Zilog Z80A CPU:
Most Z80 opcodes are one byte long, not counting a possible byte or word operand. The four opcodes CB, DD, ED and FD change
the meaning of the opcode following them. You may find it helpful to use this reference section in conjunction with the 'Z80 Datasheet',
'The Undocumented Z80 Documented' and 'Z80 CPU Users Manual', each of which are available from
Sean Young's web site, and the
Z80 CPU Official Support Page, maintained by
There are 7 subsections available:
DD and FD Opcodes,
The R Register,
Undocumented Flags and
- CB Opcodes
There are 248 different CB opcodes. The block CB 30 to CB 37 is missing from the official list. These instructions, usually
denoted by the mnemonic SLL, Shift Left Logical, left the operand and make bit 0 always one. These instructions are quite
commonly used. For example, Bounder and Enduro Racer use them.
- ED Opcodes
There are a number of unofficial ED instructions, but none of them are very useful. The ED opcodes in the range 00-3F and
80-FF (except for the block instructions of course) do nothing at all but taking up 8 T states and incrementing the R register
by 2. Most of the unlisted opcodes in the range 0x40 to 0x7f do have an effect, however. The complete list:
ED40 IN B,(C) ED60 IN H,(C)
ED41 OUT (C),B ED61 OUT (C),H
ED42 SBC HL,BC ED62 SBC HL,HL
ED43 LD (nn),BC ED63 LD (nn),HL
ED44 NEG ED64 * NEG
ED45 RETN ED65 * RETN
ED46 IM 0 ED66 * IM 0
ED47 LD I,A ED67 RRD
ED48 IN C,(C) ED68 IN L,(C)
ED49 OUT (C),C ED69 OUT (C),L
ED4A ADC HL,BC ED6A ADC HL,HL
ED4B LD BC,(nn) ED6B LD HL,(nn)
ED4C * NEG ED6C * NEG
ED4D RETI ED6D * RETN
ED4E * IM 0/1 ED6E * IM 0/1
ED4F LD R,A ED6F RLD
ED50 IN D,(C) ED70 IN (C)
ED51 OUT (C),D ED71 * OUT (C),0
ED52 SBC HL,DE ED72 SBC HL,SP
ED53 LD (nn),DE ED73 LD (nn),SP
ED54 * NEG ED74 * NEG
ED55 * RETN ED75 * RETN
ED56 IM 1 ED76 * IM 1
ED57 LD A,I ED77 * NOP
ED58 IN E,(C) ED78 IN A,(C)
ED59 OUT (C),E ED79 OUT (C),A
ED5A ADC HL,DE ED7A ADC HL,SP
ED5B LD DE,(nn) ED7B LD SP,(nn)
ED5C * NEG ED7C * NEG
ED5D * RETN ED7D * RETN
ED5E IM 2 ED7E * IM 2
ED5F LD A,R ED7F * NOP
The ED 70 instruction reads from port (C), just like the other instructions, but throws away the result. It does change the flags in the
same way as the other IN instructions, however. The ED 71 instruction OUTs a zero byte to port (C). These instructions 'should', by
regularity of the instruction set, use (HL) as operand, but since from the processor's point of view accessing memory or accessing
I/O devices is the same thing except for activation of the /IORQ line instead of the /MREQ line, and since the Z80 cannot access
memory twice in one instruction (disregarding instruction fetch of course), it can't fetch or store the data byte.
The instructions ED 4E and ED 6E are IM 0 equivalents: when FF was put on the bus (physically) at interrupt time, the Spectrum
continued to execute normally, whereas when an EF (RST 28) was put on the bus it crashed, just as it does in that case when the
Z80 is in the official interrupt mode 0. In IM 1 the Z80 just executes a RST 38 (opcode FF) no matter what is on the bus.
All the ED xx RET? instructions copy IFF2 to IFF1, even RETI (ED 4D), which the official documentation does not note. The only
difference between RETI and RETN is that peripheral devices which allow daisy-chaining of interrupts (eg the Z80 PIO) recognise
the ED 4D sequence as 'end of interrupt' and then know that they can allow a further interrupt to be passed to the processor.
