Tiny4CPU

Tiny4CPU is a lightweight and educational 4-bit CPU emulator designed for simplicity. This project provides a hands-on exploration into the fundamentals of computer architecture, offering a platform to experiment with assembly language programming in a compact environment.

Dive into the world of microprocessors with Tiny4CPU and witness the power of 4 bits in action.

Code for the upper example:

; Usage of instruction OUT

; Init Register X to 0
Init:
LDX #0

; Main Loop
Loop:
ADX #1
OUT #0
JMP Loop

Features

4-bit architecture with its own assembler and disassembler
16-instruction set for data manipulation and control flow
2 general-purpose registers and memory space of 8 bytes
Ability to adjust CPU clock cycles for synchronization in milliseconds
Assembly language support
Modular and extensible design
Comprehensive documentation
Examples for each instruction

How to write a program for the Tiny4CPU

Tiny4CPU programs are written in T4C Assembly and saved as *.t4c files. Use the Tiny4CPU Assembler to assembly your program to a *.bin file. After that, load your ROM *.bin file using the Tiny4CPU executable, including the file path and the clock time in milliseconds.

The recommended clock time is 1000 milliseconds.

Example:

./assembler .../examples/output.t4c
./tiny4cpu .../examples/output.bin 1000

Instruction set architecture (ISA)

Instruction Set

Tiny4CPU has 16 opcodes (operation code, OP or instruction machine code), which are all one byte long and stored big-endian. Big-endian means that the most-significant-byte is saved in memory first (i.e. the lower memory address). Currently only 10 of potentially 16 opcodes are defined.

Mnemonic	Instruction	Description
LDX	0000 kkkk	Load Register X with immediate nibble value k
LDY	0001 kkkk	Load Register Y with immediate nibble value k
ADX	0010 kkkk	Add immediate nibble value k to Register X
ADY	0011 kkkk	Add immediate nibble value k to Register Y
SUX	0100 kkkk	Subtract immediate nibble value k from Register X
SUY	0101 kkkk	Subtract immediate nibble value k from Register Y
OUT	0110 kkkk	Set output to nibble value from register (1)
JXZ	0111 kkkk	Jump to memory address k if Register X is zero
JYZ	1000 kkkk	Jump to memory address k if Register Y is zero
JMP	1001 kkkk	Unconditional jump to memory address k
---	1010	Unused
---	1011	Unused
---	1100	Unused
---	1101	Unused
---	1110	Unused
---	1111	Unused

(1) Use value #0 for Register X or value #1 for Register Y

Registers

Register	Size (bits)	Description
PC	4	Program Counter
IR	8	Instruction Register
IAR	8	Instruction Address Register
ACC	4	Accumulator
MAR	4	Memory Address Register
RX	4	General-purpose register X
RY	4	General-purpose register Y

Memory

Mono-directional 4-bit address-bus from the microprocessor unit (MPU) to memory
The memory layout consists of one Memory Address Register (MAR). The MAR is connected to two 2x4 decoder, enabling addressing of up to 16 memory locations
Each memory location holds 4 bits of data, resulting in a total memory size of 8 bytes

References

License

digitsensitive/Tiny4CPU

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