offsetlen queries

[eekee]

Documentation for the offsetlen instruction left me unsure whether it shifts the source data or not. testoffsetlen.tasm was uninstructive as its test data is uniform. I assumed it shifts because of the "offset" in the name, but output from my tests seems to indicate it truncates. I'm wondering, is it meant to shift, or shall I correct the documentation?

here's my intended documentation:

"offsetlen"
-0-++0
defines a masking window, only trits visible 
through the window are written to the destination.

usage:
	offsetlen|A,B,C
if A is "off", the remaining trits in the destination are preserved.
if it is "on", they are overwritten with 0.
B is the offset for the window's low-value edge (the right side).
C is the size of the window, the length extending toward high values.
affected operations are read and write to IO and memory.
the window remains active until reset with "offsetlen|off,0,9".
see also: romread* setdata IOread* IOwrite*

test code:
https://github.com/eekee/SBCTVM_Mk2_VMUSER/blob/master/offsetlen.tasm
https://github.com/eekee/SBCTVM_Mk2_VMUSER/blob/master/offsetlen.tasm

[ThomasTheSpaceFox]

The users manual has information on this on page 28

so essentially what it does is act like a sort of mask.

i.e. if i have a value in ram, say: +0+---++0
and i set offsetlength as such:
tasm: offsetlen|off,1,1
and i have register 1 set to ---------
then i copy that memory value to register 1 and we now have register 1 as:
-------+-

here is a gist with a short bit of tasm code used to verify the example:
https://gist.github.com/ThomasTheSpaceFox/0e623cb5b799cdf988705230a39b851d

does that make any sense.