A Cortex-M native code Forth along with an RTOS

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zeptoforth is a bare-metal Cortex-M Forth which includes an preemptively-multitasking, priority-scheduled RTOS. It currently supports the Raspberry Pi Pico (and other RP2040-based boards with Winbond Quad SPI flash), STM32L476, STM32F407, and STM32F746 DISCOVERY boards, and STM32F411 "Black Pill" boards, but work is planned on porting it to other boards in the near future.

zeptoforth is a portable subroutine-threaded / native code inlining Forth for Cortex-M0+/M4/M7 microcontrollers which includes a preemptively multitasking RTOS designed to be able to compile to and run from both flash and RAM (the kernel of course exists in flash).

The library of code coming with zeptoforth includes support for the following:

  • Lambda expressions
  • Double-cell and fixed-point arithmetic, including the usual mathematical functions
  • SysTick
  • Interrupt-driven serial IO drivers
  • A simple GPIO abstraction layer that is maximally uniform across supported platforms
  • GPIO and, where applicable, EXTI drivers
  • Preemptive multitasking
  • Action scheduling including synchronous messaging between actions on single tasks
  • A disassembler
  • Moving the exception vector table into RAM so it can be arbitrarily set
  • Task notifications
  • Semaphores
  • Locks
  • Message-oriented channels
  • Message-oriented rendezvous channels (aka "fchannels")
  • Message-oriented synchronous bidirectional reply channels (aka "rchannels")
  • Message-oriented ISR-friendly channels (aka "schannels")
  • Byte-oriented streams
  • Maps, including counted string and integer-keyed maps
  • Temporary buffers
  • An allocator
  • Memory pools
  • Task pools
  • Action pools (for the single-task event scheduler)
  • A line editor
  • LED drivers
  • Random number generator drivers (except on STM32F411 "Black Pill" boards)
  • Pseudorandom number generation support (using the TinyMT32 PRNG)
  • swdcom support for non-UART-based terminal support

On the Raspberry Pi Pico (and other compatible RP2040 boards) it also supports the following:

  • Programmable input/output (PIO)
  • Multicore execution; note that this can be combined with multitasking on each core and multitasking constructs can be shared by both cores

Note that the random number generator is not supported on the RP2040.

On the STM32F746 DISCOVERY board and the Raspberry Pi Pico (and other compatible RP2040 boards) it also supports the following:

  • Quad SPI Flash memory
  • A block interface
  • A block editor


This patch-level release fixes an issue with the ADC on the Raspberry Pi Pico where an incorrect ADC channel was being used by adc@ .

gzip - 6.38 MB - 06/20/2022 at 03:12


  • 1 × A zeptoforth binary, whether just a kernel or one containing compiled Forth code (the latter is highly recommended)
  • 1 × A compatible board, such as the STM32L476 DISCOVERY, STM32F407 DISCOVERY, STM32F746 DISCOVERY, or Raspberry Pi Pico boards
  • 1 × A means to flash said board (e.g. ST-Link, or in the case of the DISCOVERY boards, a USB to USB Mini cable); in the case of the
  • 1 × A means to communicate with said board over serial or ST-Link (to enable the use of swdcom)

  • Patch-level release

    Travis Bemann6 天前 0 comments

    This patch-level release fixes an issue with the ADC on the Raspberry Pi Pico where an incorrect ADC channel was being used by adc@ .

  • Patch-level release 0.42.1

    Travis Bemann6 天前 0 comments

    This patch-level release adds the ability to set the sampling time for ADC channels on the STM32 platforms, which is necessary for some ADC channels such as the internal temperature sensor to function properly. It also fixes a significant bug where two ADC channels, one of which was unconfigured, were being read on STM32 platforms by adc@ rather than one, which was causing garbage values to be generated on the STM32L476 in particular (as on the other STM32 platforms the extra value seems to have gotten discarded).

  • Patch-level release

    Travis Bemann06/19/2022 at 04:37 0 comments

    This patch-level release fixes a bug with the ADC for the stm32f411 platform where the temperature sensor and Vrefint ADC channels were enabled at the same time as the Vbat ADC channel by default, which is not permitted per design, and no means were provided to change this without manually modifying registers.

  • Release 0.42.0

    Travis Bemann06/18/2022 at 22:13 0 comments

    This release adds one-shot ADC support for the supported platforms. It also adds an optimization to compilation for the RP2040 (Raspberry Pi Pico). Note that there is a notable erratum for the STM32F407, STM32F411, and STM32F746, where when at least reading the temperature value, the first value reported by adc@ needs to be discarded; the reason for this is not clear so far.

