Indirect Addressing

The Z80 chip also supports indirect addressing, which is different from immediate and direct modes. This is used when data is not stored directly in the instruction, but instead in the memory location pointed to by a register pair.

What Is Indirect Mode?

In this mode, instead of specifying an explicit memory address, we load from or store to the address held in a register pair, such as HL, BC, or DE.

This is especially useful when working with memory blocks, moving data, or performing pointer-style operations.

Instruction Patterns

The Z80 provides several variations of indirect addressing. Here's a breakdown of key forms:

Example: Using HL as a Pointer

We can use indirect mode to store multiple values to memory without writing explicit addresses.

       ORG 256
       LD BC,35900       ; Start address for storing
       LD A,65           ; ASCII code for 'A'
       LD (BC),A         ; Store 'A' at BC
       INC BC
       LD (BC),A         ; Store 'A' at next byte
       INC BC
       LD (BC),A         ; Store 'A' at next byte


This stores three A characters into memory starting at address 35900.

A Better Version Using a Loop

       ORG 256
       LD BC,35900       ; Pointer to memory
       LD A,65           ; ASCII for 'A'
       LD E,3            ; Number of times to store

NXT:    LD (BC),A         ; Store A
       INC BC            ; Move pointer forward
       DEC E             ; Decrement counter
       JR NZ,NXT         ; Repeat until E = 0


This version is shorter and clearer, using a loop to perform repeated indirect memory writes.

Real-World Example: Writing a Message to Screen Using Indirect Mode

To see indirect addressing in action, let’s build a program that stores a short message in memory, then reads and prints it one character at a time using a pointer.

This simulates how a real system might walk through a string or data block stored in RAM.

What It Demonstrates

• How to store a string in memory using LD (HL),A
• How to step through memory using INC HL
• How to use (HL) as a dynamic pointer

Print "HI" Using Indirect Addressing

       ORG 256

       LD HL,30000         ; Point to where we'll store the string
       LD A,'H'
       LD (HL),A           ; Store 'H'
       INC HL
       LD A,'I'
       LD (HL),A           ; Store 'I'
       INC HL
       LD A,0              ; Null terminator (optional for stopping later)
       LD (HL),A

       ; Reset pointer to start of string
       LD HL,30000

READ:   LD A,(HL)           ; Load character from memory
       CP 0
       RET Z               ; End of string
       CALL PRINT
       INC HL
       JR READ

PRINT:  RST 8
       DEFB 158            ; MCAL print character in A
       RET


Notes

• HL walks through the memory where the string is stored.
• The CP 0 and RET Z stop the loop at the null terminator.
• Each character is printed using MCAL 158 (RST 8 / DEFB 158).

Why This Is Realistic

This method mimics how real operating systems, firmware, or BIOS routines process:

• Strings stored in RAM
• Buffers for network or file I/O
• Character or byte streams

Indirect addressing provides a powerful, general-purpose way to walk through data structures without hardcoding every address.

Easy String Storage

Surely there’s an easier way to store a string of characters? Absolutely, and in fact, assembly provides several ways to make this more convenient.

Here’s one of the simplest:

message:
    DEFB 'HELLO WORLD',0

or

message:
    DB 'HELLO WORLD',0


This uses an assembler directive, not a Z80 CPU instruction. DB stands for Define Byte, and it tells the assembler to place the individual ASCII values for each character (including the null terminator 0) directly into memory at the current location.

This approach allows you to define entire strings inline with your code, and the assembler handles turning them into the correct bytes.

Summary

• Indirect mode uses the value stored in a register pair as a pointer to memory.
• Common register pairs: HL, BC, DE
• Useful for memory access, structure traversal, and efficient looping
• Helps avoid hardcoding memory addresses

Exercises

Exercise 4.1
Write a program that stores the values 1 to 5 in consecutive memory locations using indirect mode and a loop.

Exercise 4.2
Explain what the following code does and why indirect mode is used:

       LD HL,40000
       LD A,(HL)
       INC HL
       LD A,(HL)