ 2] %D2. 6. 2;

/ / Test 1: max(15, 32)set RAM[ 0] 15,set RAM[ 1] 32;r epeat 14 {

t i ckt ock;}out put ; / / t o t he Max. out f i l e

/ / Test 2: max(47, 22)set PC 0, / / Reset pr og. count erset RAM[ 0] 47,set RAM[ 1] 22;r epeat 14 {

t i ckt ock;}out put ;

/ / t est 3: max( 12, 12)/ / E tc .

/ / Load t he pr ogr am and set up:l oad Max. asm,

out put - f i l e Max. out ,compare- t o Max. cmp,out put - l i st RAM[ 0] %D2. 6. 2

RAM[ 1] %D2. 6. 2RAM[ 2] %D2. 6. 2;

/ / Test 1: max(15, 32)set RAM[ 0] 15,set RAM[ 1] 32;r epeat 14 {

t i ckt ock;}

out put ; / / t o t he Max. out f i l e

/ / Test 2: max(47, 22)set PC 0, / / Reset pr og. count erset RAM[ 0] 47,set RAM[ 1] 22;

r epeat 14 {t i ckt ock;

}out put ;

/ / t est 3: max( 12, 12)

/ / E tc .

Using test scriptsS d


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g p

Execute the nextsimulation step

Execute step afterstep repeatedly

Pause thesimulation

Reset

the script

Load ascript

Script = a series ofsimulation steps, eachending with a semicolon;

Speedcontrol

Important point: Whenever an assemblyprogram (.asm file) is loaded into theemulator, the program is assembled on thefly into machine language code, and this isthe code that actually gets loaded. In theprocess, all comments and white space are

removed from the code, and all symbolsresolve to the numbers that they stand for.

The default script (and a deeper understanding of the CPU emulator logic)


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p ( p g g )

If you load a program file without firstloading a script file, the emulatorloads a default script (always). The

default script consists of a loop thatruns the computer clock infinitely.

Note that these run/stop buttons dontcontrol the program. They control thescript, which controls the computersclock, which causes the computerhardware to fetch and execute theprograms instructions, one instruction

per clock cycle.



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Part V:

Debugging

Breakpoints: a powerful debugging tool


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p p gg g

The CPU emulator continuously keeps track of: A: value of the A register

D: value of the D register

PC: value of the Program Counter

RAM[ i ] : value of any RAM location

t i me: number of elapsed machine cycles

Breakpoints: A breakpoint is a pair where variable is one of

{A, D, PC, RAM[ i ] , t i me}and i is between 0 and 32K.

Breakpoints can be declared either interactively, or via script commands.

For each declared breakpoint, when the variable reaches the value, theemulator pauses the programs execution with a proper message.

Breakpoints declaration


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1. Open thebreakpointspanel

3. Add, delete,or updatebreakpoints

2. Previously-declaredbreakpoints

Breakpoints declaration


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2. Enter the valueat which the breakshould occur

1. Select the systemvariable on which you

want to break

Breakpoints usage


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1. New

breakpoint

3. When the A register will be 2, orRAM[ 20] will be 5, or 12 time units(cycles) will elapse, or RAM[ 21] will

be 200, the emulator will pause theprograms execution with an

appropriate message.

A powerful debugging tool!

2. Run theprogram

Postscript: Maurice Wilkes (computer pioneer) discovers debugging:


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As soon as we started programming, we found to our

surprise that it wasn't as easy to get programs rightas we had thought. Debugging had to be discovered. I

can remember the exact instant when I realized that

a large part of my life from then on was going to bespent in finding mistakes in my own programs.

(Maurice Wilkes, 1949).

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