  • Release 0.41.0

    Travis Bemann06/16/2022 at 01:59 0 comments

    This release adds UART support beyond mere console support with the uart module to all of the supported platforms, with the sole exception being that LPUART1 on the STM32L476 is not currently supported. Note that all of the supported UART's can have their baud rates changed, and can be accessed even when the console is set to swdcom.

    This release also adds unofficial support for STM32F411 Nucleo 64 boards, which is activated by installing an STM32F411 "Black Pill" full, mini, full_swdcom, or mini_swdcom binary, rebooting, and then executing the following with a serial terminal set to 38400 (note that the baud is significant; the baud is not the default 115200 baud used by zeptoforth) or, if you installed an swdcom build, with swdcom:

    true constant platform-nucleo64

    After this the serial console will switch to 115200 baud, so one will need to close one's serial terminal and reopen it set to that baud unless you are using swdcom. Also, after this, the led module will be configured for use with the STM32F411 Nucleo 64. Note that another effect of making this change, even if you will be only using swdcom and not the serial console, is that the system clock will increase by a factor of 3.125, such that ms and other words that rely on the systick will count time correctly.

  • Release 0.40.0

    Travis Bemann06/11/2022 at 03:18 0 comments

    This release adds an ARMv6-M assembler (which can also be used on ARMv7-M platforms), adds /mod and u/mod for M4 and M7 platforms (they already existed for the RP2040), changes the order of display of .s to make it more standard, and adds ." and .\" at the REPL level (so they do not require one to be in a compilation state).

  • Release 0.39.0

    Travis Bemann06/05/2022 at 03:01 0 comments

    This release adds support for the STM32F411 "Black Pill" board.

  • Release 0.38.0

    Travis Bemann06/04/2022 at 02:25 0 comments

    This release reworks find and find-all so they no longer take masks (which in practice were unnecessary) and exposes looking up words by execution token to the user as find-by-xt.

  • Patch-level release 0.37.2

    Travis Bemann06/02/2022 at 02:34 0 comments

    This patch-level release includes the ability to disable the output of ACK, NAK, and BEL. It also adds the ability to use page up and page down to switch blocks in the block editor.

  • Patch-level release 0.37.1

    Travis Bemann05/30/2022 at 20:23 0 comments

    This patch-level release fixes depth so it does not include the value at the bottom of the data stack (which is a garbage value) and .s so it does not print it, and on the Raspberry Pi Pico restores stack validation in the outer interpreter.

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Enjoy this project?



teraz wrote 05/05/2022 at 10:37 point

please add password for loging (meybe OTP)

password and hardware switch for replace firmware

  Are you sure? yes | no

Travis Bemann wrote 05/06/2022 at 13:51 point

What do you mean? zeptoforth has no logging independent of what it writes to serial. Also, about securing the firmware, zeptoforth has zero built-in security by design, and even if it did, the only way to prevent writing to firmware via SWD on some platforms would be to set bits to lock out SWD, and to my knowledge there is no means to lock out uploading firmware via the USB mass storage device on the Raspberry Pi Pico.

  Are you sure? yes | no

teraz wrote 05/07/2022 at 12:21 point

I see console, i need password protect for no everyone can write in console/terminal

  Are you sure? yes | no

Travis Bemann wrote 05/07/2022 at 16:17 point

If you want to lock out the console on bootup except for someone entering a password I would recommend something like:


: hash ( c-addr u -- x ) ( add your hash algorithm here ) ;

: init ( -- )



        cr ." Password: " refill token ?dup if

            hash [ s" your-password" hash ] literal =

        else drop false then



Replace your-password with the password you intend to use (because it is hashed at compile-time this password will not be in the compiled code in flash). Also replace the hash algorithm with something sane; for testing purposes I used:

: hash ( c-addr u -- x ) 0 0 -rot ?do dup 7 lshift swap 25 rshift or over c@ xor swap 1+ swap loop nip ;

However, that's not secure by any means. (Note that truly using a secure hash rather than just 32 bits of it would require significant modifications of the above code.

Also note that this will only keep out the most basic of attackers, because there is nothing stopping them from simply reflashing the whole board or reaching into the board with SWD (some MCU's have bits for locking out SWD but these are MCU-specific and i am not familiar with the operation of these myself).

