Memory: On-chip vs Off-chip

• On-chip (SRAM):
  • Storage on the processor chip to decrease latency for critical/ frequently used data
  • Works as a cache for the main memory(DRAM)
  • Usually single cycle access
• Off-chip (DRAM):
  • Much cheaper than SRAM
  • Usually has much more latency (5–15 cycles)

Memory Space

• The memory space is devided into 6 pre-defiened regions
  • Code:
    • Typically, on-chip flash (not on-chip SRAM)
    • To store program code or data
  • SRAM:
    • Primarily used to store data (e.g., heaps, stacks), or code
  • Peripheral
  • External device
  • External RAM
  • System

Interfacing Peripherals

• Usually, each on-chip peripheral device has a few registers (e.g., control. status, data input, and data output regsiters)
• Two approaches to exchange data between a processor core and a peripheral:
  • Port-mapped I/O:
    • uses special CPU instructions specifically designed for I/O operations: Special_instruction Reg, Port
  • Memory-mapped I/O:
    • A simpler and more convenient way to interface I/O devices
    • Does not need any special instructions
    • Each device register is assigned to a memory address space
    • Uses native CPU load/store instructions: LDR/STR Reg, [Reg, #imm]
    • If we write to the special memory location assigned to a device register (STR), the written data is sent to the corresponding I/O device (by PCIe write operations)

MMIO