Porting uLisp to a new platform

Here are some instructions for porting uLisp to a new platform that it doesn't currently support.


31st January 2022: Updated this description to reflect changes to the latest version of uLisp, 4.0/4.1.


To port uLisp to a new platform it needs to satisfy the following requirements:

  • At least 2 Kbytes of RAM.
  • At least 32 Kbytes of program memory; probably more for 32-bit platforms.
  • A core for the Arduino IDE.
  • Support for serial via the Arduino IDE Serial Monitor.

Check that you can upload and run the Blink sketch from File->Examples->01.Basics in the Arduino IDE.

Select the most appropriate starting point

Choose an existing version of uLisp that's close to the platform that you're planning to support. Currently the choice is:

  • AVR uLisp: for 16-bit AVR processors. No floating-point or array support. Images are saved in the on-chip EEPROM on ATmega328P, or flash on ATmega1284P or AVR128DA/DB.
  • ARM uLisp: for 32-bit ARM chips. Images are saved to DataFlash if available, or to flash on M0.
  • ESP uLisp: for 32-bit ESP8266 and ESP32 platforms. Images are saved to flash using EEPROM emulation. Includes Wi-Fi extensions.
  • RISC-V uLisp: for 32/64-bit RISC-V chips.

Specify the workspace

The parameters for each board supported by a version of uLisp are defined by a set of #define statements in the uLisp source beginning:

// Workspace

For example:

  #define WORKSPACESIZE (1060-SDSIZE)     /* Objects (4*bytes) */
  #define EEPROMSIZE 256                  /* Bytes */
  #define STACKDIFF 160
  #define CPU_ATmega4809

Add a section to the list of supported platforms; for example:

#elif defined(MY_PLATFORM)
  #define WORKSPACESIZE 3072              /* Cells (8*bytes) */

Identify the platform

MY_PLATFORM should be the #define used by the Arduino IDE to identify the platform. You can find this as follows:

Locate the Arduino core for your platform. This will be in a directory such as:


In the package for your board locate the boards.txt file.

Locate the section of definitions for your board, and then locate the line build.board; for example:


The symbol for your board is then got by appending ARDUINO_ onto the front of this; for example:


If in doubt invent a name, and put a:


at the start of the source file.

Define the workspace size

Add a line:

#define WORKSPACESIZE 3072

to specify the number of Lisp objects to allocate. On 16-bit platforms each object is 4 bytes, and on 32-bit platforms each object is 8 bytes.

On some platforms the Arduino core displays a warning if you use more than 75% of the RAM, so the best approach is to give a small value at first, and then increase it until you get the low-memory warning. Alternatively, increase it until you get erratic behaviour due to not enough stack space.

Specify the stack difference

Some versions of uLisp give a stack overflow error, end exit gracefully, when the stack is used up. The parameter STACKDIFF is used to fine-tune this:

#define STACKDIFF 160

It specifies how much stack should be available before the error is displayed. If in doubt set this to 0

Specify the CPU

This is optional, but the CPU symbol is used later in the uLisp source to select features specific to a particular CPU.

#define CPU_ATmega4809

At this point you should be able to compile the uLisp source and see what errors you get.

Specify the number of available serial ports

If you don't do this you will get an error such as:

'serial1read' was not declared in this scope

The simplest thing at this stage is to specify that there is only one serial port; you can add support for Serial1, Serial2, etc later if they are available.

Locate the section in the uLisp source beginning:

// Streams

In the definition of gstreamfun() edit this section as follows:

  else if (streamtype == SERIALSTREAM) {
    if (address == 0) gfun = gserial;
    #if !defined(MY_PLATFORM)
    else if (address == 1) gfun = serial1read;

Add a #if to the definition of serial1write() as follows:

#if !defined(MY_PLATFORM)
inline void serial1write (char c) { Serial1.write(c); }

In the definition of pgstreamfun() edit this section as follows:

  else if (streamtype == SERIALSTREAM) {
    if (address == 0) pfun = pserial;
    #if !defined(MY_PLATFORM)
    else if (address == 1) pfun = serial1write;

You should now be able to compile uLisp without errors.

Open the Serial Monitor and check that you get a uLisp prompt. Try typing an input and check that an appropriate response is echoed.

This is the first step; there's more you'll need to do to provide support for all uLisp's features on your platform.

To check that uLisp is working correctly try some of the benchmarks at: