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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.

ALU/MU

Program Counter