Inline Assembler

This page shows you how you can write assembly code and have it assembled by the Web IDE and uploaded as a function. However there are other ways to get your code running fast! Please check out the Performance page for more information!

The Web IDE allows you to write inline assembler in the right-hand pane. You can use a series of normal JS Strings:

var adder = E.asm("int(int)",
"movs    r1, #3",
"adds    r0, r0, r1", // add two 32 bit values
"bx  lr");            // return

Or can use JavaScripts templated strings to avoid having to cope with newlines:

var adder = E.asm("int(int)",`
movs    r1, #3
adds    r0, r0, r1 // add two 32 bit values
bx  lr             // return

Which can then be used like a normal function:


This is handled as follows:

  • When you click Send to Espruino the Web IDE pulls out the E.asm call
  • It runs the strings you supplied through its assembler (which doesn't support all the ARM's opCodes yet)
  • It then creates code to load the assembler into Espruino as a String
  • It uses E.nativeCall to create a JavaScript function using the code that was generated.


  • This only works on ARM-based microcontrollers - ARM Thumb assembler will not work on ESP8266/ESP32.
  • The assembler is only partially implemented so will only parse some opcodes at the moment. If you find something missing please let us know!
  • If this is a bit hardcore for you, there are other options that allow you to write JS or C - please check out the Performance page for more information!

For an ARM Thumb reference, see this link

Argument Specifiers / Call Signature

The first argument of E.asm (and E.nativeCall) is the signature of the call, of the type:

returnType (argType1,argType2,...)

Allowed types are void, bool, int, double, Pin (a pin number), JsVar (a pointer to a JsVar structure).

Note: due to restrictions inside Espruino, you cannot have more than 5 arguments to the function.

Calling Convention (what you can use!)

For a better explanation of this, see Wikipedia

The registers r0, r1, r2 and r3 contain the first 4 32 bit arguments, and r0 is used to return the 32 bit result. Other arguments are passed on the stack (which we're not covering here).

Registers r0-r3 are free to use - however any other registers have to be restored to their previous values before the function returns to the caller. This is usually done using push {r4,r5,r6,...} and pop {r4,r5,r6,...}.

Accessing Data - iterating

By itself, the assembler isn't too useful. What you need is to be able to access your data. Luckily Espruino makes it pretty easy. Above, we used to call assembler for every element in an array - you can use this on ArrayBuffers like Uint8Array too. This is not part of the EcmaScript 5 spec (but is in EcmaScript 6).

>var a = new Uint8Array([1,2,3,4,5]);

However this will still return an ArrayBuffer (which will use up more RAM). If you don't want to return anything (maybe you're writing it out to GPIO (see below) ), use forEach:


Or you can use Array.reduce to pass an argument between calls to the assembler, for instance to sum all the items in the array:

// create and fill up array buffer
var a = new Int16Array(100);
for (var i in a) a[i]=i;

// effectively this is 'function (a,b) { return a+b; }'
var adder = E.asm("int(int,int)", `
  adds    r0, r0, r1
  bx  lr

// Call our assembler on every item and return the result
var sum = a.reduce(adder);

// prints 4950

Accessing Data - directly

When Espruino allocates data in a 'flat string' (a contiguous area of memory) you can access it by grabbing its address with E.getAddressOf and passing it into your assembler code:

// Write 1,2,3,4 into the address pointed to by the first argument
var setData = E.asm("void(int)",`
movw    r1, #1
strb    r1, [r0, #0]
movw    r1, #2
strb    r1, [r0, #1]
movw    r1, #3
strb    r1, [r0, #2]
movw    r1, #4
strb    r1, [r0, #3]
bx  lr

// Allocate a flat string of 10 bytes
var NUM_BYTES = 10;
var flat_str = E.toString({data : 0, count : NUM_BYTES});
// wrap it in a Uint8Array
var arr = new Uint8Array(E.toArrayBuffer(flat_str));
// Get the address in RAM of the actual flat string
var addr = E.getAddressOf(flat_str, true);
// Print array contents
console.log("Before : ",arr.join(","));
// Modify array contents
// Print them again
console.log("After ",arr.join(","));


