Hack Assembly Horizontal Line Drawer

This project demonstrates how to draw a horizontal line on the Hack computer screen using both Hack assembly and C.
It includes low-level bitwise address computations and pixel manipulation in simulated screen memory.

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Overview

The Hack computer’s display is a bitmap memory region starting at address 16384 (SCREEN).
Each word (16 bits) represents 16 horizontal pixels, and the screen is 512×256 pixels, meaning each row has 32 words.

This project draws a horizontal line from coordinates (x1, y) to (x2, y) by:

1. Computing the start and end word addresses for a given row.
2. Generating bit masks to fill pixels between x1 and x2.
3. Writing to the Hack’s screen memory directly.

You’ll find both:

• /assembly_code → Hack assembly version
• /c_code → equivalent C implementation for clarity and testing

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Core Formula

Each pixel is stored in memory at:

$$\text{address} = \text{SCREEN} + 32 * y + (x / 16)$$ $$\text{bit} = x \ \text{mod} \ 16$$

Registers Used (Assembly)

Register	Purpose
R0	y-coordinate
R1	x1 (becomes x1 mod 16 after division)
R2	x2 (becomes x2 mod 16 after division)
R3	color (1 = white, 0 = black)
R4	start word address
R5	end word address
R6	loop counter
R7	temporary register (bit mask)

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How It Works

1. Multiply y by 32 to get the row offset.
2. Divide x1 and x2 by 16 to find the start and end words.
3. Add SCREEN base address to get absolute memory addresses.
4. Fill bits between x1 and x2 using set_pixel() function.

Hack Assembly Implementation

The Hack Assembly implementation should be tested inside the Nand2Tetris CPU emulator.

Version 1 contains more optimized however not memory saving approach, and Version 2 uses less registers, however speed is compromised.

The corresponding registers for x1, x2, y and color (1 for black, 0 for white) should be set.

C Implementation

The C version emulates Hack screen memory in an array and applies the same bitwise logic:

#define WIDTH 512
#define HEIGHT 256

#define WORDS_PR (WIDTH / 16)
#define SIZE (HEIGHT * WORDS_PR)

extern unsigned short	screen[SIZE]; // 8192 words, like in hack

void	clear_image(unsigned char color);
void	set_pixel(int x, int y, unsigned char color);
void	horizontal_line(int y, int x1, int x2, unsigned char color);
void	vertical_line(int y, int x1, int x2, unsigned char color);
void	write_bmp(const char *filename);

To test the C version simply compile the project:

make #compile the project

./draw_test #run the program

display output.bmp #display the result

Tools Used

• Nand2Tetris Hack Computer
• CPU Emulator
• GCC for the C version