[ggchappell]

I agree that this clever and useful, but I get the feeling that it could have been more so.

I haven't done much assembly in a while, but I was heavily into it once upon a time, and I recall that values with lots of 1s were useful. There is not quick way to generate those here. This means that we can write a single instruction to set any single bit using an inclusive OR and the proper immediate value, but we cannot write a single instruction to clear any single bit.

The reason I think a bit more cleverness might have helped is that there are so many values with multiple encodings. Anything where the 8-bit value ends with 0 has a different encoding as well. For example, a rotation of 0000 and an 8-bit value of 00000100 gives the same result as a rotation of 1111 and an 8-bit value of 00000001 (right?). Perhaps some of the redundant instructions could have been used to represent things ending in lots of 1s?

Regardless, an interesting and informative post. :-)

[eckzow]

Clearing an arbitrary bit actually is supported (as mentioned in the article: "you can set, clear, or toggle any bit with one instruction").

The specific details require you to dig a bit past explaining just the immediate encoding, but in the clearing case there's a dedicated instruction for clearing the bit specified by the immediate:

  BIC - Bit Clear (immediate) performs a bitwise AND
  of a register value and the complement of an immediate
  value, and writes the result to the destination register.

As I mentioned in my other post, zero-rotation encodings are gamed out as well (to allow byte repetition).

[ggchappell]

> BIC - Bit Clear (immediate)

Ah, didn't catch that.

[tlb]

You can get trailing ones by subtracting 1:

0x01000000 - 1 = 0x00ffffff

so you can generate any number of trailing ones with 2 instructions.