Philipp Klaus Krause

The Taiwanese company Padauk makes small 8-bit microcontrollers, the smallest of which are available at 0.01 € even in small quantities. Even the larger ones are just a few cents; a particularly interestign feature is the hardware multithreading support available in larger devices. Until recently, the only available toolchain was Padauk's non-free toolchain based around their "MINI-C" IDE (which despite, the name, ist just a bit of C-like syntactic sugar coating for assembler, and in no way a C implementation).

In 2019, an effort to provide a free alternative resulted in a full free toolchain. Documentation beyond that provided by Padauk was created by reverse-engineering. A free design for a programmer along with firmware was created. Assembler, simulator and a Small Device C Compiler (SDCC) backend were written.

The Padauk microcontrollers (µC) are small 8-bit Systems-on-a-Chip (SoC). Their program memory (PROM or Flash) is 0.5 KW to 4 KW with word sizes from 13 to 16 bit. Their data memory is 60 B to 256 B with 8-bit Bytes. The µC only have few peripherals; however, some of the larger devices are barrel processors with support for up to 8 hardware threads. This allows the emulation of even timing-critical peripherals in software. Padauk also supplies a programmer and a non-free "MINI-C" IDE. There is a lack of documentation when it comes to aspects not needed for users of MINI-C. In particular, there is no documentation of opcodes and very little information on the programming protocol. There are 4 subarchitectures, which we name by the word size in the program memory pdk13, pdk14, pdk15 and pdk16. There is some variation in the form of optional instructions within the subarchitectures.

In 2019, a full free toolchain for these µC was created. The necessary documentation was reverse-engineered. A free programmer design and firmware was created. For the pdk13, pdk14 and pdk15, we wrote free assemblers and simulators. We also wrote an SDCC backend for these. SDCC is a free C compiler that emphasizes standard-compliance and generating efficient code for small devices; while not up to the level of GCC and LLVM it tends to hold up well against many non-free compilers targetting small devices (see e.g. the FOSDEM 2018 talk "The free toolchain for the STM8"). While stack handling on the Padauk µC is much better than on e.g. small Mirochip PIC devices, it is still not efficient (in particular, there is no stack-pointer-relative addressing mode). Thus SDCC does not place local variables on the stack by default, which makes functions non-reentrant and is not standard-compliant, but a common choice in such cases (see e.g. the mcs51 backend in SDCC or Keil for MCS-51). However, this approach does not work well for devices with hardware-multithreading (i.e. few pdk14 and all pdk16).

The ColecoVision and Sega Master System are popular video game systems of the 1980s. The central part of the free toolchain is the Small Device C Compiler (SDCC), which features some optimizations particularly suited to the irregular architectures, allowing SDCC to generate efficient code for the Z80. Hardware similarities between the ColecoVision and the Sega 8-bit systems allow to target both in game development, which is supported by cross-platform libraries.

The ColecoVision and Sega Master System are popular video game systems of the 1980s. When they appeared, game development was limited to commercial developers and usually done in assembler only. Today, free tools allow anyone to develop for them with relative ease, also in C. The ColecoVision and Sega Master System are popular video game systems of the 1980s. The central part of the free toolchain is the Small Device C Compiler (SDCC), which features some optimizations particularly suited to the irregular architectures, allowing SDCC to generate efficient code for the Z80 (and some other architectures relevant to retrogaming, such as the CPU used in the Game Boy). SDCC is also used as part of larger retrocomputing / -gaming tools, such as z88dk and 8bit Workshop. Besides all being Z80-based, the ColecoVision and the Sega 8-bit systems also use similar graphics and sound chips. This allows to target the simultaneously in game development, which is supported by cross-platform libraries. Some new games have been released on cartridges. Sometimes cartridges from old games are reused, but there are also new PCB designs and newly made game cartridges.

The STM8 is STMicroelectronics' current 8-bit microcontroller architecture. In recent years a free toolchain or the STM8 architecture appeared and quickly surpassed the non-free ones. The central part of the toolchain is the Small Device C Compiler (SDCC), which features some optimizations suited to highly irregular architectures such as the STM8 The talk introduces all parts of the free toolchain for the STM8 and also discusses remaining gaps.

The STM8 is a popular 8-bit architecture by ST Microelectronics commonly used in household electronics, automotive application and industrial controls. For quite a while there were no free tools, and the irregular architecture makes it hard to support in GCC or LLVM. In recent years free tools for it started to appear and now form a free toolchain that surpassed preexisting non-free ones. The most important part is the Small Device C Compiler (SDCC). New tree-decomposition-based algorithms from recent compiler research have been implemented in SDCC, including a new register allocator particularly suited to irregular architectures with few registers. SDCC quickly surpassed the non-free compiler in standard compliance and OS support and generates substantially faster integer code. Programs can be flashed by stcgal (via a serial link on STM8 devices that have a bootloader) and stm8flash (via the SWIM interface of ST-LINK hardware). OpenOCD and GDB allow on-target debugging via the ST-LINK. IDEs complete the development environment. However, stcgal still needs non-free binary blobs for use with some devices and the ST-LINK has non-free firmware. SDCC still falls short in floating-point performance. While there are some ports of free RTOSes that use the free toolchain for the STM8, more would be desirable.

SDCC is a standard (ANSI C89, ISO C90, ISO C99, ISO C11) C compiler targeting 8-bit architectures (MCS51, DS80C390, Z80, Z180, Rabbit 2000, Rabbit 3000A, GBZ80, TLCS-90, HC08, S08, STM8, PIC). It works on many host systems (Linux, Windows, Mac OS, Solaris, NetBSD, FreeBSD, ...). The talk starts with a view on SDCC from a user perspective - standard compliance, targets. There is a short interlude on how the SDCC project is set up. The second part is about some of SDCC's unusual optimizations currently not found in any other compiler, in particular an optimal register allocator and bytewise register allocation.

The presentation will be held jointly with sdcc developer Maarten Brock.