Implementation Summary Notes

Current Implementation Status:

The Z80 emulator has matured significantly with comprehensive instruction coverage, accurate flag handling, and proper timing. All major instruction groups are implemented with careful attention to undocumented behaviors.

Key Improvements Since v1:

X/Y Flag Handling: Fixed throughout - now correctly set from result for most operations, from operand for CP, and from WZ high byte for BIT n,(HL)
Interrupt Mode 0: Proper implementation with instruction buffer execution via InterruptController interface
R Register: Correctly incremented on M1 cycles including post-prefix fetches
Prefix Interactions: DD→ED, FD→DD, etc. properly handled with correct cycle additions
Block Instructions: Enhanced flag calculations for INI/IND/OUTI/OUTD with accurate undocumented behavior
IXH/IXL/IYH/IYL: Fully implemented undocumented register access via DD/FD prefixes

Remaining Minor Issues:

ED Undefined Opcodes: Comprehensive switch statement handles most, but could be more elegant
Cycle Verification: VerifyInstructionTiming() exists but only called when DEBUG_TIMING is true
Mode 0 Complexity: While functional, Mode 0 implementation is complex with two-phase execution

Correctly Implemented Features:

All Documented Instructions: Complete Z80 instruction set
Undocumented Instructions: SLL, IXH/IXL/IYH/IYL access, DDCB/FDCB register copy
Undocumented Flags: X and Y flags with proper behavior for all instruction types
Interrupt Handling: All three modes with proper timing and edge detection
WZ Register: Correctly maintained for flag calculations
Prefix Chains: Complex prefix sequences handled correctly
Block Instructions: Full implementation with proper flag calculations

zen80 Z80 Implementation Function Mapping

Implementation Status Legend

FULL: Complete implementation with all documented behaviour
NOT IMPL: Not implemented
NOP: Defaults to NOP behaviour
HANDLED: Handled through special logic
UNDOC: Undocumented instruction implemented
ENHANCED: Implementation with accurate undocumented behavior

Main Instructions (Unprefixed)

