Move (output instruction)
MVM: Masked Move
Moves only the bits selected by a mask, leaving the rest of the destination untouched.
Also called: masked move, move through a mask.
MVM(number, a, b)Arguments
| Name | What it is | Accepts | Default |
|---|---|---|---|
| destination | What the instruction acts on | a register | required |
| a | Source | a number or a tag | 0 |
| b | Mask | a number or a tag | -1 |
Arguments are positional and this table is generated from the simulator's own instruction registry, so it always matches what the engine accepts.
What it does
MOV overwrites the whole destination. MVM overwrites only the bits the mask selects and leaves every other bit of the destination exactly as it was.
That matters whenever a register has more than one owner. A drive command word where your program owns the lowest four bits and the HMI owns the rest is the everyday case: a MOV would write your four bits and wipe theirs in the same instruction, and the fault shows up as the panel mysteriously losing settings whenever the machine starts.
The mask is read as bits, so a mask of 15 is the lowest four bits (1 plus 2 plus 4 plus 8) and a mask of 256 is bit 8 on its own. Everything the mask does not select comes through from the old value of the destination.
A rung you can run
Write your bits into a word somebody else also writes to
ONS(Panel_Sets_Bit)MOV(Drive_Command,256)
ONS(Run_PB)MVM(Drive_Command,3,15)
ONS(Stop_PB)MVM(Drive_Command,0,15)Press Panel_Sets_Bit first: that puts 256 in the word, standing in for a bit the HMI owns. Now press Run_PB and Stop_PB and watch the low four bits change between 3 and 0 while 256 stays where it is. Replace either MVM with a MOV and the panel's bit is gone the first time you press a button.
Run this rung in the simulatorWhat it is called on other controllers
| Platform | Equivalent |
|---|---|
| Rockwell (Studio 5000) | MVM |
| Siemens (TIA Portal) | No single instruction: AND the destination with the inverse mask, then OR the new bits in |
| Mitsubishi (GX Works) | No single instruction: WAND and WOR in sequence |
| CODESYS and IEC 61131-3 | No single instruction: (dest AND NOT mask) OR (source AND mask) |
Common mistakes
- Leaving the mask at all bits. MVM with a mask of -1 selects every bit and is then simply a slower MOV, which quietly removes the protection you added the instruction for.
- Expecting the unselected bits to be cleared. They are not, and that is the whole point: they keep whatever the destination already held. If you want them zeroed, clear the register first.
Practise it
Interactive exercises that use MVM. Each one checks your ladder against a real scan.