Lesson 2: Understanding Registers - The Microcontroller's "Lockers"

📌 Learning Objectives

1. Understand what registers are
2. Learn to read and write registers
3. Understand the concept of register addresses
4. Master the purpose of common JoyKit registers

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🎯 What is a Register?

A Useful Analogy

Imagine your desk has a row of drawers, where each drawer:

• Has its own number (address)
• Can store things (data)
• Can be accessed to put in or take out items (read/write)

Registers are the microcontroller's "drawers" for temporarily storing data!

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📦 Basic Properties of Registers

1. Address

Each register has a unique address, represented in hexadecimal:

Register	Address	Purpose
R0	0x00	General purpose
R1	0x01	General purpose
R25	0x19	IO configuration
...	...	...

2. Bit Width

JoyKit registers are 16-bit, with data range:

• Minimum: 0000 (hex) = 0
• Maximum: FFFF (hex) = 65535

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🔧 Register Read/Write Instructions

1. Write to Register (0x10)

Write data to a specified register:

Instruction	OP00	OP01	OP10	OP11
10	Register address	Data	00	00

Example: Write 0x00FF to register R0

📚 With Comments

000 10 0000 00FF  ; R0 = 0x00FF

📋 Clean Code

000 10 0000 00FF

2. Read from Register (0x14)

Read IO data into a register:

Instruction	OP00	OP01	OP10	OP11
14	Target register	00	00	00

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🧩 Common Registers

1. IO Configuration Register (R25, 0x19)

Used to set whether IO ports are input or output:

R25 Value	Description
0000	All set to output
00FF	All set to input
000F	Lower 4 bits input, upper 4 bits output

2. System Registers

• R0-R15: General purpose registers, freely usable
• R25: IO configuration (just discussed)
• R31-R33: LED color data (R31=Red, R32=Green, R33=Blue)

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🔬 Classroom Demo: Register Value "Relay"

Experiment Goal

Demonstrate data transfer between registers.

Program Code

📚 With Comments

; Register data relay experiment
000 10 0000 00AA  ; R0 = 0xAA
001 11 0000 0001  ; R1 = R0  (R1 = 0xAA)
002 11 0001 0002  ; R2 = R1  (R2 = 0xAA)
003 12 0002 0001  ; R2 += 1  (R2 = 0xAB)
004 01 00FF 0000  ; Display R2 value with LEDs
005 02 03E8 0000  ; delay
006 0E 0003 0000  ; loop

📋 Clean Code

000 10 0000 00AA
001 11 0000 0001
002 11 0001 0002
003 12 0002 0001
004 01 00FF 0000
005 02 03E8 0000
006 0E 0003 0000

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📝 Quick Exercises

1. Write an instruction to store 0x1234 in R5
2. Write an instruction to copy R5's value to R6
3. Write an instruction to increment R6 by 1

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🎉 Summary

What we learned today:

• ✅ Registers are the microcontroller's "lockers"
• ✅ Each register has an address
• ✅ Use instruction 0x10 to write to registers
• ✅ R25 configures IO input/output
• ✅ Data can be transferred between registers

Next lesson, we'll learn about IO Input/Output to control real hardware!