Arm64 File I/O Demystified: Read, Write & Command-Line Args #raspberrypi #assembly #inputoutput

Introduction to Input and Output Devices

Overview of File Handling on Raspberry Pi

• The video introduces the topic of input and output devices, focusing on reading and writing files while handling arguments.

• It demonstrates connecting to the Raspberry Pi Connect service via remote shell, maximizing terminal visibility for better interaction.

Writing a File: Example 53

Code Structure for File Creation

• The program begins with a global directive and a start label, defining a variable filename in the data section as an ASCII string.

• A buffer is declared to store text that will be written to the file; several segments are outlined for creating, opening, writing, closing the file, and exiting.

System Calls for File Operations

• To create a file, it specifies the file descriptor using AT_FDCWD, setting its value to -100 in register x0.

• The address of filename is loaded into register x1; permissions are set using octal notation (777), indicating full access rights.

Execution Summary of Example 53

Program Execution Insights

• After saving changes in nano, executing the program does not print output but creates a file with content from the buffer.

• Verification is done using the cat command to display contents of the created file.

Reading a File: Example 54

Code Structure for File Reading

• This example involves editing 54_file_read.s, starting similarly with global directives and declaring variables including filename.

• A buffer of 1024 bytes is allocated in BSS for reading file contents; segments include opening, loading into buffer, printing contents, and closing.

System Calls for Reading Operations

• The code opens the file with read-only permissions (0), preserving its descriptor in register x20 for subsequent operations.

Execution Summary of Example 54

Program Execution Insights

• Executing this program reads an existing file's content and prints it on screen after successfully opening it.

Handling Arguments: Example 55

Code Structure for Argument Processing

• In this example (55_args.s), an uninitialized variable arg1 is declared; argument loading utilizes stack pointer offsets.

Looping Through Arguments

• A loop reads byte by byte until encountering a null terminator (ASCII zero), storing each byte sequentially into a buffer before printing.

Conclusion and Final Remarks

Summary of Key Learnings

• The video concludes by encouraging viewers to like, comment, subscribe, and enable notifications.