Notes

Components Registers We can combine registers to reduce the amount of wires needed. Using a data bus (wiring at the top) and a binary decoder, we can select which register to read/write to. Memory Assembly Assembly is a human-friendly representation of code: binary values that a computer can understand. An assembler converts ASM instructions into machine code, which is given to the CPU as input. Arithmetic Operations For example, a simple computer architecture could use 00 to represent arithmetic operations. To decide which type of operation to execute (subtraction, multiplication, addition, etc), the 3rd and 4th bits could be used. Using a index, we can build an inefficient, but simple circuit to do this. This type of circuit is an Arithmetic Logic Unit (ALU). ...

Flat-File Databases A “table” written into a single file. The most common file type for this purpose is CSV. The CSV format reads each line as a row, and each comma-separated value as a column. The first row on a CSV file is used to describe the data in each column. If a , is present on the dataset, surround that entry with " to ’escape’ it. These files can be read and written to using languages like c-language and python. ...

GDB Debugging Compile with Debug Information To allow gdb access to the source code (Provides debug symbols - Do not share publicly as it contains the source code). gcc -g <file> Look for / Fix Bugs First, initialize gdb with the executable to debug. gdb ./<executable> --tui After gdb is ready, we can use the following commands: Command Description lay next Switch to the next layout (Enables TUI mode if disabled - Allows for reading the code while debugging both in C and ASM). ref Refresh (if a program prints to the terminal, it can break gdb’s interface).` q Quit gdb. b main Add a breakpoint at the main function. b Place a breakpoint at the current line. b <N> Place a breakpoint at line N. b +<N> Place a breakpoint N lines down. b <fn> Place a breakpoint at fn function. d Delete all breakpoints. d <N> Delete breakpoint number N. clear <fn> Clear the breakpoint set to fn function. n Execute up to the next line in C. If a function call is found, execute the function completely. s Execute up to the next line in C. (Jump over) s <N> Run N lines. u Same as n, but if in a loop, execute until the loop exits. nexti Execute up to the next instruction (line in ASM). r Run the program until a breakpoint or error is reached. c Continue running the program until a breakpoint or error is reached. x/i $pc Examine the previous instruction (View memory). info registers Read the CPU registers used by the program. bt See the call stack up to the current line. (How we got here, so to speak) print sizeof(<variable>) Check the size of a struct/variable/pointer. p <var> Print variable var value. info break List breakpoints. Check for Memory Leaks Use valgrind to check for lost memory. ...

A diode allows current to only flow in one direction in a circuit. Schematic Anode (+) --|>|-- Cathode (-) Examples [Conventional Current (+) -> (-)] (+)------|>|------(-) Current can flow - The diode is now a conductor. (+)------|<|------(-) Current can't flow - The diode is now an insulator. Use Cases Protect a circuit (if a battery is connected incorrectly, for example) Convert AC to DC current Fun fact: An LED, for example, is a Light-Emitting Diode. How a Diode Works Conductors and Insulators An atom contains the following elements: ...

Symmetric Encryption User A sends a password to user B. The password is used to encrypt the messages. A secure way to share the password is required. Asymmetric Encryption Users A and B have a public key and a private key. The public keys are shared, and they are used to encrypt the messages. The users can use their private keys to decrypt the messages. GPG Create a Set of Keys gpg --full-gen-key Select ECC (sign and encrypt) - The most secure option Select default curve Encrypt a File gpg --encrypt -r email@example.org <file> # Encrypt with the recipient (-r) key. Decrypt a File gpg --decrypt --output <file-output> <file> # Use the private key to decrypt a file. Encrypt a Message echo "Very safe message" | --encrypt --armor -r email@example.org -armor Saves the encrypted info in plain text. (Great for blog posts or copying/pasting) Decrypt a Message GPG automatically figures out which private key to use. The encrypted file includes some metadata. ...