Jump to content
Sign in to follow this  
Mefistotelis

Integer formats in VI files

Recommended Posts

What could there be to discuss in Integer formats?

There's the 32-bit signed integer, many today applications use only this one and all is ok. Though LV is a programming environment, so it should have more types - 8-bit, 16-bit and 32-bit signed and unsigned variations. And today it would be hard to skip 64-bit, also signed and unsigned. So we have 8 types, and that's it, right?

Those are the types allowed for user code; and they are also used by binary formats of LV, but that's definitely not all.

Every engineer knows the binary-coded-decimal format - and of course it is used as well. Version numbers are always stored with it, ie 0x1400 means version 14, not 20.

What else? Well, imagine you have a software from 16-bit era and want to make it compatible with modern standards. You previously used the minimal amount of bits to store various counts. But today, the 8-bit and 16-bit values may not be enough to store huge multidimensional arrays, counts and sizes. What do you do? You may just replace the values with longer ones, breaking compatibility. But - you may also invent a new, variable size integer format. Even many of them.

And here they come:

- a 16-bit value, in which if the highest bit is set, then another 16-bit value should be read and the highest bit is toggled off. Nice solution, but we do not get full 32-bit range.

- a 16-bit signed value, but if it is -0x8000 - then the real value is stored in following 32-bit area. So it may take 6 bytes, but at least allows full 32-bit range.

- a 8-bit value, but if it is 255 - then the real value is stored in following 16-bit area, and if it is 254 - real value follows in 32-bit area. So has either 1, 3 or 5 bytes, and allows full 32-bit range. For some reason, 64-bit values are not supported (at least not in VL14 - I didn't looked at newer versions). There is also signed variation.

In total, we now have 13 different ways of storing integer.

Now, most of the values are stored in big endian format, but single ones here and there are left in little endian. So we do need to include endianness as well. Though I didn't found any of the variable size values in little endian. Yet.

 

In case you're interested in how exactly the variable size values are stored, here is my implementation of reading and creating these:

https://github.com/mefistotelis/pylabview/blob/master/LVmisc.py

 

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Sign in to follow this  

×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.