z88dk CPU — Z80
Default classic and newlib CPU for most targets.
z88dk uses Zilog mnemonics. Opcode bytes, flags, and T-states are Z80, not 8085.
Opcode reference
Full grids: references/opcodes.md.
| Topic | Authority |
|---|---|
| Flags and T-states | clrhome Z80 table |
| Native encodings | src/z80asm/dev/cpu/cpu_test_z80_strict_ok.asm |
| Undocumented forms z80asm accepts | cpu_test_z80_ok.asm (not *_strict_*) |
| Assembler reject | cpu_test_z80_err.asm — rg only, never bulk-read |
Documented encodings in this skill were checked against cpu_test_z80_strict_ok.asm (173 representatives, 0 misses). If a fixture and clrhome disagree on bytes, the fixture wins. If they disagree on flags or T-states, clrhome wins.
clrhome also shows Z180-only ops. Those are not Z80. See cpu-z180.
Conventions
- Emit Zilog (
ld a,b,jp nz,label). Do not emit IntelMOV/JNZin tree sources. - Immediate placeholders in the map:
*= d8 or displacement,**= d16/a16. - Conditional T-states are
taken/not-taken(example13/8fordjnz). - Strict z80asm forbids synthetics and undocumented index halves /
sll. - Hand-written library asm does not run through
z88dk-copt.
Registers
15 ...... 8 7 ...... 0
A F → AF / af' (ex af,af')
B C → BC / bc'
D E → DE / de'
H L → HL / hl'
15 ............... 0
IX IY
SP PC
I R
exx swaps BC/DE/HL with the shadow set. ex af,af' swaps AF. Newlib sdcc often reserves IY. Classic sccz80 may use IX and IY.
Flag register (F)
| Bit | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
|---|---|---|---|---|---|---|---|---|
| S | Z | Y | H | X | P/V | N | C |
| Flag | Meaning |
|---|---|
| S | Sign |
| Z | Zero |
| Y / X | Undocumented copies of result bits 5 and 3 |
| H | Half-carry |
| P/V | Parity or overflow (one bit) |
| N | Subtract (1 after sub/dec/sbc/cp) |
| C | Carry |
Map column order is SZHPNC. 8085 has separate P and V, plus K. Do not copy 8085 masks onto Z80.
Condition codes: nz z nc c po pe p m. No k / nk.
Flag rules agents must not get wrong
| Group | SZHPNC | Notes |
|---|---|---|
inc/dec 16-bit (inc bc … dec sp, inc ix) |
------ |
Does not set Z. Do not dec bc / jp nz. Test through A (ld a,b / or c) |
inc/dec 8-bit (incl. (hl)) |
SZHV0- / SZHV1- |
C unchanged. 8-bit dec r does set Z |
rlca rla rrca rra |
--0-0C |
Z unchanged. Never rla / jp z |
add hl,rp / add ix,rp |
--H-0C |
Z unchanged |
adc hl,rp / sbc hl,rp |
SZHV0C / SZHV1C |
16-bit with borrow/carry. Not on 8085 (sbc hl,de) |
8-bit add/adc |
SZHV0C |
|
8-bit sub/sbc/cp |
SZHV1C |
|
and |
SZ1P00 |
H=1, P=parity |
or / xor |
SZ0P00 |
|
djnz |
------ |
B decrements inside the op. No flags |
ld a,i / ld a,r |
SZ0*0- |
P/V ← IFF2 |
ldi/ldd |
--00*- |
P/V set if BC≠0 after the step |
ldir/lddr |
--000- |
P/V reset when the block ends |
scf |
--0-01 |
|
ccf |
--*-0* |
N=0, C inverted |
pop af |
(loaded) | Restores the stored F |
Timing notes (clrhome)
haltis 4 T (8085 is 5).jp ccis always 10 T (8085 is 10/7).jr ccis 12/7.djnzis 13/8.call ccis 17/10.ret ccis 11/5.pushis 11 T.popis 10 T.ldirfamily: 21 T per repeat, 16 T on the last. Selector stand-in may use21×n.- Indexed ALU
(ix+d)is 19 T.inc (ix+d)is 23 T.
Style in z88dk libsrc
- Prefer documented ops. Use undocumented
ixh/sllonly with a measured win, and not under strict assemble. - Prefer synthetics when the assembler expands them cleanly (see below).
- Match neighbour file whitespace (spaces in most math cores).
- One major function per file — see
style-libsrc-layout. - Use
exxfor a second 32-bit value in float/math cores. 8085 cannot.
Synthetic opcodes (z80asm, normal mode)
z80asm expands many source forms into real ops. Strict / -no-synth forbids free synthetics.
Word copies — full set
Two 8-bit lds each (example ld de,hl → ld d,h / ld e,l).
| Allowed | Forbidden in this set |
|---|---|
ld among bc / de / hl — any → any |
af, sp |
Prefer ld bc,hl / ld hl,bc to park and restore one pair.
Other synthetics exist (example ld a,(hl+)). Listings (-l) show the expansion. tool-z80asm.
Contrast with 8085
Do not use exx, IX/IY, djnz, sbc hl,de, CB/ED/DD/FD prefixes, or block I/O in asm/8085/ trees.
On Z80, CB/DD/ED/FD are prefixes. On 8085 those bytes are different one-byte extended ops (rst v, jp nk, ld hl,(de), jp k).
16-bit dec rr sets K on 8085 (underflow to −1). On Z80 it sets nothing. Portable 16-bit count test is or through A.
Assembler capability (last resort)
Fixtures under src/z80asm/dev/cpu/ answer: does z80asm accept this source line for -mz80, and what encoding does it emit?
| File | Meaning |
|---|---|
cpu_test_z80_ok.asm |
Assembles successfully; comment after ; is expected encoding |
cpu_test_z80_err.asm |
Must fail (; Error) |
*_strict_* |
Strict mode: synthetics and most undocumented ops forbidden |
*_ixiy_* |
IX/IY swap build |
ok is not always one native chip opcode (normal mode may allow synthetics).
How to read lines: skill tool-z80asm. rg one mnemonic. Never bulk-read huge *_err.asm files.
Pitfalls
dec bcdoes not set Z. Useld a,b/or c/jp nz, or two 8-bit loops, orsbc hl,bcif HL is free and flags may change.rla/rra/rlca/rrcado not set Z.djnzdoes not set flags. A followingjp ztests an older Z.jp ccis always 10 T. Do not copy 8085 10/7.ld a,iP/V is IFF2, not parity of A.sllandixh/ixlare undocumented. Strict assemble rejects them.mlt/slp/in0are Z180. They are not on this CPU.eienables after the next instruction.- No copt pass on library asm. Scan for
ex de,hl/ pair copies before you finish a hot routine.
Preference order (Z80-only code)
- Documented ops that z80asm strict accepts.
exxfor a second long. IX/IY when an index or frame is real.sbc hl,de/adc hl,bcfor 16-bit add/sub with carry.djnzandldir/cpirwhen the count is in B or BC.- Synthetics
ld de,hland friends in normal mode. - Undocumented
ixh/sllonly when the size or T-state win is measured.
Related
- Full map: references/opcodes.md
- Assembler:
tool-z80asm - 8085 (different flags and prefixes):
cpu-8085 - Z80N extras:
cpu-z80n - Z180 extras:
cpu-z180 - Measure:
methodology-measure,tool-ticks