ELEC6254: io: add

Postgraduate/ELEC6254 Foundations of Embedded IoT Systems/2020-11-01-io.md

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+# I/O
+
+## CPU-Memory Register Transfer
+
+- **Control bus:** carry read / write / output enable signals
+- **Data bus:** transfer the content in register into or out of the CPU
+- **Address bus:** select a particular register in memory
+
+## Port-Mapped I/O
+
+PMIO uses special instructions (e.g. `IN`, `OUT`) to transfer data between the computer system and the outside world.
+
+- Special bus line, insns, and separate memory space is required
+- Do not consume memory address space
+
+## Memory-Mapped I/O
+
+MMIO uses special memory locations in the normal address space.
+
+- Use any insn (e.g. `MOV`, `ADD`) to access devices
+- COnsume memory address space
+
+### Direct Memory Access
+
+DMA is a special form of MMIO: peripheral devices read / write the memory without going through the CPU.
+
+- Complex design structure; separation of hardware controllers for DMA systems
+
+## Parallel vs. Serial
+
+Parallel interface: sends multiple bits at the same time.
+
+- Easy to progeram, high speed
+- High coat, cable length is limited
+
+Serial interface: send bits one by one
+
+- Long distance data transfer
+- Fewer pins are required
+- Slower than parallel