Author Topic: Shifting data block down a nibble?  (Read 330 times)

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Offline c4ooo

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Shifting data block down a nibble?
« on: March 20, 2017, 11:47:20 pm »
How do I shift a block of data down a nibble? IIRC there was an instruction that swapped nibbles or something that made this easy. The shifted data will never be >256 bytes long.

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Offline SpiroH

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Re: Shifting data block down a nibble?
« Reply #1 on: March 22, 2017, 10:02:10 am »
Hi @c4ooo,
Can you be a bit more specific? AFAICT a nibble is just a set of 4 bits glued together; why don't you just shift-right by a constant of multiple of 4, or do you something else on your mind/question?

Offline harold

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Re: Shifting data block down a nibble?
« Reply #2 on: March 22, 2017, 11:21:54 am »
The relevant instruction is RRD, it's not very fast but it seems like nothing is ever fast on z80..

You can do something like
Code: [Select]
  xor a
_shiftloop:
  rrd
  dec hl
  djnz _shiftloop
Maybe inc hl, depends on the endianness of your buffer.
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Offline c4ooo

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Re: Shifting data block down a nibble?
« Reply #3 on: March 22, 2017, 07:46:57 pm »
Hi @c4ooo,
Can you be a bit more specific? AFAICT a nibble is just a set of 4 bits glued together; why don't you just shift-right by a constant of multiple of 4, or do you something else on your mind/question?
I need to shift the buffer down/up a nibble (4 bits). So something like 0xABCD -> 0x0ABCD0

The relevant instruction is RRD, it's not very fast but it seems like nothing is ever fast on z80..

You can do something like
Code: [Select]
  xor a
_shiftloop:
  rrd
  dec hl
  djnz _shiftloop
Maybe inc hl, depends on the endianness of your buffer.
Thanks, will investigate later.
-German Kuznetsov
The impossible chemical compound.

Offline Xeda112358

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Re: Shifting data block down a nibble?
« Reply #4 on: March 25, 2017, 08:18:15 pm »
harold's method is essentially what I did to shift the screen left or right by multiples of 4. It's slow, but faster than shifting by 1 four times.