Z80 assembly: Rotates and shifts

Move bits left or right. The bit that leaves the byte can enter the carry flag.

RLCA · RLA · RRCA · RRC · SLA · SRA · SRL

A rotate moves bits around a byte or through the carry flag. A shift moves bits and fills the empty place with 0, or keeps the sign bit for SRA. Carry records the bit that moved out.

Accumulator rotates take 4 T-states. CB-prefixed register rotates and shifts take 8. These CB forms set S, Z and P/V from the result; accumulator rotates preserve those three flags.

Run speed

What happened

Program listing

Address	Machine code	Label	Instruction	Comment

The accumulator example rotates 81h. Then B uses CB-prefixed rotate and shift instructions. Step one instruction to follow each outgoing bit.

Registers, flags and program bytes

Flags

C is the carry flag. For RLA and RR, its old value moves into the byte. The new C receives the bit that leaves. This page ignores undocumented X and Y bits.

Program memory

Execution trace

#	Address	Instruction	Result	T-states

Watch the bits move

10000001

Operation	Before	After	Carry out
RLCA	81h	03h	1
RLA	03h, C=1	07h	0
SRA B	80h	C0h	0
SRL B	C0h	60h	0

SRA keeps bit 7. This preserves the sign of a signed value. SRL shifts in 0, so it is a logical shift.

This lesson uses documented rotate and shift instructions. The undocumented SLL operation is not included.

Mnemonic glossary

Program mnemonics and directives

Name	Stands for	Meaning here
LD	Load	Loads test values into A and B.
RLCA	Rotate Left Circular Accumulator	Moves bit 7 to bit 0 and carry.
RLA	Rotate Left Accumulator through Carry	Moves carry into bit 0.
RRCA	Rotate Right Circular Accumulator	Moves bit 0 to bit 7 and carry.
RRC	Rotate Right Circular	Rotates B and updates result flags.
SLA	Shift Left Arithmetic	Shifts B left and fills bit 0 with 0.
SRA	Shift Right Arithmetic	Shifts B right and keeps bit 7.
SRL	Shift Right Logical	Shifts B right and fills bit 7 with 0.
HALT	Halt	Stops this lesson’s CPU.