Appendix 2 — Registers, Flags, and Conditions

This appendix collects the machine-state tables you look up often while reading or writing Z80 code.

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

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.

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 unused / undocumented varies usually ignore
4 H half-carry from bit 3 to bit 4 BCD support
3 unused / undocumented varies 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. Always check the instruction’s own rules.

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 only supports NZ, Z, NC, and C.

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

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

If the target is farther away, use JP instead.