io

Input/Output

• Ports
  • GPIO = General Purpose Input Output
  • UART
• Memory layout for IO
  • Flat - Directly assess devices at a memory address, efficient but less secure. More prevalent in modern system.
  • Segmented: Memories divided into separate segments, some are dedicated for devices. Better isolation.

Device

• Components - three (sets) of registers
  • Control - configure
  • Status - interrogate
  • Data - interact
• Reading from a previously written register does not necessarily yield the same result!

Interrupt

• Polling vs Interrupt
  • Polling - heavy lifting is on CPU
  • Interrupt - move a bit of intelligence to device, async and aperiodic
• Pins (usually low enable)
  • IRQ pin: maskable interrupt request
  • XIRQ pin: un-maskable, always handled
• PIC = Programmable Interrupt Controller, decodes which device interrupts. Typically handles all the maskable interrupts.
• Multiple IRQ lines and priority
• Nested Interrupt
• Procedure
  • Device → event
  • Device “raises” interrupt
  • CPU finish instruction
  • Save critical state: PC, regs, stack pointer, flags, CCR (condition code regs), usually pushed to stack.
  • Jump to exception handler, Vector Table (a list of addresses of functions and devices)
• ISR = Interrupt Service Routine, part of device driver
  • Invoked in supervisor mode, hence we want ISR to be “really short and sweet”
  • Save registers (writing asm in C…)
  • Read status or data, save them to buffer
  • RTI = Return from the Interrupt (unlike in C, RTS = Return from the Subroutine)