- DD and FD Opcodes
The DD and FD opcodes precede instructions using the IX and IY registers. If you look at the instructions carefully, you see how they work:
2A nn LD HL,(nn)
DD 2A nn LD IX,(nn)
7E LD A,(HL)
DD 7E d LD A,(IX+d)
A DD opcode simply changes the meaning of HL in the next instruction. If a memory byte is addressed indirectly via HL, as in the second
example, a displacement byte is added. Otherwise the instruction simply acts on IX instead of HL (a notational awkwardness, that will only
bother assembler and disassembler writers: JP (HL) is not indirect; it should have been denoted by JP HL). Instructions which use H or L
access the high and low halves of IX; those which reference both (HL) and either H or L replace HL by (IX+d), but still use H or L. For
example, DD 66 01 is LD H,(IX+01). Very many programs use these 'undocumented' IX instructions. FD works in exactly the same way to
DD, but with IY instead of IX. Many DD or FD opcodes after each other will effectively be NOPs, doing nothing except repeatedly setting
the flag "treat HL as IX" (or IY) and taking up 4 T states (But try to let MONS disassemble such a block.)
It is also possible to have doubly-shifted DD CB and FD CB opcodes (if DD or FD precedes an ED instruction, the DD or FD is ignored,
meaning that the ED instructions never operate on IX or IY). With the CB instructions, the situation is more interesting. Every DDCB
instruction operates on (IX+nn), but also copies the result to the register used in the original instruction, except when it is (HL). For example;
CB CE SET 0,(HL)
CB C0 SET 0,B
DD CB nn CE SET 0,(IX+nn)
DD CB nn C0 SET 0,(IX+nn) ; copy result to B
There is no standard way to denote these doubly shifted opcodes. Also, note that the offset byte is the third under all circumstances: for the
singly shifted opcodes, the offset byte is after the opcode byte (eg DD 2A nn is LD HL,(nn)), whilst the doubly shifted opcodes have the offset
byte before the opcode byte.
- The DAA Instruction
The purpose of the DAA (Decimal Adjust Accumulator)
instruction is to make an adjustment to the value in
the A register, after performing a binary mathmatical operation, such that
the result is as if the operation were performed with BCD (Binary Coded
Decimal) maths. The Z80 achieves this by adjusting the A register by a value
which is dependent upon the value of the A register, the Carry flag,
Half-Carry flag (carry from bit 3 to 4), and the N-flag (which defines
whether the last operation was an add or subtract).
The algorithm used is as follows:
- If the A register is greater than 0x99, OR the Carry flag is SET, then
The upper four bits of the Correction Factor are set to 6,
and the Carry flag will be SET.
The upper four bits of the Correction Factor are set to 0,
and the Carry flag will be CLEARED.
- If the lower four bits of the A register (A AND 0x0F) is greater than 9,
OR the Half-Carry (H) flag is SET, then
The lower four bits of the Correction Factor are set to 6.
The lower four bits of the Correction Factor are set to 0.
- This results in a Correction Factor of 0x00, 0x06, 0x60 or 0x66.
- If the N flag is CLEAR, then
ADD the Correction Factor to the A register.
SUBTRACT the Correction Factor from the A register.
- The Flags are set as follows:
Carry: Set/clear as in the first step above.
Half-Carry: Set if the correction operation caused a binary carry/borrow
from bit 3 to bit 4.
For this purpose, may be calculated as:
Bit 4 of: A(before) XOR A(after).
S,Z,P,5,3: Set as for simple logic operations on the resultant A value.
Note that the corrected result of values not arising from simple maths
operations on BCD arguments may be non-BCD or give inappropriate flags.
Algorithms can be devised which offer more reliable results, but do not
accurately reflect the behaviour of the Z80.
- The R Register
This is not really an undocumented feature, but a thorough description of it is not easy to find. The R register is a counter that is updated
during every Z80 M1 cycle (approximately equivalent to every instruction), so long as DD, FD, ED and CB are to be regarded as separate
instructions, so shifted instructions increase R by two. There's an interesting exception: doubly-shifted opcodes, the DD CB and FD CB ones,
also increase R by two. LDI increases R by two, LDIR increases it by 2 times BC, as does LDDR etcetera. R is set to zero when the Z80 is reset.
Both LD A,R and LD R,A use the value of R after it has been increased (eg an XOR A/LD R,A sequence sets the value of R to zero,
and [reset]/DI/LD A,R sets A to 0x03).
The highest bit of the R register is never changed: this is because in the old days everyone used 16 Kbit chips. Inside the chip the bits where grouped in
a 128x128 matrix, needing a 7 bit refresh cycle. Therefore Zilog decided to count only the lowest 7 bits. You can easily check that the R register is
really crucial to memory refresh. Assemble this program:
L1: XOR A
It will take about three minutes to run. Look at the upper 32K of memory, for instance the UDG graphics. It will have faded. Only the first few bytes
of each 256 byte block will still contain zeros, because they were refreshed during the execution of the loop.
The ULA took care of the refreshing of the lower 16K (This example won't work on the emulator,
of course!) R is increased by 1 during interrupt or NMI acknowledge.
