Computer I/O Interface: Functions, Mapping, and Data Transfer
Functions and Need of I/O Interfaces
Functions of I/O Interfaces
- Synchronizes the speed difference between the CPU and I/O devices.
- Selects and addresses the appropriate I/O device for communication.
- Provides control and timing signals for data transfer.
- Buffers data during transfer, managing data flow between the CPU and devices.
- Converts data formats (e.g., serial to parallel, digital to analog, and vice versa).
- Detects and reports errors during data transfer.
- Handles interrupts to inform the CPU when I/O devices need attention.
- Standardizes communication protocols for device compatibility.
Need for I/O Interfaces
- Bridges the operational differences between the CPU (fast, electronic) and peripherals (slow, electromechanical).
- Facilitates efficient, reliable, and standardized data exchange between the system and external devices.
- Ensures user interaction and data storage/retrieval from external devices.
- Expands system functionality by supporting various peripherals.
Memory-Mapped I/O vs. I/O-Mapped I/O Comparison
| Feature | Memory-Mapped I/O | I/O-Mapped I/O (Isolated I/O) |
|---|---|---|
| Address Space | Shared with memory | Separate from memory |
| Instructions | Uses memory instructions (e.g., LDA, STA) | Uses special I/O instructions (IN, OUT) |
| Address Size | Typically 16-bit (same as memory) | Typically 8-bit (dedicated I/O ports) |
| Registers | Any register can communicate with I/O device | Usually only accumulator communicates |
| Addressable Devices | Limited by memory address space | Up to 256 I/O ports (in 8-bit addressing) |
| Programming | Simpler, same as memory access | Requires special instructions |
| Performance | Faster, supports all addressing modes | May be slower, limited addressing modes |
| Complexity | Simpler logic, but address conflicts possible | More complex logic, but avoids conflicts |
| Security | Potential for address conflicts | More secure and stable |
Key Differences: Isolated vs. Memory-Mapped I/O
This section elaborates on the key distinctions between Isolated (I/O-Mapped) and Memory-Mapped I/O:
- Address Space: Isolated I/O uses a separate address space for I/O devices, while memory-mapped I/O shares the address space with memory.
- Instructions: Isolated I/O requires special instructions (IN, OUT), whereas memory-mapped I/O uses standard memory instructions.
- Efficiency: Memory-mapped I/O allows faster access and simplified programming, but it reduces available memory addresses. Isolated I/O offers greater flexibility and reliability but can be slower and more complex to program.
Program-Controlled I/O and Interrupt-Driven Advantages
Program-Controlled I/O
In Program-Controlled I/O, the CPU actively checks (polls) the status of the I/O device and waits in a loop until the device is ready for data transfer. The CPU executes instructions to move data between the device and memory, keeping the CPU busy during the entire operation.
Advantages of Interrupt-Driven I/O
Interrupt-driven I/O is more advantageous because it allows the CPU to execute other tasks while waiting for the I/O device to become ready. When the device is ready, it sends an interrupt signal, prompting the CPU to handle the data transfer. This approach significantly reduces CPU idle time and improves overall system efficiency.
Resolving Simultaneous Interrupt Requests
The processor uses a priority scheme to resolve simultaneous interrupt requests, ensuring that the interrupt with the highest priority is serviced first. Common methods for resolving these requests include:
- Fixed priority
- Dynamic priority
- Polling
- Vectored interrupts
- Interrupt nesting
Vectored Interrupt Technique Explained
In vectored interrupts, each interrupting device sends a unique code (known as a vector) to the processor when it requests service. The processor then uses this code to directly jump to the corresponding Interrupt Service Routine (ISR). This method makes interrupt handling significantly faster and more efficient compared to polling.
Types of I/O Data Transfer: Pros and Cons
| Type | Advantage | Disadvantage |
|---|---|---|
| Programmed I/O | Simple to implement | CPU is busy and inefficient |
| Interrupt-Driven I/O | CPU can perform other tasks; efficient | More complex to program |
| Direct Memory Access (DMA) | High-speed transfer without CPU involvement | Complex hardware; CPU loses bus control |
Direct Memory Access (DMA) Method and Registers Used
DMA Method
Direct Memory Access (DMA) is a feature of computer systems that allows certain hardware subsystems to access main system memory (RAM) independently of the central processing unit (CPU). This capability significantly improves system performance by offloading the CPU from data transfer tasks, especially for high-volume I/O operations.
DMA allows peripheral