Your CPU’s Workspace: The Registers
Think of the Z80’s registers as a small, specialized crew of fast workers who hold all your data. They are faster than memory, so we want them to do most of the work.
The Main Workers (8-bit)
| Register | Analogy | Key Function |
|---|---|---|
| A (Accumulator) | The Calculator | Where math always happens. If you add, subtract, or compare, it happens here. |
| B, C, D, E, H, L | The General Storage | Used to hold temporary numbers or counts. |
The Main Address Book (16-bit)
| Register Pair | Analogy | Key Function |
|---|---|---|
| HL | The Memory Pointer | Always holds a 16-bit address (a location in memory) so the CPU knows where to read or write data. |
The Only Command You Need: LD (Load)
In Z80, data movement is done with the LD (Load) instruction. Think of this as the “Copy” command.
Syntax: LD destination, source
Example 1: Loading a Simple Number (Immediate Value)
LD A, 10 ; Load the decimal value 10 into the Accumulator (A)
LD HL, 5000H ; Load the memory address 5000 (hex) into the HL pointer
Example 2: Loading Data from Memory
When you see parentheses (), it means “go to the address stored inside the register.”
LD A, (HL) ; Load the number *at* the address in HL into A
LD (HL), B ; Store the number in B *at* the address in HL
Creating “Hello World” (The Concept)
To display text, we need the CPU to: 1) Know where the text is, and 2) Call a built-in OS routine to do the work.
The Steps:
- Define your text in memory (a DB or Define Byte instruction).
- Point the HL register to the start of that text.
- Use the CALL instruction to jump to the OS’s print function address.