Appendix 8 — Registers, Flags and Conditions

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Main Registers

Register	Width	Usual role	Notes
A	8	accumulator	main byte arithmetic/logic destination
F	8	flags	holds S Z H P/V N C; not a general data register
B	8	general purpose	often used as a loop counter
C	8	general purpose	also used with port I/O
D	8	general purpose	often paired with E
E	8	general purpose	often paired with D
H	8	general purpose	high byte of HL
L	8	general purpose	low byte of HL
BC	16	register pair	counts, addresses, A-only indirect through (BC)
DE	16	register pair	data/address pair, A-only indirect through (DE)
HL	16	primary pointer pair	main indirect memory register
IX	16	index register	indexed access with displacement
IY	16	index register	second indexed access register
SP	16	stack pointer	points into the hardware stack
PC	16	program counter	address of next instruction
I	8	interrupt vector high byte	used in interrupt mode 2
R	8	refresh register	normally not useful in everyday code

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Shadow Registers

Register set	What it is
AF'	shadow accumulator and flags
BC', DE', HL'	shadow copies of the main 16-bit working pairs

You reach these through EX AF,AF' and EXX, not through ordinary LD forms.

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Flags Register

Bit	Name	Meaning when set	Common beginner use
7	S	result is negative in signed interpretation	signed comparisons
6	Z	result is zero	JP Z, JR NZ, loop exits
5	undocumented result bit	commonly copies result bit 5	usually ignore
4	H	half-carry from bit 3 to bit 4	BCD support
3	undocumented result bit	commonly copies result bit 3	usually ignore
2	P/V	parity or overflow, depends on instruction	signed overflow / parity / block ops
1	N	last arithmetic op was subtraction	mostly internal / BCD support
0	C	carry out or borrow	unsigned comparisons, rotates, shifts

Not every instruction updates every flag.

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Condition Codes

Condition	Meaning	Flag test
Z	zero	Z = 1
NZ	not zero	Z = 0
C	carry	C = 1
NC	no carry	C = 0
M	minus	S = 1
P	plus	S = 0
PE	parity even / overflow	P/V = 1
PO	parity odd / no overflow	P/V = 0

These appear in conditional JP, JR, CALL and RET forms. JR supports NZ, Z, NC and C.

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Signed and Unsigned Landmarks

Width	Unsigned range	Signed range (two's complement)
8-bit byte	0 to 255	-128 to 127
16-bit word	0 to 65535	-32768 to 32767

Useful byte landmarks:

Value	Unsigned	Signed
$00	0	0
$7F	127	127
$80	128	-128
$FF	255	-1

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Relative Branch Range

Instruction family	Range
JR CC,TARGET	-128 to +127 bytes from the next instruction
DJNZ TARGET	-128 to +127 bytes from the next instruction

Targets outside that range require JP.