Unit 3: I/O organization
Computer System Architecture notes · PTU syllabus (BSIT301/BSBC204)
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Unit summary
Input/output organisation connects the CPU and memory with the outside world. This unit covers the I/O interface unit, port types, the I/O bus, isolated vs memory-mapped I/O, programmed I/O, interrupt-initiated I/O, the DMA controller and I/O processors, synchronous and asynchronous data transfer, and strobe and handshaking.
After this unit you can
- Explain the I/O interface, ports and I/O bus
- Compare isolated and memory-mapped I/O
- Compare programmed, interrupt-initiated and DMA transfer
- Explain synchronous and asynchronous transfer, strobe and handshaking
PTU syllabus topics
- I/O interface unit
- port types
- I/O bus
- isolated vs memory-mapped I/O
- programmed I/O
- interrupt-initiated I/O
- DMA controller and IOP
- synchronous and asynchronous data transfer
- strobe and handshaking
Programmed I/O
CPU keeps checking the device
High: busy waiting
Interrupt-driven I/O
Device signals when ready
Medium
DMA
Controller moves data directly to memory
Low: CPU only sets it up
Topic 1
The I/O interface unit
- Need: peripherals differ in speed, data formats and signals from the CPU; an interface resolves these differences.
- Functions: decode device addresses, buffer data, convert formats, provide status and control signals, handle interrupts.
- Ports: data port (data in and out), status port (device status), control port (commands); serial and parallel ports.
Topic 2
The I/O bus
- I/O bus: data lines, address lines and control lines connecting the processor to interfaces; each interface checks the address to respond.
- I/O commands: control, status, data output, data input.
Topic 3
Isolated vs memory-mapped I/O
Address space
Separate for I/O and memory
Shared — I/O registers have memory addresses
Instructions
Special IN and OUT instructions
Ordinary memory instructions
Memory available
Full memory space
Reduced by I/O addresses
Example
Intel x86 port I/O
ARM processors, most modern systems
Topic 4
Modes of data transfer
Programmed I/O
CPU repeatedly checks device status (polling)
Wastes CPU time
Interrupt-initiated I/O
Device interrupts CPU when ready
Overhead for each transfer
Direct memory access (DMA)
DMA controller transfers blocks directly between memory and device; CPU only starts and ends
Needs extra hardware; bus contention
- DMA operation: CPU programs the DMA controller (address, count, direction); controller requests the bus (bus request), CPU grants it (bus grant); transfer in burst or cycle-stealing mode; interrupt at completion.
- I/O processor (IOP): a processor dedicated to I/O tasks, executing its own I/O programs — relieves the CPU further.
Topic 5
Synchronous and asynchronous data transfer
- Synchronous: sender and receiver share a common clock.
- Asynchronous: units have separate clocks; timing signals coordinate transfer.
Signals
One control line (strobe) from source or destination
Two lines — data valid and data accepted
Confirmation
No acknowledgement
Each step acknowledged
Reliability
Lower
Higher
- Asynchronous serial transfer: start bit, data bits, parity bit, stop bits — used in UART communication.
Key terms
- I/O interface
- Hardware linking CPU and a peripheral
- Memory-mapped I/O
- I/O registers accessed as memory locations
- Polling
- CPU repeatedly checking device status
- DMA
- Direct memory access without CPU involvement in each transfer
- Handshaking
- Two-way control signalling for data transfer
Quick revision
- Interface functions; data, status, control ports; I/O bus.
- Isolated vs memory-mapped I/O.
- Programmed, interrupt-driven, DMA; burst and cycle stealing; IOP.
- Synchronous vs asynchronous; strobe vs handshaking; serial frame.
Important exam questions
Practice questions written to the PTU exam pattern for this unit's syllabus: short answers (Section A style) and long answers (Sections B and C style).
Short-answer questions
- Q1.Why is an I/O interface needed?
- Q2.Distinguish isolated and memory-mapped I/O.
- Q3.What is polling?
- Q4.What is cycle stealing in DMA?
- Q5.What is an IOP?
- Q6.Distinguish strobe and handshaking.
Long-answer questions
- Q1.Explain the I/O interface, ports and I/O bus.
- Q2.Compare isolated and memory-mapped I/O.
- Q3.Explain programmed I/O, interrupt-initiated I/O and DMA.
- Q4.Explain asynchronous data transfer using strobe and handshaking.
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