Nibble-swap Pasm2 code

evanh

Has anyone got a smaller solution than this for performing a nibble swap?: $12345678 -> $21436587

        splitb  val
        splitw  val
        movbyts val, #%01_00_11_10
        mergew  val
        mergeb  val

Wuerfel_21

split/merge instructions are circular (5x mergew will bring you back to the same value you started with. splitw moves in the opposite direction. splitb/mergeb are just 2x splitw/mergew), so perhaps

        mergeb  val
        movbyts val, #%%1032
        splitb  val

Not tested, may be nonsense Yep, works.

evanh

Damn! That's cool! Thanks.

garyri

There is a need for documentation of how to perform useful operations.
This is how this p2 gem works.

1. mergeb   val    (Merge bits of 4 bytes into 8 nibbles.)
$12345678 = 0001_0010_0011_0100_0101_0110_0111_1000
            0         0         0         0           0
             0         0         1         1          3
              0         1         0         1         5
               1         1         1         1        F
                 0         0         0         1      1
                  0         1         1         0     6
                   1         0         1         0    A
                    0         0         0         0   0

2. movbyts   val,%01_00_11_10    (Swap order of 16 bit words.)
                1  0  3  2   (This causes a change of nibble order in step 3.)
               -- -- -- --
$035F16A0  --> 16 A0 03 5F

3. splitb   val    (Split bits of 8 nibbles into 4 bytes.)

$16A0035F = 0001_0110_1010_0000_0000_0011_0101_1111
            0    0    1    0    0    0    0    1       21
             0    1    0    0    0    0    1    1      43
              0    1    1    0    0    1    0    1     65
               1    0    0    0    0    1    1    1    87

@wuerfel_21 Great question.
@evanh Great answer.

evanh

Handy tool! Thanks Gary. I was struggling visualising the actions of those instructions too.

Electrodude

Shave off the implicit AUGS:

        mergeb  val
        rol     val, #16
        splitb  val

Wuerfel_21

There is no AUGS there, #%%1032 Is a regular 9 bit immediate

Electrodude

@Wuerfel_21 said:
There is no AUGS there, #%%1032 Is a regular 9 bit immediate

Sorry, I can't read.

evanh

@Electrodude said:

        mergeb  val
        rol     val, #16
        splitb  val

That is more readable though. MOVBYTS always has to be read carefully to know what gets moved where.
Another readable form:
```

    mergeb  val
    rolword val, val, #1
    splitb  val

```
EDIT: Grr, formatting is messed. It won't clean up.