Bit banger is built around an ATtiny15 microcontroller, which runs at 1.6 MHz and has 1 kB of flash ROM and a claustrophobic 32 bytes of RAM. In fact, those 32 bytes are the CPU registers. Only the most basic AVR instructions are supported; they occupy at least two bytes each, and can obviously not be compressed since they are executing from ROM, so a maximum of 512 instructions will fit inside the chip (fewer if static data is needed).
The microcontroller supports interrupts, but they would have been too costly to use. Instead, the entire demo is cycle counted.
At a clock rate of 1.6 MHz, the visible part of each line of the VGA signal swooshes by in exactly 36 clock cycles. The entire line, including horizontal blanking, is 51 clock cycles wide. During this time, both graphics and sound must be generated.
I quickly arrived at the following overall design: Three registers make up a 24-bit frame buffer, organized as a 3×8 grid. Every 60 raster lines, these registers are rotated one bit, to prepare for the next row of the grid. At three different positions along the visible part of the line, the MSB of the corresponding frame buffer register is interpreted as an instruction to either keep or invert the current colour; the resulting colour is then transmitted onto an output pin. At the end of the visible line, black is selected.
In the gaps between these four positions and the two places where the horizontal sync signal is flipped, sound must be generated and emitted. The ATtiny15 luckily has a PWM output that runs on a separate peripheral clock at a staggering 25.6 MHz, which is high enough for 8-bit audio output. Writing a sample to the PWM output is a simple one-cycle instruction; the challenge is to calculate the value of the sample during the remaining clock cycles.
hardcore+BA, and the song is a straight-up jam. Happy Friday!