HKcpu

The HKcpu is an 8-bit educational CPU designed for academic and learning purposes. Its goal is to help users understand how a real CPU works through interactive simulation and visualization. If you want to try it now, go to run it part.

Architecture

It features:

• 3 General Purpose Registers (GPRs): RA, RB, RC

  Used for arithmetic, logic, and general data manipulation.

• 6 Special Purpose Registers (SPRs): IR, AR, MA, PC, FG, RO

  • IR – Instruction Register: Holds the current instruction being executed.
  • AR – Argument Register: Holds the current argument of the instruction.
  • MA – Memory Address Register: Interface for memory read/write addresses.
  • PC – Program Counter: Points to the next instruction to be executed.
  • FG – Flags Register: Stores comparisons flags (==, <, > and !=).
  • RO - Register Output: Holds the output value to be displayed on the screen or interface.

• 8-bit Memory Address Width (256 bytes addressable)

• 22 MICs (Micro Instructions Code): follow MICs

  • Time Control:

    • RST - Reset: Reset the CPU registers and states.
    • HLT - Halt: Stop the clock to end execution.

  • Program Counter:

    • CE - Counter Enable: Enable counter to increment on the next clock.
    • CO - Counter Output: Put the counter value on data bus.
    • CL - Counter Load: Loads the data bus value to counter value on the next clock.

  • Registers:

    • RAI - RA Input: Loads the data bus value to RA register.
    • RBI - RB Input: Loads the data bus value to RB register.
    • RCI - RC Input: Loads the data bus value to RC register.
    • RAO - RA Output: Puts the RA value on data bus.
    • RBO - RB Output: Puts the RB value on data bus.
    • RCO - RC Output: Puts the RC value on data bus.

  • ALU:

    • AOP1-AOP3 - ALU Operation (3bits) Defines which ALU operation will be used.
    • AO - ALU Output: Puts the ALU result on data bus.

  • Logic Control:

    • IRI - IR Input: Loads the data bus value to IR register.
    • ARO - AR Output: Puts the ARO value on data bus.
    • ARI - AR Input: Loads the data bus value to AR register.

  • Memory:

    • MAI - MA Input: Loads the data bus value to MA interface register.
    • MI - Memory Input: Loads the data bus value to the specified address of memory.
    • MO - Memory Output: Puts the specified addres of memory value on data bus.
    • OE - Output Enable: Loads the data bus value to OE interface register.

• 1 Microcode PLA (Programmable Logic Array) used to store Micro Instructions (microcode.txt contains the Logisim PLA table)..

• 1 ALU supporting 4 arithmetic and 4 comparison operations: ADD, SUB, MUL, DIV, ==, !=, >, <

• 25 Instruction Codes Supported: Follow Instructions

  • 0x00 NOP - No Operation: Don't execute any operation on clock.
  • 0x01 ADD - Addition ALU Operation: RA + RB => RC
  • 0x02 SUB - Subtraction ALU Operation: RA - RB => RC
  • 0x03 MUL - Multiplication ALU Operation RA * RB => RC
  • 0x04 DIV - Division ALU Operation: RA / RB => RC
  • 0x05 LDA ADDR - Load A: Load the value of memory at the address specified by the argument, and put on RA.
  • 0x06 LDB ADDR - Load B: Load the value of memory at the address specified by the argument, and put on RB.
  • 0x07 LDC ADDR - Load C: Load the value of memory at the address specified by the argument, and put on RC.
  • 0x08 SVA ADDR - Save A: Save RA value on memory at the address specified by the argument.
  • 0x09 SVB ADDR - Save B: Save RB value on memory at the address specified by the argument.
  • 0x0A SVC ADDR - Save C: Save RC value on memory at the address specified by the argument.
  • 0x0B MAB - Move A to B: Copy the value of RA to RB.
  • 0x0C MAC - Move A to C: Copy the value of RA to RC.
  • 0x0D MBA - Move B to A: Copy the value of RB to RA.
  • 0x0E MBC - Move B to C: Copy the value of RB to RC.
  • 0x0F MCA - Move C to A: Copy the value of RC to RA.
  • 0x10 MCB - Move C to B: Copy the value of RC to RB.
  • 0x11 JMP ADDR - Jump: Jump PC to argument value.
  • 0x12 CMP - Compare: Compare RA and RB and set FG.
  • 0x13 JEQ ADDR - Jump If Equal: Using FG, jump to argument value if RA and RB were equal in the last CMP.
  • 0x14 JNE ADDR - Jump If Not Equal: Using FG, jump to argument value if RA and RB were not equal in the last CMP.
  • 0x15 JBT ADDR - Jump If Bigger Than: Using FG, jump to argument value if RA were bigger than RB in the last CMP.
  • 0x16 JLT ADDR - Jump If Less Than: Using FG, jump to argument value if RA were less than RB in the last CMP.
  • 0x17 OUT ADDR - Output: Puts the value of memory at the address specified by the argument to RO.
  • 0xFE RST - Reset: Execute RST MC.
  • 0xFF HLT - Halt: Execute HLT MC.

Run It

You can run the CPU simulation using Logisim for visual, step-by-step execution, or use the emulator for faster results. If you want to write more complex programs, you can also use the assembler.

Assembler and Emulator

To compile the assembler and emulator, run the following scripts:

• assembler/build.sh
• emulator/build.sh

Make sure the bin folders exist in both directories before running the build scripts.

Once compiled, you can run the Assembler and Emulator directly from their respective bin folders. To view usage instructions, simply execute them without any arguments.

If you want example programs to study or test without writing your own assembly, check the example directory.

Logisim Simulation

To explore the internal architecture and step-by-step execution, open the file hkcpu.circ using Logisim Evolution 1.9.0.

To use the CPU in Logisim:

• Manually program the ROM with instructions.

• Reset and start the CPU clock.

• For more advanced programs, you can load precompiled HEX or BIN files into the ROM.