The Computer

A Z80 computer has three main parts: a CPU, memory and I/O ports. The CPU does the work, fetching instructions and carrying them out. Memory holds the program and the data it works with. I/O ports connect the CPU to the outside world: a keyboard, a display, a storage device, a sensor.

The three parts and the buses between them. Sixteen address lines fix the memory ceiling at 64 KB; eight data lines fix the unit of transfer at one byte.

Bits, Bytes and Words

A bit is a single binary digit: 0 or 1. A byte is eight bits: the smallest unit of data the Z80 can read, write or operate on directly. A byte holds values from 0 to 255. Two consecutive bytes form a word, a 16-bit value ranging from 0 to 65,535.

The % prefix marks a binary number: %01110101 is the binary representation of $75 (117 in decimal). The full explanation of binary (how bits combine to make values, two's complement for signed numbers and bit-by-bit arithmetic) is in Appendix 7.

Hexadecimal

Hexadecimal is base 16. It uses sixteen digits: 0–9 for values 0–9, then A–F for values 10–15. Atom marks hex numbers with a $ prefix. Exactly four bits map to one hex digit, so a byte fits in two hex digits.

Hex digit	Value	Binary pattern
0	0	0000
1–9	1–9	0001–1001
A	10	1010
B	11	1011
C	12	1100
D	13	1101
E	14	1110
F	15	1111

So $75 is the byte %0111 0101, a 7 (0111) followed by a 5 (0101). And $FF is %1111 1111, which is 255. A four-digit hex number like $8000 is a 16-bit address. The full conversion tables are in Appendix 7.

Memory

The Z80 can address 65,536 bytes of memory, arranged as a flat array of 65,536 numbered slots that each hold one byte. The number that identifies a slot is its address.

The full address range runs from $0000 to $FFFF, address zero to address 65,535.

ROM (read-only memory) holds fixed data and keeps its contents when the power goes off. Bootloaders, fixed routines and lookup tables live here. RAM (random-access memory) can be freely read and written, but loses its contents when the power goes off; variables, the stack and anything your program creates or modifies go here.

The hardware designer decides which addresses connect to which chips. A system's memory map describes which ranges connect to which hardware. A typical small Z80 board might look like this:

$0000-$1FFF   ROM   (8 KB - startup code)
$2000-$7FFF   RAM   (24 KB - program and data)
$8000-$FFFF   -     (unmapped, or more RAM, or memory-mapped I/O)

At reset, the program counter starts at $0000. That address must contain valid code. On the board above, it is in ROM.

The memory map of the board above, drawn to scale across the full address space.

I/O Ports

The Z80 has a separate address space for hardware peripherals, reached with the IN and OUT instructions. In the usual programming model, an 8-bit port number selects the device. The CPU also drives an upper byte on its address pins during I/O; hardware may ignore it or use it for extra selection. Chapter 9 covers that distinction in detail.

The CPU and Its Registers

To carry out instructions, the CPU needs fast internal storage: the registers. The Z80 has about 26 bytes of register storage built directly into the chip, much faster than external RAM.

Here is the complete Z80 register set:

Register	Width	Role
A	8 bits	Accumulator. Most arithmetic and logic results end up here.
F	8 bits	Flags. Individual bits record the outcome of the last instruction that affected them. Conditional instructions test them; `PUSH AF` moves the whole byte.
B	8 bits	General purpose. Frequently used as a loop counter.
C	8 bits	General purpose. Forms the low byte of BC during register-addressed `IN`/`OUT`.
D	8 bits	General purpose.
E	8 bits	General purpose.
H	8 bits	General purpose. High byte of HL.
L	8 bits	General purpose. Low byte of HL.
BC	16 bits	B and C as a pair. Used for 16-bit counts and addresses.
DE	16 bits	D and E as a pair. Often used as a destination address when copying data.
HL	16 bits	H and L as a pair. The primary address register — most indirect memory access goes through HL.
IX	16 bits	Index register. Used for indexed memory access (base address + offset). Splits into IXH and IXL.
IY	16 bits	Index register. Same role as IX; a second independent index. Splits into IYH and IYL.
SP	16 bits	Stack pointer. Points to the top of the hardware stack.
PC	16 bits	Program counter. Always holds the address of the next instruction to execute. Jumps, calls and returns are what write to it.
I	8 bits	Interrupt vector register. Used with interrupt mode 2.
R	8 bits	Refresh register. Incremented automatically as each instruction is fetched.

When B and C are used as the pair BC, B holds the high byte and C holds the low byte, the same pattern as DE (D high, E low) and HL (H high, L low). So if HL = $1A2B, then H = $1A and L = $2B.

The Z80 also has a hidden second copy of A, F, B, C, D, E, H and L called the shadow registers, covered in Chapter 8. A compact register reference is in Appendix 8.

The whole register set. Each of the four main pairs is two 8-bit registers and one 16-bit register at the same time.

The Fetch-Execute Cycle

The CPU does one thing, over and over: read the byte at the address in PC, interpret it as an instruction, carry it out and advance PC to the next instruction.

Some instructions are one byte long, some are two, three or four. After executing an instruction, PC advances by exactly as many bytes as that instruction occupied, unless the instruction itself changes PC, which is what jumps and calls do.

The cycle the CPU repeats until it halts.

A First Look at the Machine

The following is a complete Z80 program, ten bytes of raw instructions starting at address $0000:

asm

$0000:  3E 05        ; load 5 into register A
$0002:  47           ; copy A into register B
$0003:  3E 03        ; load 3 into register A
$0005:  80           ; add B to A  ->  A = 8
$0006:  32 00 80     ; store A at address $8000
$0009:  76           ; halt

When the CPU resets, PC is $0000. It fetches $3E, recognises it as a two-byte "load constant into A" instruction, reads the next byte ($05), loads 5 into A and advances PC to $0002. It continues instruction by instruction until it reaches $76 (HALT). HALT suspends normal instruction execution until an interrupt or reset. Address $8000 now holds the value 8.

Endianness

The Z80 is little-endian: the low byte goes at the lower address, the high byte at the higher address.

Storing the word $1A2B at address $8000:

Address   Contents
$8000       $2B    <- low byte first
$8001       $1A    <- high byte second

You saw this in the first program: the address $8000 is encoded in the instruction at $0006 as two bytes $00 $80 (low byte $00 first, high byte $80 second).

Storing $8000. The low byte goes to the lower address, so the two bytes appear in the instruction stream in the opposite order to the way the value is written.

The Flags Register

Each flag is a single bit, set (1) or clear (0). The two you will use from the very beginning are:

Symbol	Name	Set when
Z	Zero	The result of the last operation was zero
C	Carry	The last addition produced a carry out of bit 7, or the last subtraction required a borrow

The Z80 has four more flags: S (sign), H (half carry), P/V (parity/overflow) and N (subtract). The full flags reference is in Appendix 8.

LD instructions do not affect flags at all. Arithmetic and comparison instructions do. Chapter 5 explains exactly which instructions set which flags.

Exercise

Parts of the system. A table with one row for each action below should identify the active parts of the computer (CPU, memory or I/O), what travels on the data bus, and what the address selects.

Fetch the opcode at $0120.
Read a byte from RAM at $8004.
Write $3C to port $10.
Add B to A.

Exercise notes

The Computer ​

Bits, Bytes and Words ​

Hexadecimal ​

Memory ​

I/O Ports ​

The CPU and Its Registers ​

The Fetch-Execute Cycle ​

A First Look at the Machine ​

Endianness ​

The Flags Register ​

Exercise ​