I⧸O and Devices

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A CPU accesses physical memory over a Bus

Devices access memory over I/O bus with DMA
Devices can appear to be a region of memory
- Memory-mapped IO
A crossbar can connect any input to any output

A bridge connects two different buses

The I⧸O Advanced Programmable Interrupt Controller handles interrupts from devices

What is memory?

Communicating with a device

Memory-mapped device registers

Some ranges of physical addresses correspond to device registers
Load and store instructions get status or sends instructions to a device

Device memory

Device may have memory and the OS can write directly to the device through I/O Bus rather than RAM

Direct Memory Access (DMA)

Place instructions to device in main memory

Example

Parallel Port

Every bit corresponds to a pin

There are three primary types of device registers

Status
- Tells something about device current state
- Read
Command
- Issue a command to the device
- Write
Data
- Used to transfer data to and from device

Using Physical Addresses

volatile int32_t *device_control = (int32_t *) 0xc00c0100;
*device_control = 0x80; /* give command */
int32_t status = *device_control; /* get back status */

OS must map device addresses to virtual addresses and ensure they are non-cacheable

volatile keyword tells compiler to not optimize
- Read and write directly to RAM
Assign physical address at boot time to avoid conflicts