LDDR

LDDR

Load and Decrement with Repeat

Description

Performs ldd until BC = 0, effectively copying BC bytes of data from HL to DE, where HL and DE point to the end of their respective blocks.

ldd
ret po
jr -5

Uses

• Copying lots of data

  • Sprite routines
  • Copying graphical data from some buffer to the actual screen
  • Filling lots of space with one (or more) bytes

Results

Register/Flag	16-bit (non-ADL)	24-bit (ADL)
S flag	Not affected
Z flag	Not affected
H flag	Reset
P/V flag	Set if BC ≠ 0 after the operation; else reset
N flag	Reset
C flag	Not affected

Allowed Instructions

Instruction	Opcode	CC (ADL/non-ADL)	CC (.S)	CC (.L)
lddr	$ED, $B8	2F + (1R + 1W + 1)* BC	3F + (1R + 1W + 1)* BC	3F + (1R + 1W + 1)* BC

Notes

• Interrupts can be triggered while this instruction is in progress (unless they are disabled using DI, of course).
• Since most of the time it’s more convenient to specify the beginning of a block of data than the end, LDIR is significantly more popular than LDDR.
• Assuming you are copying hard-coded data and BC is NOT already set to the desired number of bytes, it is only faster to use hard-coded LDD s instead of ld bc,BytesToCopy \ lddr if you are copying two bytes. Copying three bytes, the cycle times and size of the code are exactly the same (6 bytes, 6F+3R+3W+3).
• If you want to copy a few more bytes than whatever number is in BC, it is both smaller and significantly faster to use INC BC several times before LDDR than a few LDD s after it. (INC BC is one byte and only 1F (plus the extra 1R + 1W + 1 from LDDR) whereas LDD is two bytes and 2F+1R+1W+1. So you just save one byte and 1F.)

Examples

• Filling a block of memory with a single byte

  ld hl,EndOfBlock
  ld de,EndOfBlock-1
  ld bc,SizeOfBlock-1
  ld (hl),ByteToCopy
  lddr
  

See Also

CPDR, LD, LDD, LDI, LDIR