Z80 assembly: Loading data

Load one byte from memory, move it through registers, and write it to a new address.

LDr,nr,r′A,(HL)(HL),A

LD means Load. It copies a value. These forms do not change flags. LD r,n takes 7 T-states. LD r,r′ takes 4. The two memory forms take 7.

Source byte

Run speed

What happened

Program listing

Address	Machine code	Label	Instruction	Comment

The source byte starts at 010Dh, immediately after the 13-byte code block. The program copies it to 010Eh. The highlighted instruction is at PC.

Registers, flags and memory

Flags

A register stores a small value inside the CPU. HL is a register pair. The value in HL is a memory address. PC means Program Counter. A T-state is one CPU clock period. This page ignores undocumented X and Y bits.

Source HL points here

Destination HL points here before writing

Execution trace

#	Address	Instruction	Result	T-states

Another way to copy one byte

; direct transfer through DE
LD HL,source
LD DE,dest
LD A,(HL)
LD (DE),A
HALT

Program	Code size	Time
This lesson’s register path	13 bytes	53 T-states
Direct transfer through DE	9 bytes	38 T-states

Both examples include address setup and HALT. The direct path needs DE, an extra register pair for the destination address.

Simplification: memory is a flat 64 KB array. Interrupts and input/output devices are not modeled.

Mnemonic glossary

Program mnemonics and directives

Name	Stands for	What it does here
LD	Load	Copies a value into a register or memory.
HALT	Halt	Stops this lesson’s CPU. Interrupts are not modeled.
ORG	Origin	Assembler directive, not a CPU instruction. Sets the code address.
DB	Define Byte	Assembler directive, not a CPU instruction. Places the source byte in memory.
DS	Define Space	Assembler directive, not a CPU instruction. Reserves the destination byte.