In ARM Thumb, you can't store full 32 bit literal values in assembler, so loading big constants is a bit harder than you'd expect. Instead of directly specifying the constant, you must define an area of memory that will contain it, and then you must reference that area (relative to the current instruction). This is even more painful because the program counter is 4 bytes ahead of current execution:

var getConst = E.asm("int()", `
  ldr    r0, [pc, #0]   // 2*2 - 4 = 0
  bx lr
.word    0x1234BEEF


In reality, you'll want to use labels and let the assembler sort this out for you:

var getConst = E.asm("int()", `
  ldr    r0, my_data
  bx lr
  .word    0x1234BEEF


Even this can cause some problems. You can only access an address that is a multiple of 4 bytes ahead of the current instruction. If you get an error when assembling such as Invalid number 'mylabel' - must be between 0 and 1020 and a multiple of 4 then you'll need to pad out the constants with a nop:

var getConst = E.asm("int()",`
  ldr    r1, my_data
  mov  r0,r1        // extra instruction makes non-2 aligned
  bx lr
  nop               // must pad
  .word    0x1234BEEF


Note: If you just need a small constant then you may be ok. You can use mov to load a value between 0 and 255:

var getConst = E.asm("int()", `
  mov    r0, #254
  bx lr

Or you can use movw to load a 16 bit values (between 0 and 65535) but movw is a double-length instruction that takes 4 bytes in total.

var getConst = E.asm("int()", `
  movw    r0, #65535
  bx lr

Storing Data

While ARM Thumb has a LDR instruction that will load from a label, there is no such thing for a store. Instead, you need to use the ADR pseudo-instruction to store the address in a register which you can then use in the store instruction.

For example the following section of code will return a value that increments after each call:

var inc = E.asm("int()",`
  adr    r1, data // Get address of 'data'
  ldr    r0, [r1] // Load the value of data into R0
  add    r0, #1   // Add one to it
  str    r0, [r1] // Save the value of R0 into 'data'
  bx lr           // Return (R0 is the value)
data:             // ... padding
  .word    0x0    // the word that we'll increment

Accessing IO

You can write directly to the hardware in order to perform IO very quickly. However how you do this depends on the CPU on your board...

STM32F1 (original Espruino Board)

Useful docs are:

GPIOA's Output data register is 0x4001080C (which sets ALL pins on that port). To set individual pins you can write to BSRR = 0x40010810 and to clear them you can write to BRR = 0x40010814

So you could write the following code to give the 3 LEDs (on A13,A14 and A15) a quick pulse.

digitalWrite([LED1,LED2,LED3],0); // set up the output state (easier done in JS!)

var pulse = E.asm("void()",`
  ldr    r2, gpioa_addr  // Get the gpio address
  movw    r3, #57344    // the bit mask for A13,A14,A15 - 0b1110000000000000 = 57344
  str    r3, [r2, #0]    // set *0x40010810 = 57344 (set pins A13-A15)
  str    r3, [r2, #4]    // set *0x40010814 = 57344 (clear pins A13-A15)
  bx    lr              // return
  .word    0x40010810
// Our data


STM32F4 (Espruino Pico)

Useful docs are:

GPIOB's Output data register is 0x40020414 (GPIOA is 0x40020014). Writing to that address sets ALL pins on that port. To set individual pins you can write to the lower 16 bits of BSRR = 0x40020418 and to clear them you can write to the upper 16 bits.

