• Understanding Basic Computer Engineering
• Understanding Binary

Objective: to gain a deeper understanding of how a CPU functions at a low level.

We're going to write an emulator for the world-famous LambdaSchool-8 computer, otherwise known as LS-8! This is an 8-bit computer with 8-bit memory addressing, which is about as simple as it gets.

An 8 bit CPU is one that only has 8 wires available for addresses (specifying where something is), computations, and instructions. With 8 bits, our CPU has a total of 256 bytes of memory and can only compute values up to 255. The CPU could support 256 instructions, as well, but we won't need them.

We'll design a memory subsystem, an ALU, and the main CPU emulator code in JS. Then we'll write code that loads a file of machine code instructions from disk, parses the file, and loads it into memory. After that, we'll start the emulator running and see it execute instructions!

For starters, we'll execute code that stores the value 8 in a register, then prints it out:

# print8.ls8

00000100 # LDI R0,8
00000000
00001000
00000110 # PRN R0
00000000
00011011 # HALT

The value on the left is the machine code value of the instruction (opcode) or its immediate argument(s) (the operands).

See the LS-8 Spec for complete CPU details. Note that the spec is a complete document, and not all its pieces need to be implemented to complete this assignment. You only need to implement the instructions referenced below.

Extend your LS8 emulator to support the following program:

# mult.ls8

00000100 # LDI R0,8
00000000
00001000
00000100 # LDI R1,9
00000001
00001001
00000101 # MUL R0,R1
00000000
00000001
00000110 # PRN R0, should print 72
00000000
00011011 # HALT

Your goal is to write a simple CPU that supports the above instructions. You will need to read the file (as an argument or a stream) via NodeJS into an array of memory addresses (RAM). Then you will create a program counter (PC) that points to the index of the current instruction, reads it, decodes it, and executes it. You should use setInterval() to create a timer (the clock) and execute one instruction per clock tick.

When the CPU processes a HALT instruction, use clearInterval() to stop the clock so that NodeJS exits.

You'll need to add the functionality for MUL (multiply the following two registers together and store the result in the current register) and PRN (print the current register as a numeric value.)

Supported instructions:

LDI    Load Immediate. This loads a numeric value into a register

MUL    Multiply the values in two registers together, and store the
       result in the first register

PRN    Print Numeric--console.log the integer value of the active register

HALT   Stop the CPU (and exit the emulator)

The following command line input:

node ls8 mult.ls8

Should produce

72

through the process of executing the machine code in the input file.

All CPUs manage a stack that can be used to store information temporarily. This stack resides in main memory and typically starts at the top of memory (at a high address) and grows downward as things are pushed on. The LS-8 is no exception to this.

• Implement a system stack per the spec. Add PUSH and POP instructions. Read the beginning of the spec to see which register is the stack pointer, and where the stack starts in memory.

Back the my day, functions were called subroutines. In machine code, this enables you to jump to an address with the CALL instruction, and then return back to where you called from with the RET instruction. This enables you to create reusable functions.

The stack is used to hold the return address, so you must implement the stack, above, first.

• Add subroutine calls. CALL and RET.