Z80

Zilog Z80A

Microprocessor from Zilog, which is used in the Amstrad CPC Computers. The Z80/Z80A was a very popular microprocessor, used in a great variety of home computers and appliances as far-fetched as satellites. It was even used in the Commodore C128 as a secondary processor in order to achieve CP/M compatibility.

The Z80 microprocessor is an 8-bit CPU with a 4-bit ALU and a 16-bit address bus capable of direct access to 64KB of memory space. The Z80 is a little-endian CPU, meaning it stores 16-bit values with the least significant byte first, followed by the most significant byte. It has a language of 252 root instructions and with the reserved 4 bytes as prefixes, access to an additional 308 instructions. The Z80 was modelled after the 8080 and contains the seventy-eight 8080 opcodes as a subset to its language.

While not in the same league as the Intel 80x86 or the Motorola 68000 series, the Z80 is extremely useful for low cost control applications. One of the more useful features of the Z80 is the built-in refresh circuitry for ease of design with DRAMs.

The Z80 comes in a 40 pin DIP package. It has been manufactured in A, B, and C models, differing only in maximum clock speed. It also has been manufactured as a stand-alone microcontroller with various configurations of on-chip RAM and EPROM.

The Z80 CPU has been manufactured by others, and various Z80s have been used in the construction of the CPC during its lifetime.

• SGS Z8400AB1
• ST Z8400AB1
• ZILOG Z8400APS
• ZILOG Z0840004PSC

Zilog ended the production of the Z80 in April 2024. The chip is still available in ample quantities through NOS chip suppliers.

Modern incarnations

Apart from surplus/new Z80-clones that are quite easy to find, many emulations depend on software implementations of the Z80:

• The T80 is a VHDL implementation of the Z80 and Z80A, finished in 2002 on OpenCores
• arnold uses InkZ80, written in C++ (apart from the author-designed C simulation)
• On OpenCores, there is also a Verilog implementation of the Z80.

Zilog itself offers the eZ80 processor, a new, 50MHz design. Kits now have reached a less-than-prohibitive price range and may be available without a business.

Legend

Flags

• - = no change
• + = change by definition (if noted, by the operation marked with '=> flags', otherwise by the only non-single-bit operation):

* S = sign, bit 7 of the result byte (accumulator or high byte for 16-bit operations)
* Z = zero, set if the result is zero (8 or 16-bit value)
* 5 = undocumented, bit 5 of the result byte
* H = half-carry, the carry (theoretical bit 4) of the low nibble of the result byte
* 3 = undocumented, bit 3 of the result byte
* P = parity (set if the result byte has an even number of bits set) or overflow (set when crossing the boundary of the signed range); always specified
* N = negative, set if the previous operation was a subtraction; always specified
* C = carry, the theoretical bit 8 of the result byte

• 0 = always reset
• 1 = always set
• X = change described under Effect
• P = parity (only for the parity flag)
• V = overflow (only for the parity flag)
• A = OR with the respective bit of the accumulator
• C = set if the counter (bc) is nonzero after decrementing

Miscellaneous

• () = indirection
• (()) = I/O port
• [] = operator precedence (to avoid confusion with indirection)
• E.B = the Bth bit of the value of expression E
• * = any bit value (0 or 1)
• wz = an internal 16-bit register connected to 16-bit operations
• tmp, tmp2 = temporary storage whose value is thrown away after each instruction

Letter A

Letter B

Letter C

Letter D

if hf or [a AND 0x0f > 9] then tmp -= 0x06
if cf or [a > 0x99] then tmp -= 0x60

if hf or [a AND 0x0f > 9] then tmp += 0x06
if cf or [a > 0x99] then tmp += 0x60

Letter E

Letter H

Letter I

Letter J

Letter L

Letter N

Letter O

Letter P

Letter R

Letter S

Letter X

Any instruction in bold is undocumented by Zilog.

Standard opcodes

CB-prefixed opcodes

ED-prefixed opcodes

The opcodes that are not mentioned in the following table are EDNOP (ED-prefixed NOP instruction). Thay have no effect but take 8 cycles and increment the register R two times. A pair of ED prefixes is also an EDNOP.

Notes:

• The opcode ED 70 reads the port indicated by the register C without keeping the result but modifies the register F
• The opcode ED 71 corresponds to the instruction OUT (C),255 on a CMOS Z80

DD or FD-prefixed opcodes

If an opcode is prefixed by DD, the instruction is changed as follows:

• HL is replaced by IX
• H is replaced by IXH
• L is replaced by IXL
• (HL) is replaced by (IX+d)

Same for the FD prefix but with IY instead of IX.

There are 3 exceptions:

• In the instruction EX DE,HL, HL will not be replaced with IX or IY. The EXX instruction is not affected either.
• If (HL) and L or H are used in the same instruction, L and H are not replaced with IXL or IXH. For instance LD L,(IX+d) stores the content of (IX+d) into L, not IXL
• If the next byte is a DD, ED or FD prefix, the current DD or FD prefix is ignored (it's equivalent to a NONI) and processing continues with the next byte. ED-prefixed opcodes cannot be altered by DD or FD prefixes.

When a DD or FD prefix is followed by a CB byte, the CB acts as a second prefix. A mandatory displacement byte comes next, and then the actual opcode. If the instruction produces output other than in the flags (i.e. all except BIT), then the result gets placed both into (IX+d) or (IY+d) and the register one would normally expect to be altered.

• Despite having different names and opcodes, RETI and RETN are in fact the exact same instruction
• Contrarily to what the syntax of the instructions JP (HL/IX/IY) suggest, PC will be loaded with the contents of the register itself, not the indexed value. Those instructions should be understood as JP HL/IX/IY
• IN (C) and OUT (C) instructions syntax is misleading as these instructions actually use the full 16-bit port address contained in BC
• While the syntax of ADD, ADC and SBC instructions all explicitely mention the A register, the SUB instruction does not mention it
• The 16-bit commands ADD HL,ss, ADC HL,ss and SBC HL,ss exist but not the command SUB HL,ss

• Official Zilog Z80 CPU user manual (2016)
• Media:Z80 CPU Technical Manual 1977.pdf
• Media:Mostek Z80 Programming Manual.pdf
• Media:Z80 CPC Timings cheat sheet.20230709.pdf
• Z80 - undocumented opcodes

• Zilog [1]
• Computer Systems based on Z80 Family
• The Z80 processor on Wikipedia
• Z80 documentation from Grimware
• Detailed look at Z80 instruction timings with the help of a Z80 netlist simulation
• Decoding Z80 opcodes
• Behaviour of the undocumented flags