Packing a string of digits into an integer quickly

It was a fairly common thing to do very similar operations on Apple ][ computers.

6502 had a BCD (binary coded decimal) mode; basically when in BCD mode arithmetic operators carried at 0x9 rather than 0xF, and operations worked nibble-wise rather than byte-wise. So it was common to represent 2 digits in one bye regardless of hex vs decimal.

I actually have been thinking about this lately because of the other half of the problem, ie ASCII <—> native types. On the Apple 2 one really cool speed trick was to store strings directly on the screen. The Apple 2 text screen was memory mapped (0x400 - 0x7FF IIRC). To write strings to the screen in assembly, you wrote the ASCII character codes to the right memory locations. But if you were trying to eke out every single cycle you could, and your strings were fixed length, you didn’t need to store them somewhere and copy them to the screen; you could use the screen as the store for your strings.

I once made a stupid program that counted to one million this way; it ran fast enough to watch it finish.

I don't understand how this can work: there are 15 (or 7 :-) other characters with the same bottom four bits as each digit. How does it distinguish between, say, '4' and 't'? Space and '0' have the same lower four bits: \0!

The BEAM VM (Erlang/Elixir) recently optimized a related procedure as well https://github.com/erlang/otp/pull/7426

Anyone know an efficient way of zipping/interleaving two bitmasks on Intel? Assume both bitmasks are in their own mask registers, and the top halves and bottom halves can be zipped into separate output registers. My current solution uses the pdep instruction.

How often these days strings are arrays of characters terminated with '\0' instead of UTF16?

Can’t you just search through and remove all the non-digit characters? (Excel user here)