xorvoid/sectorc

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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 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 ")" | indentifier | integer op = "+" | "-" | "&" | "|" | "^" | "<<" | ">>" | "==" | "!=" | "<" | ">" | "<=" | ">=" In addition, both // comment and /* multi-line comment */ styles are supported. (NOTE: This grammar is 704 bytes in ascii, 38% larger than it's 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 regaurdless 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.