Opcode	Instruction	Coverage	Implementation	Function Location
00	NOP	✅	FULL	`executeBlock0()` case 0, y=0
01	LD BC,nn	✅	FULL	`executeBlock0()` case 1, q=0, p=0
02	LD (BC),A	✅	FULL	`executeBlock0()` case 2, y=0
03	INC BC	✅	FULL	`executeBlock0()` case 3, q=0, p=0
04	INC B	✅	FULL	`executeBlock0()` case 4 + `inc8()`
05	DEC B	✅	FULL	`executeBlock0()` case 5 + `dec8()`
06	LD B,n	✅	FULL	`executeBlock0()` case 6
07	RLCA	✅	FULL	`executeBlock0()` case 7, y=0 + `rlc8()`
08	EX AF,AF'	✅	FULL	`executeBlock0()` case 0, y=1
09	ADD HL,BC	✅	FULL	`executeBlock0()` case 1, q=1 + `add16()`
0A	LD A,(BC)	✅	FULL	`executeBlock0()` case 2, y=1
0B	DEC BC	✅	FULL	`executeBlock0()` case 3, q=1, p=0
0C	INC C	✅	FULL	`executeBlock0()` case 4 + `inc8()`
0D	DEC C	✅	FULL	`executeBlock0()` case 5 + `dec8()`
0E	LD C,n	✅	FULL	`executeBlock0()` case 6
0F	RRCA	✅	FULL	`executeBlock0()` case 7, y=1 + `rrc8()`
10	DJNZ d	✅	FULL	`executeBlock0()` case 0, y=2
11	LD DE,nn	✅	FULL	`executeBlock0()` case 1, q=0, p=1
12	LD (DE),A	✅	FULL	`executeBlock0()` case 2, y=2
13	INC DE	✅	FULL	`executeBlock0()` case 3, q=0, p=1
14	INC D	✅	FULL	`executeBlock0()` case 4 + `inc8()`
15	DEC D	✅	FULL	`executeBlock0()` case 5 + `dec8()`
16	LD D,n	✅	FULL	`executeBlock0()` case 6
17	RLA	✅	FULL	`executeBlock0()` case 7, y=2 + `rl8()`
18	JR d	✅	FULL	`executeBlock0()` case 0, y=3
19	ADD HL,DE	✅	FULL	`executeBlock0()` case 1, q=1 + `add16()`
1A	LD A,(DE)	✅	FULL	`executeBlock0()` case 2, y=3
1B	DEC DE	✅	FULL	`executeBlock0()` case 3, q=1, p=1
1C	INC E	✅	FULL	`executeBlock0()` case 4 + `inc8()`
1D	DEC E	✅	FULL	`executeBlock0()` case 5 + `dec8()`
1E	LD E,n	✅	FULL	`executeBlock0()` case 6
1F	RRA	✅	FULL	`executeBlock0()` case 7, y=3 + `rr8()`
20	JR NZ,d	✅	FULL	`executeBlock0()` case 0, y=4 + `testCondition()`
21	LD HL,nn	✅	FULL	`executeBlock0()` case 1, q=0, p=2
22	LD (nn),HL	✅	FULL	`executeBlock0()` case 2, y=4
23	INC HL	✅	FULL	`executeBlock0()` case 3, q=0, p=2
24	INC H	✅	FULL	`executeBlock0()` case 4 + `inc8()`
25	DEC H	✅	FULL	`executeBlock0()` case 5 + `dec8()`
26	LD H,n	✅	FULL	`executeBlock0()` case 6
27	DAA	✅	FULL	`executeBlock0()` case 7, y=4 + `daa()`
28	JR Z,d	✅	FULL	`executeBlock0()` case 0, y=5 + `testCondition()`
29	ADD HL,HL	✅	FULL	`executeBlock0()` case 1, q=1 + `add16()`
2A	LD HL,(nn)	✅	FULL	`executeBlock0()` case 2, y=5
2B	DEC HL	✅	FULL	`executeBlock0()` case 3, q=1, p=2
2C	INC L	✅	FULL	`executeBlock0()` case 4 + `inc8()`
2D	DEC L	✅	FULL	`executeBlock0()` case 5 + `dec8()`
2E	LD L,n	✅	FULL	`executeBlock0()` case 6
2F	CPL	✅	FULL	`executeBlock0()` case 7, y=5 (X/Y from A)
30	JR NC,d	✅	FULL	`executeBlock0()` case 0, y=6 + `testCondition()`
31	LD SP,nn	✅	FULL	`executeBlock0()` case 1, q=0, p=3
32	LD (nn),A	✅	FULL	`executeBlock0()` case 2, y=6
33	INC SP	✅	FULL	`executeBlock0()` case 3, q=0, p=3
34	INC (HL)	✅	FULL	`executeBlock0()` case 4, y=6 + `inc8()`
35	DEC (HL)	✅	FULL	`executeBlock0()` case 5, y=6 + `dec8()`
36	LD (HL),n	✅	FULL	`executeBlock0()` case 6, y=6
37	SCF	✅	FULL	`executeBlock0()` case 7, y=6 (X/Y from A)
38	JR C,d	✅	FULL	`executeBlock0()` case 0, y=7 + `testCondition()`
39	ADD HL,SP	✅	FULL	`executeBlock0()` case 1, q=1 + `add16()`
3A	LD A,(nn)	✅	FULL	`executeBlock0()` case 2, y=7
3B	DEC SP	✅	FULL	`executeBlock0()` case 3, q=1, p=3
3C	INC A	✅	FULL	`executeBlock0()` case 4 + `inc8()`
3D	DEC A	✅	FULL	`executeBlock0()` case 5 + `dec8()`
3E	LD A,n	✅	FULL	`executeBlock0()` case 6
3F	CCF	✅	FULL	`executeBlock0()` case 7, y=7 (X/Y from A)
40-75,77-7F	LD r,r'	✅	FULL	`executeBlock1()`
76	HALT	✅	FULL	`executeBlock1()` opcode=0x76
80-87	ADD A,r	✅	FULL	`executeBlock2()` y=0 + `add8()`
88-8F	ADC A,r	✅	FULL	`executeBlock2()` y=1 + `adc8()`
90-97	SUB r	✅	FULL	`executeBlock2()` y=2 + `sub8()`
98-9F	SBC A,r	✅	FULL	`executeBlock2()` y=3 + `sbc8()`
A0-A7	AND r	✅	FULL	`executeBlock2()` y=4 + `and8()`
A8-AF	XOR r	✅	FULL	`executeBlock2()` y=5 + `xor8()`
B0-B7	OR r	✅	FULL	`executeBlock2()` y=6 + `or8()`
B8-BF	CP r	✅	FULL	`executeBlock2()` y=7 + `cp8()` (X/Y from operand)
C0-FF	Various	✅	FULL	`executeBlock3()` all cases