- Undocumented Flags
This undocumented "feature" of Z80 has its effect on programs like Sabre Wulf, Ghosts'n'Goblins and the Speedlock loaders. Bits 3 and 5 of the
F register are not used. They can contain information, as you can readily figure out by PUSHing AF onto the stack and then POPping some it into
another pair of registers. Furthermore, sometimes their values change. The following empirical rule (due to Gerton Lunter) gives their values
after most instructions:
The values of bits 5 and 3 follow the values of the corresponding bits of the last 8 bit result of an instruction that changed the usual flags.
For instance, after an ADD A,B those bits will be identical to the bits of the A register.
As well as the two completely undocumented flags, after some instructions, the official documentation lists the value of some flags as
'undefined'. However, these flags have predictable values: (In the list below, C is the register and c is the carry flag)
Instruction Non-standard flags
CP xx 3 and 5 copied from the argument, not the result
ADD HL,xx Consider the instruction being done in two steps:
ADC HL,xx/SBC HL,xx first the LSBs being added, then the MSBs. The
3,H,5 and S flags are set as for the second step,
and Z is set only if the entire 16-bit result is
zero. (S and Z are not changed by ADD HL,xx).
BIT n,r P/V is set to the same value as Z. S is reset
unless the instruction is BIT 7,r and bit 7
of r is set, in which case S is set.
BIT n,(HL) 3 and 5 are apparently copied from an internal
BIT n,(IX/IY+d) storage in the Z80; this is set as follows:
ADD HL,xx: H before the addition
LD r,(IX/IY+d): high byte of IX/IY+d
JR d: high byte of the jump target
LD r,r': no effect
Others have not been tested yet.
SCF/CCF/CPL 3 and 5 copied from A. CCF sets H to the value of
c before the instruction is executed.
LDD/LDDR/LDI/LDIR 3 is bit 3 of (copied value+A), whilst 5 is bit 1 of
CPD/CPDR/CPI/CPIR 3 is bit 3 of (A-(HL)-(half carry flag)); 5 is bit 1
of this value. (HL) is the value of (HL) before the
instruction, whilst H is the value of H after the
IND/INDR/INI/INIR S,5 and 3 are affected as DEC B; N is set to bit
OUTD/OTDR/OUTI/OTIR 7 of the value written to/read from the IO port. c
is found by taking C, adding one if the instruction
increments HL or decrementing it otherwise, then
adding the value written/read, and taking the carry
of this final sum. H is set to the same value as c.
P/V for the IN... and OUT... instructions can be calculated as follows: in the following, x.y refers to bit y of x and inp is the byte
read from the port. Look at bits 0 and 1 of C and inp, and obtain a temporary result. The first result column should be used for
IND/INDR/OUTD & OTDR and the second for INI/INIR/OUTI & OTIR:
C.1 C.0 inp.1 inp.0 Temp1
0 0 0 0 0/0
0 0 0 1 1/0
0 0 1 0 0/1
0 0 1 1 0/0
0 1 0 0 1/0
0 1 0 1 0/1
0 1 1 0 0/0
0 1 1 1 1/1
1 0 0 0 0/1
1 0 0 1 0/0
1 0 1 0 1/1
1 0 1 1 0/1
1 1 0 0 0/0
1 1 0 1 1/1
1 1 1 0 0/1
1 1 1 1 1/0
Now, calculate Temp2 according to the following pseudo-code:
If B.3 == B.2 == B.1 == B.0 == 0 then
Temp2 = Parity(B) xor (B.4 or (B.6 and not B.5))
Temp2 = Parity(B) xor (B.0 or (B.2 and not B.1))
(Parity(B) is the standard partity function). Finally,
P/V = Temp1 xor Temp2 xor C.2 xor inp.2
Ghosts'n'Goblins uses the undocumented flag due to a programming error. The rhino in Sabre Wulf walks backward or keeps running
in little circles in a corner, if the (in this case undocumented) behaviour of the sign flag in the BIT instruction isn't right. From the code:
AD86 DD CB 06 7E BIT 7,(IX+6)
AD8A F2 8F AD JP P,0xad8f
An amazing piece of code! Speedlock does so many weird things that all must be exactly right for it to run. Finally, the 128K ROM uses the AF
register to hold the return address of a subroutine for a while.
The Z80 has three interrupt modes, selected by the instructions IM 0, IM 1 and IM 2.
When an interrupt is due, which is signalled by the ULA taking the level-triggered /INT pin on the Z80 low, nothing happens until the last
M-cycle of the instruction currently being executed. At that point, if interrupts are enabled (IFF1 is set) then interrupt processing will begin.
