Z80 assembly: Logic and bits

Use masks to keep, clear, or toggle bits. Test one bit without changing the value.

AND · OR · XOR · BIT · SET · RES

AND keeps bits set in both values. OR sets a bit if either value has it. XOR sets a bit when the values differ. BIT tests a bit. SET makes it 1. RES makes it 0.

AND, OR and XOR update S, Z, H, P/V, N and C. BIT updates S, Z, H and P/V, while preserving C. SET and RES do not change flags.

Run speed

What happened

Program listing

Address	Machine code	Label	Instruction	Comment

The program starts with 11001100b. It combines masks, tests bit 7, clears it, then sets bit 0.

Registers, flags and program bytes

Flags

A mask is a value used to select bits. In this example, 0Fh keeps the low nibble. 80h selects bit 7. BIT sets Z when the tested bit is 0, sets H, clears N and preserves C. This page ignores undocumented X and Y bits.

Program memory

Execution trace

#	Address	Instruction	Result	T-states

Bit operation effects

Instruction	Input A	Other value / bit	Output A
AND 0Fh	CCh	0Fh	0Ch
OR 80h	0Ch	80h	8Ch
XOR 0Ch	8Ch	0Ch	80h
RES 7,A	80h	bit 7	00h
SET 0,A	00h	bit 0	01h

AND with 0Fh is one way to keep only the low four bits. OR with 80h sets the top bit.

The examples use register A. The same bit instructions can also operate on the byte at (HL). Input/output ports and interrupts are not modeled.

Mnemonic glossary

Program mnemonics and directives

Name	Stands for	Meaning here
LD	Load	Loads the first byte into A.
AND	Logical AND	Keeps bits set in both inputs.
OR	Logical OR	Sets bits present in either input.
XOR	Exclusive OR	Sets bits that differ.
BIT	Bit test	Tests one bit without changing A.
SET	Set bit	Changes the selected bit to 1.
RES	Reset bit	Changes the selected bit to 0.
HALT	Halt	Stops this lesson’s CPU.