CB-Prefixed Instructions

Opcode Range	Instruction	Coverage	Implementation	Function Location
CB 00-07	RLC r	✅	FULL	`executeCB()` x=0, y=0 + `rlc8()`
CB 08-0F	RRC r	✅	FULL	`executeCB()` x=0, y=1 + `rrc8()`
CB 10-17	RL r	✅	FULL	`executeCB()` x=0, y=2 + `rl8()`
CB 18-1F	RR r	✅	FULL	`executeCB()` x=0, y=3 + `rr8()`
CB 20-27	SLA r	✅	FULL	`executeCB()` x=0, y=4 + `sla8()`
CB 28-2F	SRA r	✅	FULL	`executeCB()` x=0, y=5 + `sra8()`
CB 30-37	SLL r	✅	UNDOC	`executeCB()` x=0, y=6 + `sll8()`
CB 38-3F	SRL r	✅	FULL	`executeCB()` x=0, y=7 + `srl8()`
CB 40-7F	BIT n,r	✅	FULL	`executeCB()` x=1 + `bit()` (WZ properly set for (HL), X/Y from WZ high)
CB 80-BF	RES n,r	✅	FULL	`executeCB()` x=2
CB C0-FF	SET n,r	✅	FULL	`executeCB()` x=3

ED-Prefixed Instructions

Opcode	Instruction	Coverage	Implementation	Function Location
ED 00-3F	Undefined	✅	NOP	`executeED()` explicit switch cases
ED 40	IN B,(C)	✅	FULL	`executeED()` x=1, z=0, y=0
ED 41	OUT (C),B	✅	FULL	`executeED()` x=1, z=1, y=0
ED 42	SBC HL,BC	✅	FULL	`executeED()` x=1, z=2, q=0 + `sbc16()`
ED 43	LD (nn),BC	✅	FULL	`executeED()` x=1, z=3, q=0
ED 44	NEG	✅	FULL	`executeED()` x=1, z=4 + `neg()`
ED 45	RETN	✅	FULL	`executeED()` x=1, z=5 (IFF2→IFF1)
ED 46	IM 0	✅	FULL	`executeED()` x=1, z=6, y=0
ED 47	LD I,A	✅	FULL	`executeED()` x=1, z=7, y=0
ED 48	IN C,(C)	✅	FULL	`executeED()` x=1, z=0, y=1
ED 49	OUT (C),C	✅	FULL	`executeED()` x=1, z=1, y=1
ED 4A	ADC HL,BC	✅	FULL	`executeED()` x=1, z=2, q=1 + `adc16()`
ED 4B	LD BC,(nn)	✅	FULL	`executeED()` x=1, z=3, q=1
ED 4C	NEG	✅	FULL	`executeED()` duplicate handling
ED 4D	RETI	✅	FULL	`executeED()` x=1, z=5
ED 4E	IM 0/1	✅	FULL	`executeED()` duplicate handling
ED 4F	LD R,A	✅	FULL	`executeED()` x=1, z=7, y=1
ED 50-5F	Various	✅	FULL	`executeED()` all implemented
ED 60-6F	Various	✅	FULL	`executeED()` all implemented
ED 67	RRD	✅	FULL	`executeED()` + `rrd()`
ED 6F	RLD	✅	FULL	`executeED()` + `rld()`
ED 70-7F	Various	✅	FULL	`executeED()` all implemented
ED 80-9F	Undefined	✅	NOP	`executeED()` explicit switch cases
ED A0	LDI	✅	ENHANCED	`ldi()` with complex X/Y flag calculation
ED A1	CPI	✅	ENHANCED	`cpi()` with complex X/Y flag calculation
ED A2	INI	✅	ENHANCED	`ini()` with full undocumented flag behavior
ED A3	OUTI	✅	ENHANCED	`outi()` with full undocumented flag behavior
ED A4-A7	Undefined	✅	NOP	`executeED()` explicit switch cases
ED A8	LDD	✅	ENHANCED	`ldd()` with complex X/Y flag calculation
ED A9	CPD	✅	ENHANCED	`cpd()` with complex X/Y flag calculation
ED AA	IND	✅	ENHANCED	`ind()` with full undocumented flag behavior
ED AB	OUTD	✅	ENHANCED	`outd()` with full undocumented flag behavior
ED AC-AF	Undefined	✅	NOP	`executeED()` explicit switch cases
ED B0	LDIR	✅	ENHANCED	`ldir()` with repeat logic and R increment
ED B1	CPIR	✅	ENHANCED	`cpir()` with repeat logic and R increment
ED B2	INIR	✅	ENHANCED	`inir()` with repeat logic and R increment
ED B3	OTIR	✅	ENHANCED	`otir()` with repeat logic and R increment
ED B4-B7	Undefined	✅	NOP	`executeED()` explicit switch cases
ED B8	LDDR	✅	ENHANCED	`lddr()` with repeat logic and R increment
ED B9	CPDR	✅	ENHANCED	`cpdr()` with repeat logic and R increment
ED BA	INDR	✅	ENHANCED	`indr()` with repeat logic and R increment
ED BB	OTDR	✅	ENHANCED	`otdr()` with repeat logic and R increment
ED BC-FF	Undefined	✅	NOP	`executeED()` explicit switch cases

