1. Implementing Concurrency Abstractions
for Programming
Multi-Core Embedded Systems
in Scheme
Ruben Vandamme
Promotor: Prof. Dr. Wolfgang De Meuter
Advisors: Dr. Coen De Roover
Christophe Scholliers

2. 2
Overview

Embedded systems

Event-driven XMOS chip

Interpreter requirements

Bit Scheme

Modifying Bit Scheme to support XMOS

Demonstration

Contributions & Conclusion

3. 3
Embedded software

Increasingly important
●
Digital watches, microwaves, cars, etc
●
98% of processors used are embedded

Different from PC and server software
●
Interacts with the outside world
●
Reading sensors, buttons, communicating, etc

Polling: frequently check condition

Interrupts: asynchronous signals
●
Less overhead, less power

4. 4
Interrupts

Dedicated hardware for frequent tasks
●
PWM, UART, I²C,…
●
Timing sensitive tasks
●
Interrupts are often used as an interface

Source of various bugs
●
Stack overflow, interrupt overload, …
●
John Regehr (2005)
Safe and structured use of interrupts in real-time
and embedded software

5. 5
Event-driven chip

XMOS XS1-G4
●
No interrupts or polling

Multi-core, multi-threaded
●
Threads supported in HW
●
Guaranteed execution time
●
Message passing

Transputer

Programmed in XC
●
Based on CSP

6. 6
Interpreter requirements

Use less than 64 KB memory (per core)
●
Most interpreters need an order of magnitude
more memory
●
MiniScheme, Pico, etc.

Contain a real-time garbage collector

We need to extend it to
●
exploit concurrency provided by hardware
●
export hardware functionality
Input & output, timing, ...

7. 7
Bit Scheme

Fits in 64KB memory
Byte code + interpreter + runtime memory

Byte code based
Compiler can remove unneeded functions

No runtime error handling
Keeps interpreter small

Real-time garbage collector

Served as a basis for our XMOS Scheme

8. 8
Exploiting XMOS concurrency

We run four interpreters run in parallel
●
One on each core
●
Modified compiler and interpreter accordingly

Exploit all memory and IO possibilities
(par
(core CORE_0
...)
(core CORE_1
...)
(core CORE_2
...)
(core CORE_3
...))

9. 9
Communication primitives added

Use message passing over channels
●
cout, cin
●
Primitives use hardware

Doesn't support composite types
●
Serialization needed
Core 0 Core 1
(par
(core CORE_0
(cout CORE_1 99))
(core CORE_1
(display (cin CORE_0)))) Core 2 Core 3

10. 10
IO primitives added

Initialize and configure IO
pon, poff, pconf_in, pconf_out

Perform IO
pout, pin

Wait for an event on IO pins
peq, pneq

Use hardware functionality
(define PORT_CLOCKLED 525056)
(pon PORT_CLOCKLED)
(pconf_out PORT_CLOCKLED 0)
(pout PORT_CLOCKLED 15)

11. 11
Time primitives added

We added a notion of time
●
Execute actions at a certain point in time
●
(timer)
Returns the current time in clockticks
●
(after time)
Blocks thread until current time is after time
●
Both primitives call hardware functionality
(define now (timer))
(define clock 100000000)
(after (+ now (∗ 5 clock)))
(display ”5 seconds later”)

12. Handling multiple events at once
12

Certain primitives are blocking
pne, peq, cin

Threads need to be able to handle more
than one event at a time
(select
((select_pne buttons 15)
(lambda (buttonsvalue)
(display buttonsvalue)))
((select_cin CORE_1)
display)
(else (display ”default”)))

13. 13
Compilation
(par
(core CORE_0 ...) BC0
(core CORE_1 ...) BC1
(core CORE_2 ...) BC2
(core CORE_3 ...) BC3
)
Step 1
Scheme compiler
compiles Scheme → bytecode

14. 14
Compilation
(par BC0 BC1
Interpreter Interpreter
Interpreter
(core CORE_0 ...) BC0
(core CORE_1 ...) BC1
(core CORE_2 ...) BC2
(core CORE_3 ...) BC3
) BC2 BC3
Interpreter Interpreter
Step 1
Scheme compiler
compiles Scheme → bytecode Step2
XMOS toolchain
compiles bytecode + interpreter
→ XMOS executable

15. 15
Demonstration

Case study

LED Pulse Width Modulation in Scheme

Communication over Xbee
●
Via UART implemented in Scheme
App UART
Buttons RX
UART
PWM
TX

16. 16
Contributions & Conclusion

Ported a Scheme interpreter to the new
XMOS chip

Exploit the concurrency of XMOS chip

Added new primitives
●
IO, message passing, time, …

Allow to program hardware from Scheme

Modified compiler accordingly

17. 17
Questions