Overview

This is a 16-bit CPU that I built some time ago (around the end of 2021) as a project to better understand how computers work. It works through the Logisim circuit simulator. To use it, just import the CPU.circ file into Logisim.

In addition to the CPU design, I have also written a very basic Assembly language to make writing code easier. The source files should end with .bones and the compiled files end with .sesh.
To use these files, with Logisim open, right click over the RAM component and click on "Load Image", then import the .sesh file into it.

Architecture

The CPU has 4 general use registers and 1 address register. The address register is used in SAVE and LOAD operations. The memory contains 16384 addressable addresses, with each address being able to hold 16 bits of information. It also offers out-of-the-box support for a 32x32 display and a keyboard to serve as input. There are 23 instructions available, ranging from logical operations to instructions related to the interface.

Assembly Language

The CPU instruction set is very simple, and it's composed of 23 commands. Here is a quick explanation of these:

ALU Operations

The commands that fall under this category are:

Command	Syntax	Description	Binary output	Hex output
ADD	ADD Ra, Rb, Rdest	Adds Ra to Rb and stores the result in Rdest	0100 1000 RaRb 00DD	0x48xy
SUB	SUB Ra, Rb, Rdest	Subtract Rb from Ra and stores the result in Rdest	0100 1001 RaRb 00DD	0x49xy
MUL	MUL Ra, Rb, Rdest	Multiply Ra to Rb and stores the result in Rdest	0100 1010 RaRb 00DD	0x4Axy
DIV	DIV Ra, Rb, Rdest	Divides Ra by Rb and stores the result in Rdest	0100 1011 RaRb 00DD	0x4Bxy
STPU	STPU Rn, Rdest	Increases Rn by 1 and stores the result in Rdest	0100 0101 00RR 00DD	0x45xy
STPD	STPD Rn, Rdest	Decreases Rn by 1 and stores the result in Rdest	0100 0110 00RR 00DD	0x46xy
NOT	NOT Rn, Rdest	Negates Rn and stores the result in Rdest. If Rn is 1, output is 0, else output is FFFF - (Rn - 1)	0100 0111 00RR 00DD	0x47xy
COPY	COPY Rn, Rdest	Copies the value from Rn into Rdest	0100 1111 00RR 00DD	0x4Fxy

JUMP Operations

JUMP operations modify the Program Counter (PC) if a specific condition is met. In this tiny Assembly language, the destiny (or new PC value) is given by the value of the address register. The value of the address register is modified by calling the ADDRESS command. Usually, a jump operation will look like the following:

ADDRESS 0x0009;  // Loads the value 0x0009 in the address register
JLT R1, R3; // Jump to 0x0009 if the value of R1 is smaller than R3

Command	Syntax	Description	Binary output	Hex output
JGT	JGT Ra, Rb	Modify PC to the value of the address register if Ra is greater than Rb	1010 0000 R1R2 0000	0xA0x0
JLT	JLT Ra, Rb	Modify PC to the value of the address register if Ra is smaller than Rb	1010 0001 R1R2 0000	0xA1x0
JNQ	JNQ Ra, Rb	Modify PC to the value of the address register if Ra is different from Rb	1010 0010 R1R2 0000	0xA2x0
JEQ	JEQ Ra, Rb	Modify PC to the value of the address register if Ra is equal to Rb	1010 0011 R1R2 0000	0xA3x0

RAM modifiers

Memory modifier operations act by modifying the RAM. They are usually written in the following fashion:

ADDRESS 0x001A;  // Loads the value 0x0009 in the address register
LOAD R0; // Save the value of the of the 0x0009 RAM address into the R0 register.

CONST 0x0005, 0x001A; // Writes the value 0x5 in the 0x001A address of the RAM.

Command	Syntax	Description	Binary output	Hex output
ADDRESS	ADDRESS addr	Stores the addr value into the address register	11AA AAAA AAAA AAAA	0xwxyz
SAVE	SAVE Rn	Saves the value of the Rn register into the memory address pointed by the address register	1000 1000 0000 00SS	0x880x
LOAD	LOAD Rn	Loads the value present in the memory address pointed by the address register into the Rn register	1000 0000 0000 00DD	0x800x
CONST	CONST value, loc	Saves the constant value CONST in the loc address of the RAM	value	value

Interface

In order to create a form of user interface, a set of graphical commands were incorporated right into the CPU instructions.

In the following table, Rx and Ry represent X and Y coordinates of a 2D Cartesian plane.

Command	Syntax	Description	Binary output	Hex output
START	START	Turns on the RGB video device	1011 1000 0000 0000	0xb800
PAINT	PAINT Rx, Ry	Paint a white pixel in the X, Y coordinates indicated by Rx and Ry	1011 11XX XXXY YYYY	0xBxyz
ERASE	ERASE Rx, Ry	Erases any pixel existent in the X, Y coordinates indicated by Rx and Ry	1011 00XX XXXY YYYY	0xBxyz
SOUND	SOUND freq	Emits a beep with the specified frequency	1001 FFFF FFFF FFFF	0x9xyz
BUFF	BUFF Rn	Reads a character from the keyboard buffer and stores it into the Rn register	Binary output	Hex output

Commands with no arguments

In this category fall the two instructions composed by a single instruction alone. The PASS instruction is meant just to make some sort of visual code segmentation possible.

Command	Syntax	Description	Binary output	Hex output
HALT	HALT	Stops the program execution	0000 1111 0000 1111	0x0F0F
PASS	PASS	Does nothing	0000 0000 0000 0000	0x0000