DD-Prefixed (IX) Instructions

Opcode	Instruction	Coverage	Implementation	Function Location
DD 09	ADD IX,BC	✅	FULL	`executeDDFDInstruction()` with HL substitution
DD 19	ADD IX,DE	✅	FULL	`executeDDFDInstruction()` with HL substitution
DD 21	LD IX,nn	✅	FULL	`executeDDFDInstruction()` with HL substitution
DD 22	LD (nn),IX	✅	FULL	`executeDDFDInstruction()` with HL substitution
DD 23	INC IX	✅	FULL	`executeDDFDInstruction()` with HL substitution
DD 24	INC IXH	✅	UNDOC	`handleIXHIXL()` + `inc8()`
DD 25	DEC IXH	✅	UNDOC	`handleIXHIXL()` + `dec8()`
DD 26	LD IXH,n	✅	UNDOC	`handleIXHIXL()`
DD 29	ADD IX,IX	✅	FULL	`executeDDFDInstruction()` with HL substitution
DD 2A	LD IX,(nn)	✅	FULL	`executeDDFDInstruction()` with HL substitution
DD 2B	DEC IX	✅	FULL	`executeDDFDInstruction()` with HL substitution
DD 2C	INC IXL	✅	UNDOC	`handleIXHIXL()` + `inc8()`
DD 2D	DEC IXL	✅	UNDOC	`handleIXHIXL()` + `dec8()`
DD 2E	LD IXL,n	✅	UNDOC	`handleIXHIXL()`
DD 34	INC (IX+d)	✅	FULL	`executeDD()` special case handling
DD 35	DEC (IX+d)	✅	FULL	`executeDD()` special case handling
DD 36	LD (IX+d),n	✅	FULL	`executeDD()` special case handling
DD 39	ADD IX,SP	✅	FULL	`executeDDFDInstruction()` with HL substitution
DD 44-45,4C-4D,54-55,5C-5D,60-65,67-6D	LD with IXH/IXL	✅	UNDOC	`handleIXHIXL()` full register access
DD 46-7E	LD r,(IX+d)	✅	FULL	`executeDD()` special case handling
DD 70-77	LD (IX+d),r	✅	FULL	`executeDD()` special case handling
DD 84-85,8C-8D,94-95,9C-9D,A4-A5,AC-AD,B4-B5,BC-BD	ALU with IXH/IXL	✅	UNDOC	`handleIXHIXL()` full ALU operations
DD 86-BE	ALU ops (IX+d)	✅	FULL	`executeDD()` special case handling
DD CB	DDCB prefix	✅	HANDLED	`executeDD()` → `executeDDCB()`
DD DD	Double DD	✅	NOP	`executeDD()` returns 4
DD E1	POP IX	✅	FULL	`executeDDFDInstruction()` with HL substitution
DD E3	EX (SP),IX	✅	FULL	`executeDDFDInstruction()` with HL substitution
DD E5	PUSH IX	✅	FULL	`executeDDFDInstruction()` with HL substitution
DD E9	JP (IX)	✅	FULL	`executeDDFDInstruction()` with HL substitution
DD ED	ED after DD	✅	HANDLED	`executeDD()` → `executeED()` + 4
DD F9	LD SP,IX	✅	FULL	`executeDDFDInstruction()` with HL substitution
DD FD	FD after DD	✅	HANDLED	`executeDD()` → `executeFD()` + 4

