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https://github.com/xorvoid/sectorc

A C Compiler that fits in the 512 byte boot sector of an x86 machine
https://github.com/xorvoid/sectorc

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A C Compiler that fits in the 512 byte boot sector of an x86 machine

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README 

        
# 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:



```

6gUAwAdoADAfaAAgBzH/6DABPfQYdQXoJQHr8+gjAVOJP+gSALDDqluB+9lQdeAG/zdoAEAfy+gI

AegFAYnYg/hNdFuE9nQNsOiqiwcp+IPoAqvr4j3/FXUG6OUAquvXPVgYdQXoJgDrGj0C2nUGV+gb

AOsF6CgA68Ow6apYKfiD6AKrifgp8CaJRP7rrOg4ALiFwKu4D4Srq1fonP9ewz2N/HUV6JoA6BkA

ieu4iQRQuIs26IAAWKvD6AcAieu4iQbrc4nd6HkA6HYA6DgAHg4fvq8Bra052HQGhcB19h/DrVCw

UKroWQDoGwC4WZGrW4D/wHUMuDnIq7i4AKu4AA+ridirH8M9jfx1COgzALiLBOucg/j4dQXorf/r

JIP49nUI6BwAuI0G6wyE0nQFsLiq6wa4iwarAduJ2KvrA+gAAOhLADwgfvkx2zHJPDkPnsI8IH4S

weEIiMFr2wqD6DABw+gqAOvqicg9Ly90Dj0qL3QSPSkoD5TGidjD6BAAPAp1+eu86Ln/g/jDdfjr

slIx9osEMQQ8O3QUuAACMdLNFIDkgHX0PDt1BIkEMcBaw/v/A8H9/yvB+v/34fb/I8FMAAvBLgAz

wYQA0+CaANP4jwCUwHf/lcAMAJzADgCfwIUAnsCZAJ3AAAAAAAAAAAAAAAAAAAAAAAAAAAAAVao=

```



## 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



![Moving Sinwave](img/sinwave.png)



## 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   = 

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](https://xorvoid.com/sectorc.html)



## Why?



In 2020, cesarblum wrote a Forth that fits in a bootsector: ([sectorforth](https://github.com/cesarblum/sectorforth))



In 2021, jart et. al. wrote a Lisp that fits in the bootsector: ([sectorlisp](https://github.com/jart/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.