Add a blog post about micro:bit emulation

QEMU 4.0 ships the core micro:bit emulation that was implemented during
Outreachy and GSoC 2018.  This blog posts explains how to use it and
describes the current status.

Signed-off-by: Stefan Hajnoczi <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Thomas Huth <[email protected]>

_posts/2019-05-21-microbit.md

@@ -0,0 +1,132 @@
+---
+layout: post
+title:  "QEMU 4.0 adds micro:bit emulation support"
+date:   2019-05-22 12:45:00 +0200
+categories: ['qemu 4', microbit, gsoc, outreachy, internships]
+---
+[micro:bit](http://microbit.org/) emulation support is available from QEMU 4.0
+onwards and can be used for low-level software testing and development.  Unlike
+existing micro:bit simulators, QEMU performs full-system emulation and actually
+runs the same ARM code as the real hardware.  This blog post explains what
+full-system emulation means and why QEMU is now a useful tool for developing
+micro:bit software.
+
+The [micro:bit is a tiny ARM board](https://tech.microbit.org/hardware/)
+designed for teaching.  It is increasingly being used around the world to
+expose children to computers, programming, and electronics in a low-cost way
+with an active online community that shares project ideas, lesson plans, and
+programming tips.
+
+![micro:bit board](https://pxt.azureedge.net/blob/12e4685e5f24df67255a242bccb4d1c8e1395e5f/static/courses/csintro/making/microbit-board.png)
+
+## Simulators and emulators
+*Simulators* are used for many tasks from mobile app development to
+performance analysis of computer hardware.  It is possible to develop code
+using a simulator without having access to real hardware.  Oftentimes using a
+simulator is more convenient than flashing and debugging programs on real
+hardware.
+
+*Emulators* allow programs written for one computer system to run on a
+different computer system.  They use techniques like [machine code
+interpreters](https://en.wikipedia.org/wiki/Interpreter_%28computing%29) and
+[just-in-time
+compilers](https://en.wikipedia.org/wiki/Just-in-time_compilation) to execute
+guest programs that do not run natively on the host computer.  Each CPU
+instruction must be correctly implemented by the emulator so it can run guest
+software.
+
+## How existing micro:bit simulators work
+Simulators can be implemented at various layers in the software stack.  The
+[MakeCode editor](https://makecode.microbit.org/#editor) for JavaScript
+development includes a micro:bit simulator:
+
+![MakeCode editor](/screenshots/makecode.png)
+
+This simulator does not execute any ARM code and is therefore not running
+the same CPU instructions as a real micro:bit.  Instead it reuses the JavaScript
+engine already available in your web browser to execute micro:bit JavaScript
+programs.  This is achieved by providing the micro:bit JavaScript APIs that
+micro:bit programs expect.  The programs don't need to know whether those APIs
+are implemented by the real micro:bit software stack or whether they are
+actually calling into the MakeCode simulator.
+
+In the screenshot above the micro:bit program calls `showString("Hello
+world!")` and this becomes a call into the MakeCode simulator code to
+render images of LEDs in the web browser.  On real hardware the code path is
+different and eventually leads to an LED matrix driver that lights
+up the LEDs by driving output pins on the micro:bit board.
+
+## Full-system emulation
+Unlike the MakeCode simulator, QEMU emulates the micro:bit CPU and boots
+from the same ARM code as the real micro:bit board.  The simulation happens at
+the CPU instruction and hardware interface level instead of at the JavaScript
+API level.  This is called *full-system emulation* because the entire
+guest software environment is present.
+
+What are the advantages of full-system emulation?
+* Programs written in any language can run (MicroPython, mbed C/C++, etc)
+* Boot, device driver, and language run-time code can be tested
+* Bugs in lower layers of the software stack can be reproduced
+* CPU architecture-specific bugs can be reproduced (stack and memory corruption bugs)
+* A debugger can be connected to inspect the entire software stack
+
+The main disadvantage of full-system emulation is that the performance
+overhead is higher since simulation happens at the CPU instruction level.
+Programs consist of many CPU instructions so the task of emulation is
+performance-sensitive.  Luckily the micro:bit's CPU is much less powerful than
+CPUs available in our laptops and desktops, so programs execute at a reasonable
+speed.
+
+## Running micro:bit programs on QEMU
+QEMU emulates the core devices on the micro:bit, including the serial port
+(UART) and timers.  This is enough for developing and testing low-level
+software but does not offer the LEDs, radio, and other devices that most
+micro:bit programs rely on.  These devices might be emulated by QEMU in the
+future, but for now the main use of QEMU is for developing and testing
+low-level micro:bit code.
+
+To run `test.hex`:
+```shell
+$ qemu-system-arm -M microbit -device loader,file=test.hex -serial stdio
+```
+
+Any output written to the serial port is printed to the terminal by QEMU.
+
+## Debugging micro:bit programs with QEMU and GDB
+QEMU has GDB guest debugging support.  This means GDB can connect to QEMU in
+order to debug the guest software.  This is similar to debugging a real system
+over JTAG, except no hardware is necessary!
+
+Connect with GDB to debug the guest:
+```
+$ qemu-system-arm -M microbit -device loader,file=test.hex -s
+$ gdb
+(gdb) target remote tcp:127.0.0.1:1234
+(gdb) x/10i $pc
+=> 0x161c4:	ldr	r3, [r4, #0]
+   0x161c6:	cmp	r3, #0
+   0x161c8:	beq.n	0x161d2
+   0x161ca:	ldr	r3, [pc, #48]	; (0x161fc)
+   0x161cc:	ldr	r3, [r3, #0]
+   0x161ce:	cmp	r3, #0
+   0x161d0:	bne.n	0x161d8
+   0x161d2:	movs	r0, #6
+   0x161d4:	bl	0x16160
+   0x161d8:	ldr	r0, [r4, #0]
+```
+
+Having a debugger is very powerful.  QEMU can also load ELF files in
+addition to the popular .hex files used for micro:bit programs.  ELF files can
+contain debugging information that enables source-level debugging so GDB can
+display function and variable names as well as listing the source code instead
+of showing assembly instructions.
+
+## Conclusion
+QEMU now offers a platform for developing and testing micro:bit programs.
+It is open to future extension, hopefully to emulate more devices and offer
+a graphical user interface.
+
+micro:bit emulation was contributed by Julia Suvorova and Steffen Görtz as
+part of their Outreachy and Google Summer of Code internships with QEMU.  Jim
+Mussared, Joel Stanley, and Stefan Hajnoczi acted as mentors and contributed
+patches as well.