FD-Prefixed (IY) Instructions

Opcode	Instruction	Coverage	Implementation	Function Location
FD 24	INC IYH	✅	UNDOC	`handleIYHIYL()` + `inc8()`
FD 25	DEC IYH	✅	UNDOC	`handleIYHIYL()` + `dec8()`
FD 26	LD IYH,n	✅	UNDOC	`handleIYHIYL()`
FD 2C	INC IYL	✅	UNDOC	`handleIYHIYL()` + `inc8()`
FD 2D	DEC IYL	✅	UNDOC	`handleIYHIYL()` + `dec8()`
FD 2E	LD IYL,n	✅	UNDOC	`handleIYHIYL()`
FD 44-45,4C-4D,54-55,5C-5D,60-65,67-6D	LD with IYH/IYL	✅	UNDOC	`handleIYHIYL()` full register access
FD 84-85,8C-8D,94-95,9C-9D,A4-A5,AC-AD,B4-B5,BC-BD	ALU with IYH/IYL	✅	UNDOC	`handleIYHIYL()` full ALU operations
FD 09-F9	All IY ops	✅	FULL	`executeFD()` mirrors `executeDD()` logic
FD CB	FDCB prefix	✅	HANDLED	`executeFD()` → `executeFDCB()`
FD DD	DD after FD	✅	HANDLED	`executeFD()` → `executeDD()` + 4
FD ED	ED after FD	✅	HANDLED	`executeFD()` → `executeED()` + 4
FD FD	Double FD	✅	NOP	`executeFD()` returns 4

DDCB-Prefixed Instructions

Opcode Range	Instruction	Coverage	Implementation	Function Location
DDCB d 00-3F	Shift/Rotate (IX+d)	✅	FULL	`executeDDFDCBOperation()` x=0
DDCB d 40-7F	BIT n,(IX+d)	✅	FULL	`executeDDFDCBOperation()` x=1 (X/Y from WZ high)
DDCB d 80-BF	RES n,(IX+d)	✅	FULL	`executeDDFDCBOperation()` x=2
DDCB d C0-FF	SET n,(IX+d)	✅	FULL	`executeDDFDCBOperation()` x=3
DDCB undoc	Register copy	✅	UNDOC	`executeDDFDCBOperation()` copies result to register

FDCB-Prefixed Instructions

Opcode Range	Instruction	Coverage	Implementation	Function Location
FDCB d 00-FF	All (IY+d) bit ops	✅	FULL	`executeDDFDCBOperation()`
FDCB undoc	Register copy	✅	UNDOC	`executeDDFDCBOperation()` copies result to register