Also note that this must be compiled after any other implementations of init or otherwise these will execute (provided they follow the convention of each init calling the init before it before doing its operations) after the password is entered successfully.

  Are you sure? yes | no

teraz wrote 05/19/2022 at 21:18 point

this is not otp

  Are you sure? yes | no

Travis Bemann wrote 05/20/2022 at 01:19 point

Well, anything truly OTP will be very hardware-specific. What I gave there was a general solution to passwording zeptoforth, not a means to permanently lock the user out of the device. Some devices, such as the RPi Pico, have no OTP capability in the first place.

  Are you sure? yes | no

Thomas wrote 04/13/2020 at 08:01 point

I like the story behind your project a lot! Since you're using e4thcom some of Manfred Marlow's approaches to the little Forth I'm maintaining might also be useful for your Forth. Also it's quite possible that my works with minor adaptations :-)

  Are you sure? yes | no

Travis Bemann wrote 04/13/2020 at 12:05 point

The main adaptation that would be needed is that it would need to wait until it receives ACK to send another line of code, and it would need to treat NAK as an indicator of an error indicating to stop ending any more data. This is why with e4thcom it is used in noforth mode, because noforth behaves in this fashion.

  Are you sure? yes | no

Thomas wrote 04/13/2020 at 16:09 point

I remember I had a long discussion with Manfred about ACK/NACK vs. OK/? - he didn't like the solution he made for noforth too much and made a point of using the "human readable" handshake with OK and ? (for error). What's your reason for using ACK/NACK?

  Are you sure? yes | no

Travis Bemann wrote 04/13/2020 at 18:28 point

My reason to do ACK and NAK is that it would signal success versus failure without relying on the textual content of what is output - in zeptoforth an exception can really result in anything being output while ACK and NAK could be transparently added to all prompts and all errors without any further changes being necessary - and there was a premade mode in e4thcom to use ACK and NAK.

(Note that I am responding to this post because is not letting me reply to your other post.)

  Are you sure? yes | no

Thomas wrote 04/16/2020 at 18:55 point

I didn't notice your reply, sorry. Hint: in HaD discussions you can simply reply to the post someone else replied to.

OK, now I understand your preferences. When I started this I first tried to work with an XON/XOFF handshake, at least until I figured the following out about USB-serial converters on Linux: 

  Are you sure? yes | no

Travis Bemann wrote 04/17/2020 at 15:05 point

Personally I would prefer hardware flow control, but unfortunately the USB-serial dongle I am using to communicate with the STM32F407 DISCOVERY board lacks pins for RTS/CTS, and the serial over USB connection built into the STM32L476 DISCOVERY board has no option for hardware flow control in the first place.

  Are you sure? yes | no

Travis Bemann wrote 04/16/2020 at 18:01 point

I looked at the code for and noticed that is coded for Python 2.7, which is no longer maintained. Could you do a port for Python 3? This could be very useful for people who do not run Linux (and thus e4thcom is not a feasible option, as apparently people have not had luck with FreeBSD's Linux compatibility layer with it). (I could add more screen support for also adding XON and XOFF, but screen still would not support loading files from within other files with anything like #include or #require.)

  Are you sure? yes | no

Thomas wrote 04/16/2020 at 18:39 point

Sure I can port it to Python 3 :-) To be honest, when I wrote it I didn't know much Python and 2.7 was the default on the Ubuntu 14.04 I was using then.

Do you know of anyone who uses the e4thcom 64bit binary on WSL 2?

  Are you sure? yes | no

Travis Bemann wrote 04/17/2020 at 14:51 point

I have not heard of anyone trying 64-bit e4thcom with WSL 2, but then, I have not heard of anyone who has used WSL 2 in the first place. Most of the people I know in #forth on freenode are either Linux (and not Linux on top of Windows) or BSD users.

  Are you sure? yes | no

Thomas wrote 04/18/2020 at 08:50 point

I gave it a try. It appears to work but it's not fully tested:

  Are you sure? yes | no

Thomas wrote 04/18/2020 at 13:01 point

I tried telnet to ucSim with the python3 variant - Python3 "byte array vs. str"  issues, of coursed. Unfortunately I get timeouts after fixing it. That's very difficult to debug. The serial interface transfer, which is likely the one you're most interested in, should work anyway.

