What happens when we run code?

18. What happens when we run code?#

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pick the most appropriate first, but you can also free-response to expand

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cd Documents/systems
ls

example				seawulf
github-in-class-brownaymanm-1	test
kwl				testobj.md
nand2tetris			tiny-book

18.1. on Mac/Linux#

cd nand2tetris/tools

and then

bash CPUEmulator.sh 

or in file explorer, Double click on the CPUEmulator.bat file

18.2. On Windows:#

Double click on the CPUEmulator.bat file

you can submit an issue to the coursewebsite if you figure out a CLI way to start it on windows

18.3. To work on the online supported version#

Follow this link for CPU emulator https://nand2tetris.github.io/web-ide/cpu

18.4. Starting the CPU Emulator#

Linux/MacOS

In your terminal:

cd nand2tetris/tools

and then

bash CPUEmulator.sh 

Windows

Double click on the CPUEmulator.bat file

you can submit an issue to the coursewebsite if you figure out a CLI way to start it on windows for a community badge

18.5. Loading a program#

We’re not going to do project 5, which is to build a CPU, but instead to use the test programs designed to check if a student’s CPU works with the CPU Emulator they provide to understand a simple canonical CPU.

For more on how the emulator works see the CPU Emulator Tutorial.

For much more detail about how this all works chapter 4 of the related text book has description of the machine code and assembly language.

We’re going to use the test cases from the book’s project 5:

Load Program
Navigate to nand2tetris/projects/05

18.6. How does the computer actually add constants?#

We’ll use add.hack first.

This program adds constants, 2+3.

It is a program, assembly code that we are loading to the simulator’s ROM, which is memory that gets read only by the CPU.

Run the simulator and watch what each line of the program does.

Notice the following:

to compute with a constant, that number only exists in ROM in the instructions
to write a value to memory the address register first has to be pointed to where in the memory the value will go, then the value can be sent there

The simulator has a few key parts:

address register
program counter

If you prefer to read, see section 5.2.1- 5.2.6 of nan2tetris book

This program the first instruction puts 2 in the address register from the instructions in ROM.
Then it moves the 2 through the ALU to the data register (D)
then it puts 3 on the address register
then it adds the numbers at D and A
then it puts 0 on the address register
then it write the output from the ALU (D) to memory (at the location indicated by the A register)

18.7. The max between two numbers#

To compute the maximum value max.hack:

subtracts the two numbers
compares the difference to 0
jumps to different lines based on if it is >0 or not
assigns the address regiter a value accordingly
copies one of the two values to the new location

18.8. Explore ideas#

Create some experiments in the simulator and write new programs. Here are some ideas, but you can try anything you are curious about.

What line do you change to change where it outputs the data?
How could you add a third number?
How could you add two pairs, saving the intermediate numbers?
How could you do (4+4)*(3+2)?

18.9. Why is the object file not human readable?#

https://learning.oreilly.com/library/view/beautiful-code/9780596510046/ch17.html#from_code_to_pointers

The object file displayed as random weird characters because it is written to disk in a different format than our terminal reads it in. It is the specific instructions (from the assembly) and memory addresses written to a file in binary. Our terminal reads it one byte (8bits) at a time and interprets those as characters.

Today, we will see how characters are represented as integers, then binary, and read and write files in binary of strings we know to see how it what happens and mimic something like how that object file happened by writing as binary and then reading it as characters.

18.10. Prelude: REPL#

One way we can use many interpreted languages, including Python is to treat the interpreter like an application and then interact with it in the terminal.

This is called the Read, Execute, Print Loop or REPL.

We can execute simple expressions like mathemeatical expressions:

4+5

and it will print the result

We can create variables

name = 'ayman'

but assignment does not return anything, so nothing is printed.

We can see the value of a variable, by typing its name

name

and then it prints the value

'ayman'

18.10.1. Characters to Integers#

Python provides a builtin function ord to find the Unicode key for a character.

This structure, the [] is called a list comprehension. It executes a for loop in order to build a list. In the REPL, it will then print out the list.

[ord(char) for char in name]

We get the integer representation for each one.

Note that it’s important to try multiple things if you aren’t sure the format you are getting back or to check the official documents, as Denno found by testing this with his name in lowercase.

[ord(char) for char in 'denno']

Coincidentally, the letters in his name all fall into the hundreds and only require 0 and 1 in decimal, so it looks like they could be 3 bit binary numbers.

[100, 101, 110, 110, 111]

Python also allows us to change content into a byte array:

bytearray(34)

It does not display in the 0 and 1 like a string that we might expect, but it does change the type.

bytearray(b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00')

We can then write this to a file, by getting the actual bytes of each item. First, we will save our list in a variable.

We will use the Python with keyword to open the file so that we do not to need to separately open and close the file. in the open function, the first parameter is the file name and the second is the mode. In this case, we want to write (‘w’) in binary (‘b’). f is the filestream.

name_ints = [ord(char) for char in name]
with open("name.txt",'wb') as f:
     f.write(bytes(bytearray(name_ints)))

it returns 5 in this case, because it wrote out 5 bytes, one per character.

If we exit python we can look at the file and see how it is interpreted.

exit()

Remember we wrote, as binary, the numbers, 97, 121, 109, 97, 110 to the file

cat name.txt

but it is interpretted as the string by the terminal.

ayman

python

Back in Python, we can read the file as binary

with open("name.txt",'rb') as f:
    print(f.read())  

This is binary, but the python interpreter also prints it as a string, the b indicates the object is actually binary type.

b'ayman'

If we read it not as binary, it returns as the string

with open("name.txt",'r') as f:
    print(f.read())

ayman

We can also confirm the type

with open("name.txt",'rb') as f:
    content = f.read()

type(content)
<class 'bytes'>

The with keyword is alternative to:

f = open("name.txt",'rb')
f.read()
f.close()

Practice exercise

msg = [65, 121, 109, 97, 110, 32, 83, 97, 110, 100, 111, 117, 107, 10]

bytes(bytearray(msg))
b'Ayman Sandouk\n'

18.11. Experience Report Evidence#

Add in your experience report the modifcations to the code as above.

18.12. Prepare for Next Class#

2025-04-08.md1. Review your idethoughts.md from a few weeks ago and add some summary notes.

Think about what features or extensions in your favorite IDE you like the most and that you think others may not know about be preppared to share.

18.13. Badges#

Review

Run and examine how rect.hack and max.hack in the nand2tetris/projects/05/ folder work. Make notes and answer the questions below in assemblyexplore.md.

Write an excerpt of code in a high level compiled language that would compile into this `max.hack``.
What does rect.hack do?
What did you learn trying to figure out how it works?

Practice