Support Functions

Function	Purpose	File	Notes
`execute()`	Main instruction decoder	decode.go	Tracks prefix for verification
`executeBlock0()`	Opcodes 0x00-0x3F decoder	decode.go
`executeBlock1()`	Opcodes 0x40-0x7F decoder	decode.go
`executeBlock2()`	Opcodes 0x80-0xBF decoder	decode.go
`executeBlock3()`	Opcodes 0xC0-0xFF decoder	decode.go
`executeCB()`	CB prefix handler	prefix_cb.go	Fixed: WZ set for (HL), X/Y from WZ high for BIT
`executeED()`	ED prefix handler	prefix_ed.go	Comprehensive undefined opcode handling
`executeDD()`	DD prefix handler	prefix_ddfd.go	Full IXH/IXL register access implemented
`executeFD()`	FD prefix handler	prefix_ddfd.go	Full IYH/IYL register access implemented
`executeDDCB()`	DDCB prefix handler	prefix_ddfd.go	R not incremented (correct - not M1)
`executeFDCB()`	FDCB prefix handler	prefix_ddfd.go	R not incremented (correct - not M1)
`executeDDFDInstruction()`	IX/IY instruction executor	prefix_ddfd.go
`executeDDFDCBOperation()`	DDCB/FDCB operation executor	prefix_ddfd.go
`executeBlockInstruction()`	ED block instruction dispatcher	prefix_ed.go
`handleIXHIXL()`	IXH/IXL register access	prefix_ddfd.go	Full implementation
`handleIYHIYL()`	IYH/IYL register access	prefix_ddfd.go	Full implementation
`getRegisterForIX()`	IX register mapping	prefix_ddfd.go	Maps H→IXH, L→IXL
`getRegisterForIY()`	IY register mapping	prefix_ddfd.go	Maps H→IYH, L→IYL
`getIXHIXLCycles()`	IXH/IXL timing	prefix_ddfd.go
`getIYHIYLCycles()`	IYH/IYL timing	prefix_ddfd.go
`add8()`, `adc8()`, `sub8()`, `sbc8()`	8-bit arithmetic	alu.go	Fixed: X/Y from result
`and8()`, `or8()`, `xor8()`	8-bit logic	alu.go	Fixed: X/Y from result
`cp8()`	Compare	alu.go	Fixed: X/Y from operand
`inc8()`, `dec8()`	8-bit increment/decrement	alu.go	Fixed: X/Y from result
`add16()`, `adc16()`, `sbc16()`	16-bit arithmetic	alu.go	Fixed: X/Y from high byte, WZ=HL+1
`rlc8()`, `rrc8()`, `rl8()`, `rr8()`	Rotation operations	alu.go	Fixed: X/Y from result
`sla8()`, `sra8()`, `sll8()`, `srl8()`	Shift operations	alu.go	Fixed: X/Y from result
`daa()`	Decimal adjust	alu.go	Fixed: X/Y from result
`neg()`	Negate accumulator	prefix_ed.go
`rrd()`, `rld()`	Rotate decimal	prefix_ed.go	X/Y from A, WZ=HL+1
`bit()`	Bit test	prefix_cb.go	X/Y from value (overridden for (HL))
`ldi()`, `ldd()`, `ldir()`, `lddr()`	Block transfer	prefix_ed.go	Enhanced: Complex X/Y flag calculation
`cpi()`, `cpd()`, `cpir()`, `cpdr()`	Block search	prefix_ed.go	Enhanced: Complex X/Y flag calculation
`ini()`, `ind()`, `inir()`, `indr()`	Block input	prefix_ed.go	Enhanced: Full undocumented flag behavior
`outi()`, `outd()`, `otir()`, `otdr()`	Block output	prefix_ed.go	Enhanced: Full undocumented flag behavior
`testCondition()`	Conditional test	z80.go
`getRegister8()`	Register access	decode.go
`involvesHL()`	HL instruction check	prefix_ddfd.go	Correctly identifies HL-using instructions
`parity()`	Parity calculation	alu.go
`push()`, `pop()`	Stack operations	z80.go
`fetchByte()`, `fetchWord()`	Memory fetch	z80.go	Supports Mode 0 buffer
`readWord()`, `writeWord()`	Memory access	z80.go
`handleInterrupts()`	Interrupt handler	z80.go	Full Mode 0 with instruction buffer
`executeMode0Instruction()`	Mode 0 executor	z80.go	Two-phase execution
`VerifyInstructionTiming()`	Cycle verification	timing_fixes.go	Called when DEBUG_TIMING=true