  Are you sure? yes | no

Travis Bemann wrote 04/19/2020 at 16:19 point

Could you add a copyright notice and license block to so I can incorporate it, with some minor modifications, to the zeptoforth codebase? I should note that there is no such thing as the public domain in Germany.

  Are you sure? yes | no

Thomas wrote 04/19/2020 at 18:24 point

Sure, I'll do that. MIT is OK, I s'pose?

Edit: done

  Are you sure? yes | no

Travis Bemann wrote 04/19/2020 at 18:52 point

That would work, particularly since the MIT license is GPL-compatible.

  Are you sure? yes | no

Travis Bemann wrote 04/19/2020 at 21:41 point

I put my changes to to make it work with zeptoforth in my fork of your gist, and I will include this with zeptoforth so users can upload code without relying on e4thcom.

  Are you sure? yes | no

Elliot Williams wrote 04/13/2020 at 07:41 point

Cool!  Going to have to check this out.  

Lemme see if I have a 407 Disco around here somewhere...

I agree that Forth misses lambdas, just b/c it's hard to think up good names all the time.    

Although on multitasking/timers, honestly, I end up just writing "begin do_something 100 ms again", which works as well, if your "ms" definition has a "yield" in it.  But this keeps the chip from going into a low-power mode, b/c it's always round-robinning into delay statements.  If the scheduler knew...  

So yeah.  I'm going to have a peek at your multitasking setup.

  Are you sure? yes | no

Travis Bemann wrote 04/13/2020 at 12:00 point

I would appreciate more people trying it out, even though that requires their owning the same hardware as myself. (It is hard for me to maintain the code for hardware I do not own, e.g. I cannot create zeptoforth_full binaries without physically owning the hardware in question.)

Lambdas are a feature missing, unfortunately, from most Forths, even though there are a few with them, such as Retro (of course Retro is not a conventional Forth by any means).

The multitasker is designed to put the MCU to sleep when no task is actively executing, even though it wakes up the processor when a SysTick occurs, when a byte is received over a USART, or, if data is pending in the serial TX circular buffer and the USART TX shift register becomes free. So if one writes:

: foo begin do-baz 100 ms again ;

' foo 256 256 256 spawn constant foo-task

foo-task task-enable

it will sleep the MCU, aside from it getting woken up by SysTicks and by any input the user feeds into the REPL.

  Are you sure? yes | no

Travis Bemann wrote 04/18/2020 at 17:37 point

You said you might try it out on the F407. If you had any problems with the zeptoforth_full-[version].bin file for the F407, there were issues with it being corrupt, so try it out with the latest release binaries, where I made sure to check that it now works.

  Are you sure? yes | no

crun wrote 04/12/2020 at 20:44 point

Could you explain why vs mecrisp?

Re M0, you say "it relies on key features of the Thumb-2 instruction set which are missing from the Thumb-2 instruction set". Could you clarify?

  Are you sure? yes | no

Travis Bemann wrote 04/12/2020 at 21:22 point

To be completely honest, it fits much of the same niche as Mecrisp-Stellaris, but there being one Forth for a platform has not stopped anyone from making another (e.g. there already was a Cortex-M Forth before Mecrisp-Stellaris, namely Riscy Pygness). This project exists largely because I wanted to create a bare-metal native code embedded Forth of my own, and the STM32L4 and STM32F4 series microcontrollers were an attractive target, big enough to fit a good-sized Forth with plenty of room to spare, with good documentation for peripherals, and without being effectively a PC on a chip (e.g. the Raspberry Pi boards) with peripherals that may require proprietary binary blobs and thus not amenable to development by an individual such as myself.

About the M0, the issue is that 32-bit literals are difficult to implement without 16-bit low and 16-bit high immediate MOV/MOVT instructions, and while I tried to implement literals that use a mixture of MOVS, LSLS, and ORRS instructions the problem is that when allocating space for a literal ahead of time, and then writing to it later, it is hard to predict how long that sequence of instructions will be (unless one assumes the very longest length and then fills out the rest of the space with NOPs). So as a result I am focusing on Cortex-M3/M4 for now, and will  implement support for the Cortex-M0 later if I see the need for it.

  Are you sure? yes | no

crun wrote 06/10/2020 at 07:35 point

Looking into M4 because of the dsp floating point performance. So 4th for the little F411 dip boards would be ok with me.  Interesting how to integrate Forth with the arm dsp libraries though.

  Are you sure? yes | no

alessandro.cordova wrote 04/11/2020 at 17:43 point


  Are you sure? yes | no

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