device-controller

The CPU cannot directly manipulate hardware signals (move disk arm, read magnetic flux). It needs a hardware intermediary that understands both the CPU’s bus interface and the device’s physical interface.

A device controller is a hardware component that interfaces between the system bus and the physical I/O device, containing registers for commands, data, and status, plus local buffers for data transfer.

1. CPU (via driver) writes commands and parameters to controller’s registers
2. Controller parses registers to determine operation (read/write/seek)
3. Controller executes operation on the physical device
4. Data transfers to/from controller’s local buffer
5. Controller raises interrupt when operation completes
6. CPU reads status register to check success/failure

• Contains three types of registers: control, data, and status
• Has local buffer to hold data during transfer
• Can operate asynchronously from CPU (using DMA)
• Sits on the system bus, addressed like memory (memory-mapped I/O)

• Built from: System Bus, O System
• Builds into: O Request to Hardware Operation
• Related: Device Driver, DMA, Interrupt Handler
• Contrasts with: Device Driver (software vs hardware)

• Register addresses vary by device — driver must know the correct addresses
• Local buffer size limits transfer size per operation
• Status register must be read before another command is issued
• Some controllers have buggy implementations causing race conditions