Special Registers and Features

Feature	Coverage	Implementation	Function Location
A,F,B,C,D,E,H,L	✅	FULL	Direct fields in Z80 struct
A',F',B',C',D',E',H',L'	✅	FULL	Direct fields in Z80 struct
IXH,IXL	✅	UNDOC	Direct fields, full opcode access via DD prefix
IYH,IYL	✅	UNDOC	Direct fields, full opcode access via FD prefix
I,R	✅	FULL	Direct fields, R increments on M1
SP,PC	✅	FULL	Direct fields in Z80 struct
WZ (MEMPTR)	✅	FULL	Direct field, properly maintained
IFF1,IFF2	✅	FULL	Direct fields in Z80 struct
IM	✅	FULL	Direct field in Z80 struct
Halted	✅	FULL	Direct field in Z80 struct
pendingEI,pendingDI	✅	FULL	Direct fields in Z80 struct
NMI,INT	✅	FULL	Direct fields in Z80 struct
nmiEdge	✅	FULL	Edge detection for NMI
mode0Buffer	✅	FULL	Instruction buffer for Mode 0
mode0Index	✅	FULL	Current position in Mode 0 buffer
mode0Active	✅	FULL	Mode 0 execution state
M1Hook	✅	FULL	Debug hook for M1 cycles

Interrupt Implementation

Feature	Coverage	Implementation	Function Location
NMI handling	✅	FULL	`handleInterrupts()` edge-triggered
INT Mode 0	✅	FULL	`handleInterrupts()` with InterruptController interface
INT Mode 1	✅	FULL	`handleInterrupts()` jumps to 0x0038
INT Mode 2	✅	FULL	`handleInterrupts()` vectored via I register
EI delay	✅	FULL	`Step()` with pendingEI flag
DI delay	✅	FULL	`Step()` with pendingDI flag
R increment on INT	✅	FULL	Incremented for interrupt acknowledge M1

Flag Implementation

Flag	Bit	Coverage	Implementation	Functions
Carry (C)	0	✅	FULL	All ALU functions
Add/Subtract (N)	1	✅	FULL	All ALU functions
Parity/Overflow (P/V)	2	✅	FULL	Context-dependent in ALU
X (undocumented)	3	✅	UNDOC	Correct behavior throughout
Half-carry (H)	4	✅	FULL	All ALU functions
Y (undocumented)	5	✅	UNDOC	Correct behavior throughout
Zero (Z)	6	✅	FULL	All ALU functions
Sign (S)	7	✅	FULL	All ALU functions

Test Coverage Analysis

Test Category	Status	Coverage	Notes
Unit Tests	✅	Comprehensive	All instruction groups tested
Flag Tests	✅	Complete	Including undocumented X/Y flags
Timing Tests	✅	Extensive	Conditional branches, block ops
Interrupt Tests	✅	Full	All modes, EI delay, HALT interaction
Prefix Tests	✅	Complete	DD/FD/ED/CB and combinations
Block Op Tests	✅	Enhanced	Complex flag calculations verified
Integration Tests	✅	Multiple	ZEXDOC, Spectrum ROM
Edge Cases	✅	Well covered	R register, WZ, prefix chains

Performance and Quality Metrics

Metric	Status	Details
Instruction Coverage	100%	All documented Z80 instructions
Undocumented Coverage	High	Major undocumented features implemented
Test Pass Rate	100%	All unit tests passing
ZEXDOC Compatibility	✅	Runs CP/M test suite
Spectrum ROM Boot	✅	Successfully boots 48K and 128K ROMs
Debugging Support	✅	M1 hooks, cycle verification

Summary

The zen80 Z80 emulator has evolved into a mature, comprehensive implementation with: - Complete instruction set: All documented instructions plus major undocumented features - Accurate flag handling: Including proper X/Y flag behavior for all cases - Full interrupt support: All three modes with correct behavior - Extensive testing: Unit tests, integration tests, and real-world ROM execution

The emulator successfully runs both synthetic test suites (ZEXDOC) and real-world software (Spectrum ROMs), demonstrating both theoretical correctness and practical compatibility.