Computer Architecture

CPU

The CPU (Central Processing Unit) follows the instruction cycle to execute instructions:

1. Fetch - fetch the instruction from memory;
2. Decode - decode the instruction;
3. Execute - execute the instruction.

I/O

• Input/Output (I/O) is the communication between the CPU and external devices;
• Read/Write (R/W), as in the computer memory;
• Address Bus, Data Bus.

Process

• A process is a program in execution;
• Each process is associated with a program, and a program can have multiple processes;
• The programs execute in isolated environments called processes:

• The .text and .rodata sections are read-only, so they are in the read-only memory (ROM);
• The .data is stored in the ROM, but in run-time it is copied to the RAM;
• The .bss is stored in the RAM, but it is not initialized;

• A process is like a “virtual machine” that has its own memory, registers, and stack;
• The “virtual CPU” is called thread;
• The “virtual memory” is called address space and contains:
  • executable file information, line .text, .data, .rodata, .bss, etc;
  • the stack for each thread;
• The I/O is represented by file descriptors in the file descriptor table;
• The signals module is used to communicate between processes; interruption mechanism;

OS operations that interact with the address space

• Create a new process;
• Dynamically allocate memory;
• Thread creation (it needs to be allocated space to store the stack);
• Dynamically load a library (.so / .dll);
• File mapping (map a file to memory);
• Share memory between processes;
• Share read-only memory (libraries, source code, etc) between processes, to save memory;
• Extend physical memory with disk space.