This repository contains my implementation of the Hack computer as described in "The Elements of Computing Systems" by Noam Nisan and Shimon Schocken. The project includes the complete hardware architecture of the Hack platform.
The Hack computer hardware consists of three core components.
- Instruction Memory (ROM)
- Date Memory (RAM)
- The Central Processing Unit (CPU)
Source: https://en.wikipedia.org/wiki/Hack_computer
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ROM Module: A linear array of sequential, 16-bit memory registers, starting at address 0x0000.
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Addressing: Uses a 15-bit wide address bus, allowing access to 32,768 individual memory words.
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Clock Signal: The simulation and computer emulator applications provide the system clock signal.
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Program Counter: Provides the address of the current instruction within the CPU.
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Instruction Fetching: The instruction stored in the ROM at the address specified by the instruction address bus becomes the "current" instruction at the start of the next clock cycle.
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Instruction Decoding: There is no dedicated instruction register; instructions are directly decoded from the active ROM register in each clock cycle.
RAM Module: A continuous linear array of sequential, read-write, 16-bit memory registers.
Address Segments:
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General-Purpose Data Storage: Addresses 0 (0x0000) through 16383 (0x3FFF) contain read-write registers for general program data.
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Screen I/O Subsystem: Addresses 16384 (0x4000) through 24575 (0x5FFF) function like data RAM but also control output on the computer’s virtual 256 x 512 screen. If screen output is not needed, these registers can be used for general data.
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Keyboard Memory Map Register: Address 24576 (0x6000) is a read-only register linked to the keyboard output of the host computer running the Hack emulator.
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Invalid Address Range: Addresses 24577 (0x6001) through 32767 (0x7FFF) are invalid.
System Clock Signal: RAM state transitions are synchronized with the system clock signal.
The Hack CPU comprises an Arithmetic Logic Unit (ALU), a set of registers (including the D and A registers), a Program Counter (PC), and an instruction control unit that integrates these components to coordinate the execution of instructions.
Source: https://en.wikipedia.org/wiki/Hack_computer
- A combinational logic device with two 16-bit input operands and one 16-bit output.
- Operates based on six ordered, single-bit control inputs.
- Provides two status flags: zr (zero) and ng (negative) to indicate computation results.
Taken from The Elements of Computing Systems, Chapter 2: https://docs.wixstatic.com/ugd/44046b_f0eaab042ba042dcb58f3e08b46bb4d7.pdf
- D Register (Data): A 16-bit general-purpose register; that always supplies the x operand for the ALU, though its value can be ignored for some instructions.
- A Register (Address): Can provide its value as the y operand for the ALU; also used for data memory addressing and as a target for branching instructions.
- M Register (Pseudo-register): Represents the value stored in the RAM at the address specified by the A register; not physically implemented in hardware but used conceptually for accessing memory data indirectly through the A register's address.
- A 16-bit counter whose low 15 bits specify the address of the next instruction in memory.
- Automatically increments each clock cycle unless modified by a branching instruction.
- Includes a reset input that sets the PC to 0x0000 when toggled.
- The CPU includes logic to alter instruction execution order via the PC but does not support hardware interrupts or function calls.
The Hack machine language has two types of instructions, each encoded in 16 binary digits:
Encoding:
- The most significant bit is 0.
- Bits b14 through b0 represent a 15-bit binary value corresponding to a non-negative integer between 0 and 32767.
Functionality:
- Loads the 15-bit value into the CPU's A-register.
- It enables the RAM register at the address specified by this value for read/write operations in the next clock cycle.
Encoding:
- The most significant bit is 1.
- The next two bits are conventionally set to 1 and ignored by the CPU.
- Bit a specifies the source of the y operand for the ALU.
- Bits c1 to c6 control the operands and computation performed by the ALU.
- Bits d1 to d3 determine the destination(s) for the ALU output.
- Bits j1 to j3 specify a branching condition based on the most recent computation result, allowing for arithmetic-based branching, unconditional jumps, or no branching.
Format:
- dest=comp;jump
- dest specifies where to store the result.
- comp specifies the computation to perform.
- jump specifies the branching condition.
This encoding scheme allows the Hack CPU to perform basic computations, data manipulation, and control flow, making it a simple yet powerful instruction set for implementing basic programs and algorithms on the Hack platform.
The Hack Computer Assembler translates Hack assembly language into machine code through a structured process:
- Extracts and categorizes instruction types (A, C, L).
- Removes comments and whitespace for clean processing.
- Converts parsed instructions into binary machine code.
- Uses hash maps to resolve symbols.
- The symbol table maps symbolic names (labels and variables) to memory addresses, ensuring accurate translation.
- Scans the assembly code to identify labels.
- Creates a symbol table that maps labels to their corresponding addresses.
- Prepares for the second pass by resolving labels and variables.
- Translates assembly instructions into binary using the symbol table created in the first pass.
- Replaces symbolic names with actual addresses.
The Hack Computer successfully executes applications written in Hack Machine Language.
Final Result: Pong running on the CPU Emulator.