Z80 assembly: Strings, fast fill and copy

A string is a sequence of character bytes. Fill a memory range, then copy a string with LDIR.

DB · DJNZ · LDIR

A string is stored as one byte per character. This example uses ASCII bytes for “Z80!”. A fill loop writes the same value to several addresses. LDIR copies a block forward and updates HL, DE and BC after each byte.

The fill loop takes 26 T-states for each continuing byte and 21 for the last. LDIR takes 21 T-states while repeating and 16 on its last byte.

Run speed

What happened

Program listing

Address	Machine code	Label	Instruction	Comment

The program fills 011Bh–011Eh with 2Ah. Then it copies the four bytes at 0117h–011Ah to 011Fh–0122h.

Registers and memory

Flags

ASCII assigns a number to each character. Z is 5Ah, 8 is 38h, 0 is 30h, and ! is 21h. The bytes are not stored as special string objects.

Source, fill and copy areas

Execution trace

#	Address	Instruction	Result	T-states

Block instruction or manual loop?

Task	Code	Continuing byte	Last byte
LDIR copy	2 bytes	21 T	16 T
LD A,(HL) + LD (DE),A + pointer updates + branch	7 bytes	44 T	39 T
Fill loop: LD (HL),A + INC HL + DJNZ	4 bytes	26 T	21 T

LDIR copies from a source range. It cannot repeat one source byte to fill a range, because HL moves after each copy. Use a fill loop for repeated values.

The string has a fixed length of four bytes and no zero terminator. Real programs can use a terminator or a separate length value.

Mnemonic glossary

Program mnemonics and directives

Name	Stands for	Meaning here
LD	Load	Sets up values and writes fill bytes.
INC	Increment	Moves HL to the next memory byte.
DJNZ	Decrement B and Jump if Not Zero	Repeats the fill loop four times.
LDIR	Load, Increment, Repeat	Copies the string bytes forward.
DB	Define Byte	Assembler directive. Stores the string bytes.
DS	Define Space	Assembler directive. Reserves fill and copy ranges.
HALT	Halt	Stops after the copy.