So you could write the following code to give LED1 a quick pulse (it's on pin B2).

digitalWrite(LED1,0); // set up the output state (easier done in JS!)

var pulse = E.asm("void()",`
  ldr  r2,gpiob_addr
  movw  r0,#4    // 1<<2 = pin 2 on the port
  lsl r1,r0,#16  // shift it left by 16 for the reset register
  str  r0,[r2]   // Turn on
  str  r1,[r2]   // Turn off
  bx  lr
  .word  0x40020418


You can do loops as follows. This example adds together all the numbers below and including the current one:

var a = E.asm("int(int,int)", `
  adds   r0, r0, r1
  sub    r1, r1, #1
  cmp    r1, #0
  bgt    loopStart
  bx  lr

for (var i=1;i<10;i++)
  console.log(i, a(0,i));

// 1 1
// 2 3
// 3 6
// 4 10
// 5 15
// 6 21
// 7 28
// 8 36
// 9 45


  • This example will crash Espruino for any number less than or equal to zero
  • Labels take some of the pain out of this. bgt loopStart is actually equivalent to bgt #-10: The program counter is always 4 bytes ahead of the current instruction, and instructions are (usually!) 2 bytes long. That means that to get back to the exact same instruction you must use -4 and you must subtract another 2 for each instruction you want to jump over (hence -10).


As of Espruino 1v72, setWatch can now call native code from within the interrupt - which is much faster than if the code was executed from the event loop.

For instance the following code measures the number of state changes every second on BTN:

// inc function from above
var inc = E.asm("void()",`
  adr    r1, data
  ldr    r0, [r1]
  add    r0, #1
  str    r0, [r1]
  bx lr
  .word    0x0
var dataPtr = ASM_BASE-4; // the address of 'data'

// Now call inc when the button is pressed - in an IRQ
setWatch(inc, BTN, { irq:true });

// every second...
setInterval(function() {
  console.log(peek32(dataPtr)); // print the value of the counter
  poke32(dataPtr,0); // reset the counter
}, 1000);

In order to be properly useful you'll probably want to access the current time:

  • Low precision 16 bit (32/40 kHz) on Espruino (F103): RTC DIVL (0x40002814)
  • Low precision 16 bit (32 kHz) on Espruino Pico (F401): RTC_SSR (0x40002828)
  • Higher precision 32 bit (72/80 Mhz) use SYSTICK (0xE000E018), which stops when the device sleeps.

Compiling C Code

You can also inline compile C code by using the Inline C Compiler.

Another way to compile C code is to use the toolchain that will compile Espruino:

arm-none-eabi-gcc -mlittle-endian -mthumb -mcpu=cortex-m3  -mfix-cortex-m3-ldrd  -mthumb-interwork -mfloat-abi=soft -nostdinc -nostdlib -c test.c -o test.o
# you can also add -O3 to the command-line to optimise the code

And you can dump the contents of the object file with:

arm-none-eabi-objdump -S test.o

You might get something like:

00000010 <pulse>:
  10:    4a02          ldr    r2, [pc, #8]    ; (1c <pulse+0xc>)
  12:    f44f 4360     mov.w    r3, #57344    ; 0xe000
  16:    6013          str    r3, [r2, #0]
  18:    6053          str    r3, [r2, #4]
  1a:    4770          bx    lr
  1c:    40010810     .word    0x40010810

Which you can then turn into:

var ASM_BASE=process.memory().stackEndAddress;
var ASM_BASE1=ASM_BASE+1/*thumb*/;
[0x4a02,0xf44f,0x4360,0x6013,0x6053,0x4770,0x0810,0x4001].forEach(function(v) { poke16((ASM_BASE+=2)-2,v); });
var pulse = E.nativeCall(ASM_BASE1, "void()")

Note: The best method is now to convert the raw opcodes to a base64 encoded string, and to then use the following:

var myFn = E.nativeCall(1, "void ()", atob(""))

This will save the program code into Espruino's variable storage, so save() will store it along with everything else. Note that we're passing 1 as the offset, because it is Thumb assembly.

This page is auto-generated from GitHub. If you see any mistakes or have suggestions, please let us know.