werdl/4bit

Features

• Custom 4-bit ALU, supporting AND, OR, XOR, NOT, full add and full subtract
• 256 nibbles of general use memory
• Four registers, used for I/O to/from the ALU
• Program counter with jump and jump if zero instructions

Instruction Set

Binary Value	Assembly Mnemonic	A_ARG value	B_ARG value	Operation Description	Implemented
0000	ADD	None	None	Adds reg 00 to reg 01 and stores in reg 10	Yes
0001	SUB	None	None	Subtracts reg 01 from reg 00 and stores in reg 10	Yes
0010	REG	4 bit literal	2 bit register address	Stores A_ARG in reg B_ARG	Yes
0011	NOP	None	None	None	Yes
0100	NOT	None	None	Logically inverts the contents of reg 00 and stores in reg 10	Yes
0101	XOR	None	None	Performs logical XOR on registers 00 and 01 and stores in reg 10	Yes
0110	OR	None	None	Performs logical OR on registers 00 and 01 and stores in reg 10	Yes
0111	AND	None	None	Performs logical AND on registers 00 and 01 and stores in reg 10	Yes
1000	SAV	8 bit address	4 bit literal	Saves B_ARG to memory address B_ARG	Yes
1001	LDA	8 bit address	2 bit register address	Loads memory address A_ARG to reg B_ARG	Yes
1010	WRIT	8 bit address	2 bit register address	Saves reg B_ARG to memory address A_ARG	Yes
1011	Reserved	-	-	-	No
1100	JMP	6 bit address	None	Jumps to PC address A_ARG	Yes
1101	Reserved	-	-	-	No
1110	JCU	6 bit address	2 bit register address	If register B_ARG is equal to register 11, jumps to memory address A_ARG	Yes
1111	Reserved	-	-	-	No

Notes on the instruction set

• There are 4 "namespaces" (bitfields)

Binary range	Use	# uses
0000 - 0011	Arithmetic uses	2 + NOP
0100 - 0111	Logical operations	4
1000 - 1011	Memory manipulation operations	3
1100 - 1111	Control flow operations	3 (only 1 implemented)

• I think it is Turing-complete or near to it? I am pretty sure it is with manual instruction entry on the ALU/Memory Unit, but the PC I am less sure of

Example Programs

A simple program that adds 1 to a number in memory address 0x00 and stores it in memory address 0x01

LDA 0x00 00 ; Load the number from memory address 0x00 into reg 00
REG 1 01 ; Store the literal 1 in reg 01
ADD ; Add reg 00 to reg 01 and store in reg 10
WRIT 0x01 10 ; Write the result to memory address 0x01

Circuitry

Fully designed in Logisim, using the built in TTL library and also this library. A few logic gates are included for more obscure ICs, but they will be replaced either with said ICs or with equivalent chips in series.

ALU/MU

The ALU/MU is the heart of the computer, and is where all the computation is done. It is a 4-bit ALU, with 256 nibbles of general use memory. The ALU/MU is connected to the program counter, and is where the instructions are executed. The MU contains the registers and the memory.

Program Counter

The program counter is a simple counter that increments by one each clock cycle. It is connected to the ALU/MU, and is where the instructions are fetched from memory. The CLK input loads the current instruction into the ALU/MU, and the GO input executes the instruction. It is programmable, and can jump to any address in memory, using the JMP and JCU instructions.