birb

smol stack synthesizer for embedded systems, single-header, based on work by viznut et al.

Running the demo

$ make music

Documentation

birb programs are arrays of values between 0x00 and 0x1F (5 bits), terminated with 0xFF (BIRB_END).

Programs operate on a stack, making the programs very RPN-like. Popping a value from an empty stack yields 0, and pushing a value onto a full stack overwrites the top value. The default maximum stack size is 64 but can be customized with BIRB_STACK_SIZE.

When evaluating programs, there is a special T value set which can be referenced from the birb program. This is intended to be used to produce time-varying output for synthesizing sounds.

After hitting the end of the program, top value on the stack is popped and returned from birb_eval (or 0 if the stack was empty).

Opcode Reference

Most opcodes pop 2 values from the stack (x then y) and push a single value back:

• BIRB_SHR: y >> x
• BIRB_SHL: y << x
• BIRB_AND: y & x
• BIRB_OR: y | x
• BIRB_XOR: y ^ x
• BIRB_ADD: y + x
• BIRB_SUB: y - x
• BIRB_MUL: y * x
• BIRB_DIV: y / x, or 0 if x == 0
• BIRB_MOD: y % x, or 0 if x == 0
• BIRB_DIG: (x << 4) | y. Useful for constructing larger constants, e.g. 4 3 2 1 DIG DIG DIG yields 0x1234

Other opcodes that do not follow the above pattern:

• 0x0 - 0xF: Push the given opcode onto the stack
• BIRB_T: Push the special variable T onto the stack
• BIRB_U: Push the special variable U onto the stack
• BIRB_SWP: Swap the position of the top two values on the stack
• BIRB_DUP: Duplicate the top value on the stack
• BIRB_RND: Pop x from the stack, then push hash(x) back