1. Module 2.E
Buffered I/O
Tim Rogers 2017

2. Learning Outcome #2 Bus Timing Analysis
“An ability to interface a microcontroller to various devices”
Bus Timing Analysis
9S12C Multiplexed Bus Expansion
General-Purpose I/O Ports
Interrupt Handling
Buffered I/O
Buffered, Interrupt-Driven Printer Design Example
How?

3. Objective Why? “Buffered I/O”
Devices are much slower than CPUs (yes – even the 9S12)
Problems:
Data production/consumption rates are different.
CPU slowed by device. EX: CPU has a character to print - printer is busy, so CPU must wait.
Data may be coming in to CPU before the CPU is ready to deal with it.

4. These simple relationship are called producer/consumer.
Solution: A buffer
For output devices - want the CPU to put data in a buffer as soon as it’s ready (don’t wait for the device to no longer be busy).
Buffer
Produce Data
Consume Data
These simple relationship are called producer/consumer.

5. These simple relationship are called producer/consumer.
Solution: A Buffer
For input devices - want the device to place data in a buffer as it’s produced. Then the CPU can grab it when the CPU is ready.
Buffer
Consume Data
Produce Data
These simple relationship are called producer/consumer.

6. Circular Buffers
Space is finite and we do not have dynamic memory management (i.e. malloc/free or new/delete).
Allocate a fixed size in memory, then “wrap around”
Managed with a First in First Out (FIFO) policy.

7. Circular buffers 2 pointers manage buffer allocation and deallocation
IN: points to next available location. Producer puts data here.
OUT: points to next data item in FIFO order. Consumer takes data from here.
We are going to manage the buffer with no other meta-data. IN
OUT

8. Circular Buffers OUT IN Example: BufferSize = 8 Empty condition:
IN == OUT

9. Circular Buffers A B C D OUT IN Example: BufferSize = 8
Content in buffer A B C D IN

10. (IN+1) mod BufferSize == OUT
Can the buffer be full when OUT points to index 3
(A) Yes
(B) No
(C) I really can’t tell
Can the buffer be full when OUT points to index 3
(A) Yes 
(B) No
(C) I really can’t tell
Circular Buffers
Example:
BufferSize = 8
OUT
Buffer Full:
(IN+1) mod BufferSize == OUT A B
Why? IN C G D E F

11. Circular Buffer Algorithms
Spin-wait when buffer is full
Producer process
1. Check state of buffer
2. If buffer is FULL, wait for space
3. Else, place character in buffer at location pointed to by IN
4. Increment IN pointer modulo BUFSIZE
5. Exit

12. Circular Buffer Algorithms
Spin-wait when buffer is empty
Consumer Process
1. Check state of buffer
2. If buffer is EMPTY, wait for data
3. Else, get data from location pointed to by OUT pointer
4. Increment OUT pointer modulo BUFSIZE
5. Exit