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

1. Emit Zilog (ld a,b, jp nz,label). Do not emit Intel MOV / JNZ in tree sources.
2. Immediate placeholders in the map: * = d8 or displacement, ** = d16/a16.
3. Conditional T-states are taken/not-taken (example 13/8 for djnz).
4. Strict z80asm forbids synthetics and undocumented index halves / sll.
5. 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)

• halt is 4 T (8085 is 5).
• jp cc is always 10 T (8085 is 10/7).
• jr cc is 12/7. djnz is 13/8. call cc is 17/10. ret cc is 11/5.
• push is 11 T. pop is 10 T.
• ldir family: 21 T per repeat, 16 T on the last. Selector stand-in may use 21×n.
• Indexed ALU (ix+d) is 19 T. inc (ix+d) is 23 T.

Style in z88dk libsrc

• Prefer documented ops. Use undocumented ixh / sll only 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 exx for 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

1. dec bc does not set Z. Use ld a,b / or c / jp nz, or two 8-bit loops, or sbc hl,bc if HL is free and flags may change.
2. rla / rra / rlca / rrca do not set Z.
3. djnz does not set flags. A following jp z tests an older Z.
4. jp cc is always 10 T. Do not copy 8085 10/7.
5. ld a,i P/V is IFF2, not parity of A.
6. sll and ixh/ixl are undocumented. Strict assemble rejects them.
7. mlt / slp / in0 are Z180. They are not on this CPU.
8. ei enables after the next instruction.
9. No copt pass on library asm. Scan for ex de,hl / pair copies before you finish a hot routine.

Preference order (Z80-only code)

1. Documented ops that z80asm strict accepts.
2. exx for a second long. IX/IY when an index or frame is real.
3. sbc hl,de / adc hl,bc for 16-bit add/sub with carry.
4. djnz and ldir/cpir when the count is in B or BC.
5. Synthetics ld de,hl and friends in normal mode.
6. Undocumented ixh / sll only 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