For this purpose, HALT is effectively an infinite series of NOPs, and the repeated instructions (LDIR, etc) can be interrupted after each
execution. Interrupt processing begins by resetting IFF1 and IFF2; this has two non-obvious consequences:
On the 48K Spectrum, the ULA holds the /INT pin low for precisely 32 T-states. This pin is sampled
during the last M-cycle of every instruction apart from repeated IX and IY prefixes (DD and FD). If the pin goes high again before it is sampled,
no interrupt will occur. The /INT pin must be held low for at least 23 T-states, as some IX and IY instructions take 23 T-states. If the interrupt routine
starts EI/NOP, this can cause a double interrupt, as the /INT pin will be sampled 19 (for IM 2)+4+4=27 T-states after being first sampled, when it may
still be low.
- If a LD A,I or LD A,R (which copy IFF2 to the P/V flag) is interrupted, then the P/V flag is reset, even if interrupts were enabled beforehand.
- If interrupts are disabled when a EI instruction is interrupted, then the interrupt will not occur until after the instruction following the EI, as
when IFF1 is sampled during the one and only M-cycle of the EI, it will be reset.
In IM 1, the processor simply executes an RST 38 instruction if an interrupt is requested. This is the mode the Spectrum is initalised to. In this
mode, the processor takes 13 T states to reach 0x0038: a 7 T state M1 cycle to acknowledge the interrupt and decrement SP, a 3 T state M2 cycle
to write the high byte of PC onto the stack and decrement SP again, and finally a 3 T state M3 cycle to write the low byte onto the stack and to set
PC to 0x0038.
The other mode that is commonly used on the Spectrum is IM 2. If an interrupt is requested, the processor first builds a
16-bit address by combining the I register (as the high byte) with whatever the interrupting device places on the data bus. The processor then fetches
the 16-bit address at this interrupt table entry, and finally calls the subroutine at that address. Rodnay Zaks in his book 'Programming the Z80' states
that only even bytes are allowed as low index byte, but that isn't true. The normal Spectrum contains no hardware to place a byte on the bus, and the
bus will therefore always read FF (because the ULA also doesn't read the screen if it generates an interrupt),
so the resulting index address is 256*I+255. However, some not-so-neat hardware devices put things on the data bus when they shouldn't, so later programs
didn't assume the low index byte was FF. These programs contain a 257 byte table of equal bytes starting at 256*I, and the interrupt routine is placed at
an address that is a multiple of 257. A useful, but not so much used trick on the Spectrum, is to make the table contain FF's (or use the ROM for this) and put
a byte 18 hex, the opcode for JR, at FFFF. The first byte of the ROM is a DI, F3 hex, so the JR will jump to FFF4, where a long JP to the actual interrupt
routine is put. In IM 2, it takes 19 cycles to get to the interrupt routine:
In interrupt mode 0, the processor executes the instruction that the interrupting device places on the data bus. On a standard Spectrum this will be the
byte FF, coincidentally the opcode for RST 38. But for the same reasons as above, this is not really reliable. If there is a RST n on the data bus, it
takes 12 cycles to get to 'n':
- M1: 7 T states: acknowledge interrupt and decrement SP
- M2: 3 T states: write high byte and decrement SP
- M3: 3 T states: write low byte
- M4: 3 T states: read low byte from the interrupt vector
- M5: 3 T states: read high byte and jump to interrupt routine
With a CALL nnnn on the data bus, it takes 19 cycles:
- M1: 6 T states: acknowledge interrupt and decrement SP
- M2: 3 T states: write high byte and decrement SP
- M3: 3 T states: write low byte and jump to 'n'
When the /NMI pin goes low, an internal flip-flop in the Z80 is set to note that an NMI is pending; This flip-flop is sampled at the end of every instruction,
apart from DD/FD and possibly EI/DI.
- M1: 6 T states: acknowledge interrupt
- M2: 3 T states: read low byte of 'nnnn' from data bus
- M3: 4 T states: read high byte of 'nnnn' and decrement SP
- M4: 3 T states: write high byte of PC to the stack and decrement SP
- M5: 3 T states: write low byte of PC and jump to 'nnnn'
When an NMI occurs, IFF1 is reset, thereby disallowing further maskable interrupts, but IFF2 is left unchanged. This enables the NMI service routine to
check whether the interrupted program had enabled or disabled maskable interrupts. The NMI routine should end with a RETN instruction, which in
addition to the usual RET actions copies IFF2 to IFF1, thus restoring the interrupt state of the interrupted code.
When an NMI occurs, it takes 11 T states to get to 0x0066: a 5 T state M1 cycle to do an opcode read and decrement SP, a 3 T state M2 cycle to write
the high byte of PC to the stack and decrement SP and finally a 3 T state M3 cycle to write the low byte of PC and jump to 0x0066.