Author Topic: Duplicating Bits in Register A (Read 3793 times)

Hot_Dog · « **on:** October 08, 2011, 03:08:16 pm »

If I have a number stored in Register A, such as %01110010, what's the best way to turn it into %0011111100001100? Meaning every bit in the register is doubled? I don't care what the output register (or variable) is

Deep Toaster · « **Reply #1 on:** October 08, 2011, 04:27:16 pm »

I once had to do that, but ended up just looping through it and copying each bit twice. (When I asked Kerm he couldn't think of any alternatives at the mement either.) Someone out of the people viewing this topic is probably gonna come up with some brilliant routine to do this, but I have no idea.

calc84maniac · « **Reply #2 on:** October 08, 2011, 04:28:13 pm »

Define "best way". If you want the smallest solution, it's probably something like:

Code: [Select]

  ld b,8
doubleloop:
  rlca
  adc hl,hl
  rrca
  adc hl,hl
  rlca
  djnz doubleloop

The fastest solution would be to use a lookup table, like so:

Code: [Select]

  ld h,doubleLUT >> 8
  ld l,a
  ld e,(hl)
  inc h
  ld d,(hl)

Hot_Dog · « **Reply #3 on:** October 08, 2011, 05:17:26 pm »

I didn't think about a lookup table. That's actually probably the best unless a faster one comes around

calcdude84se · « **Reply #4 on:** October 08, 2011, 07:08:24 pm »

calc84's is broken as far as I can tell because it neglects that when "duplicated" A is two bytes. (You also need to 256-byte align the table.)
A fixed version (which destroys A):

Code: [Select]

ld h,LUT>>8
add a,a
jr nc,$+3
inc h
ld e,(hl)
inc hl
ld d,(hl)

Edit: In general, except for the simplest of processes, look-up tables tend to be the fastest method. (They are also the most space-consuming.)

thepenguin77 · « **Reply #5 on:** October 08, 2011, 07:38:57 pm »

Quote from: calcdude84se on October 08, 2011, 07:08:24 pm

calc84's is broken as far as I can tell because it neglects that when "duplicated" A is two bytes. (You also need to 256-byte align the table.)

I thought that same thing at first, however:

Quote from: calc84maniac on October 08, 2011, 04:28:13 pm

Code: [Select]
inc h

He actually did it right. He just didn't order the LUT the way you expected him to

(Or for that matter, the way any other person would.)

calcdude84se · « **Reply #6 on:** October 09, 2011, 03:19:23 pm »

Nice catch. And a more efficient way to order it, too. I shall have to remember that. Perhaps I thought I had seen "inc hl" instead of "inc h". * calcdude shrugs.

Author Topic: Duplicating Bits in Register A (Read 3793 times)

Hot_Dog

Duplicating Bits in Register A

Deep Toaster

Re: Duplicating Bits in Register A

calc84maniac

Re: Duplicating Bits in Register A

Hot_Dog

Re: Duplicating Bits in Register A

calcdude84se

Re: Duplicating Bits in Register A

thepenguin77

Re: Duplicating Bits in Register A

calcdude84se

Re: Duplicating Bits in Register A