Passing params in PASM

This example shows you how to pass a start address for a set of paramaters to your new Cog as the boot parameter. You can then read each paramater, as a long or a byte, by incrementing the supplied Address and using the rdlong and rdbyte instructions to retrieve them from the Hub.

This is a pretty standard way to bootstrap your Cog with all the information it needs to do its job, and is also a handy way to provide arrays or single memory locations where the Cog can deposit the result of its work.

For example, you could define a LENGTH var, followed by a LED_PATTERN var and produce code in PASM which reads each byte from LED_PATTERN in turn and flashes the LEDs. We'll probably cover something like this later, but for now here's a simple example:

  _CLKMODE = xtal1 + pll16x
  _XINFREQ = 6_000_000

  long MY_PIN_1, MY_PIN_2     ' These will appear next to each other in memory

PUB main
  MY_PIN_1 := |< 0            ' Set our first pin to a bitmask for A0
  MY_PIN_2 := |< 1            ' And our second for A1
  cognew(@blink, @MY_PIN_1)   ' Supply the address of the first parameter  

        org         0
blink   mov         addr,       par    ' Load our address from the boot param
        rdlong      Pn,         addr   ' Read the first pin number
        add         addr,       #4     ' Increment addr by 1 long ( 4 bytes )
        rdlong      Pn2,        addr   ' Read the second pin number

        or          dira,       Pn     ' Set up pins as outputs
        or          dira,       Pn2    ' It's good to do stuff while we're waiting
                                       ' for the Hub to come back around, since we
                                       ' only just synced for a rdbyte!

        rdlong      Delay,      #0     ' Prepare the delay
        mov         Time,       cnt    ' Prep the wait time
        add         Time,       #9     ' Add the minimum wait time

:loop   waitcnt     Time,       Delay  ' Start blinking!
        xor         outa,       Pn
        xor         outa,       Pn2
        jmp         #:loop

Pn      res         1                  ' Store our first LED pin
Pn2     res         1                  ' Store our second LED pin
Addr    res         1                  ' Store our parameter address
Delay   res         1                  ' Store the delay increment
Time    res         1                  ' Store the current time for wait

Breaking it down

I'm going to assume you've read My First Pasm and skim over much of the basics.

long MY_PIN_1, MY_PIN_2

In our PASM code, we're declaring two variables next to each other. These will appear adjacent to each other in HUB memory as two sets of 4 bytes, or 2 longs. These are the longs we want to copy to our COG!

MY_PIN_1 := |< 0
MY_PIN_2 := |< 1

In our main method, we're setting the values of MY_PIN_1 and MY_PIN_2 to be bit masks, by using our old friend the bitwise decode operator |<. Practically speaking, this will convert 0 to %0 and 1 to %10 etc.

We use bit masks because these can be or'd against our output registers to flip those bits to 1. Or xord to toggle them.

It's worth noting that in SPIN and PASM you use % to denote a binary number, like %10101010 and $ to denote a hexadecimal number like $AA.

mov addr, par

The very first line of our PASM ( remember that "blink" in this case is a label, and not part of the instruction ) deals with the par register. This register always starts off containing the parameter that's passed into it via cognew. In this case we've passed it the Hub address of MY_PIN_1, and it can use this address to locate that value in the Hub when it wants to read it.

rdlong      Pn,         addr

The second line executes what's known as a HUB instruction. The instruction rdlong waits for the HUB to come around, then reads a long ( 4 bytes ) into the target location. In this instance Pn is our memory location within the COG and is where we want to copy the value of MY_PIN_1.

add         addr,       #4

Now that we've read one long from HUB memory, we want to advance to the next one. Addresses are byte-aligned, which means we can read one byte at a time. Since we want the next 4 bytes, and not just the last 3 of the previous long plus the first of the next long, we'll increment our addr register by 4. 4 bytes = 1 long.

rdlong      Pn2,        addr

Now we've got a pointer to MY_PIN_2, which is a long in memory right next to MY_PIN_1 we can use the rdlong instruction to read it into Pn2.

Hooray. We've successfully copied two setting values from HUB memory into our COG!

or          dira,       Pn

In this next line we're simply oring the value of Pn against dira- this sets it as an output. The bit that's set in the Pn mask will flip to 1 in the output register. For example 0000 or 0100 = 0100.

Everything else you should remember from My First Pasm, we set up a loop using waitcnt and jmp and use xor ( 1 xor 1 = 0, 0 xor 1 = 1 ) to blink the LEDs.

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Phil Howard
Phil is Pimoroni's software guru, instantly recognisable by his somewhat pirate-themed moustache growing attempts. Usually found buried neck deep in Python libraries, he's also been known to escape on occasion and turn out crazy new products. If you need a helping hand, he's a prolific Twitter user and rampages around the forums like a T-Rex. Ask him if you need help with Pimoroni's software libraries, or Propeller HAT.