SectorC

SectorC is a C compiler written in x86-16 assembly that fits within the 512 byte boot sector of an x86 machine. It supports a subset of C that is large enough to write real and interesting programs. It is quite likely the smallest C compiler ever written.

In a base64 encoding, it looks like this:
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=

Supported language

A fairly large subset is supported: global variables, functions, if statements, while statements, lots of operators, pointer dereference, inline machine-code, comments, etc. All of these features make it quite capable.

For example, the following program animates a moving sine-wave:

int y;
int x;
int x_0;
void sin_positive_approx()
{
  y = ( x_0 * ( 157 - x_0 ) ) >> 7;
}
void sin()
{
  x_0 = x;
  while( x_0 > 314 ){
    x_0 = x_0 - 314;
  }
  if( x_0 <= 157 ){
    sin_positive_approx();
  }
  if( x_0 > 157 ){
    x_0 = x_0 - 157;
    sin_positive_approx();
    y = 0 - y;
  }
  y = 100 + y;
}


int offset;
int x_end;
void draw_sine_wave()
{
  x = offset;
  x_end = x + 314;
  while( x <= x_end ){
    sin();
    pixel_x = x - offset;
    pixel_y = y;
    vga_set_pixel();
    x = x + 1;
  }
}

int v_1;
int v_2;
void delay()
{
  v_1 = 0;
  while( v_1 < 50 ){
    v_2 = 0;
    while( v_2 < 10000 ){
      v_2 = v_2 + 1;
    }
    v_1 = v_1 + 1;
  }
}

void main()
{
  vga_init();

  offset = 0;
  while( 1 ){
    vga_clear();
    draw_sine_wave();

    delay();
    offset = offset + 1;
    if( offset >= 314 ){ // mod the value to avoid 2^16 integer overflow
      offset = offset - 314;
    }
  }
}

Screenshot

Provided Example Code

A few examples are provided that leverage the unique hardware aspects of the x86-16 IBM PC:

• examples/hello.c: Print a text greeting on the screen writing to memory at 0xB8000
• examples/sinwave.c: Draw a moving sine wave animation with VGA Mode 0x13 using an appropriately bad approximation of sin(x)
• examples/twinkle.c: Play “Twinkle Twinkle Little Star” through the PC Speaker (Warning: LOUD)

Grammar

The following grammar is accepted and compiled by sectorc:

program     = (var_decl | func_decl)+
var_decl    = "int" identifier ";"
func_decl   = "void" func_name "{" statement* "}"
func_name   = <identifier that ends in "()" with no space>
statement   = "if(" expr "){" statement* "}"
            | "while(" expr "){" statement* "}"
            | "asm" integer ";"
            | func_name ";"
            | assign_expr ";"
assign_expr = deref? identifier "=" expr
deref       = "*(int*)"
expr        = unary (op unary)?
unary       = deref identifier
            | "&" identifier
            | "(" expr ")"
            | identifier
            | integer
op          = "+" | "-" | "&" | "|" | "^" | "<<" | ">>"
            | "==" | "!=" | "<" | ">" | "<=" | ">="

In addition, both // comment and /* multi-line comment */ styles are supported.

(NOTE: This grammar is 704 bytes in ascii, 38% larger than its implementation!)

How?

See blog post: SectorC: A C Compiler in 512 bytes

Why?

In 2020, cesarblum wrote a Forth that fits in a bootsector: (sectorforth)

In 2021, jart et. al. wrote a Lisp that fits in the bootsector: (sectorlisp)

Naturally, C always needs to come and crash (literally) every low-level systems party regardless of whether it was even invited.

Running

Dependencies:

• nasm for assembling (I used v2.16.01)
• qemu-system-i386 for emulating x86-16 (I used v8.0.0)

Build: ./build.sh

Run: ./run.sh your_source.c

NOTE: Tested only on a MacBook M1

What is this useful for?

Probably Nothing.

Or at least that's what I thought when starting out. But, I didn't think I'd get such a feature set. Now, I'd say that it might be useful for someone that wants to explore x86-16 bios functions and machine model w/o having to learn lots of x86 assembly first. But, then again, you should just use a proper C compiler and write a tiny bootloader to execute it.