memory abstraction

Concept

RAM

• array of bytes with unique physical addresses
• contiguous region: interval of consecutive addresses

Data

• transient - valid for limited time
• persistent - valid for the duration of the program
• both types can grow or shrink during execution

Role of the OS

1. allocate memory space to a new process
2. manage memory space for process
3. protect memory space of processes from each other
4. manage memory for internal use within the process

Physical addressing

• processes load and store from physical addresses
• direct addressing
• hard to run multiple processes, load and stores may clash

Relative addressing

Address relocation

• when loading the process into memory
• add an offset to all memory references
• slow -> need to calculate everything
• hard to distinguish memory refernce from other instructions

Base + Limit registers

• base register -> points to the start of the space allocated to the process
• limit register -> size of the space allocated to the process
• logical address:
  • at compile time, memory references are compiles as an offset from the base register
  • each process has a self-containted logical memory space
• need to compute physical = base + offset and check offset < limit` for every access