Mysterious Program in EPROM of Synertek SYM-1 Owned by Composer Richard Teitelbaum

ssp2.asm

{
	via3.DDRA = 1;
	via3.DDRB = 0xff; // output
	via3.PCR = 1; // CA1: Independent interrupt input-negative edge

	via2.DDRA = 0;	// input
	via2.DDRB = 0xff;	// output
	via2.PCR = 0x88;	// CA & CB: Handshake output, negative active edge

	bool moreBits = true;
	int count16 = 0;
	int inputByte0 = 0;
	int inputByte1 = 0;
	int shift0 = 0;
	int shift1 = 0;
	int bitCount = 8;

	while(true) {

		// wait until via3.B is ready
		while ((via3.IFR >> 1 & 1) == 0) {}
		via3.B = ~via3.B;
		int inputBit = via3.A & 0x80;

		int bits = 0;
		if (inputByte0 < 0)
			bits |= (1<<0);
		if (inputByte1 < 0)
			bits |= (1<<1);
		while ((via3.IFR & 0x10) == 0) {}
		via3.B ^= bits;

		shift0 <<= 2;
		shift1 <<= 2;
		if ((via3.A & 0x80) != inputBit) {
			shift0 |= 1;
			shift1 |= 1;
		}

		if ((moreBits) && (shift0 == 0xfd)) {
			count16 = 0;
			moreBits = false;
		}
		else {
			bitCount --;
			inputByte0 <<= 1;
			inputByte1 <<= 1;
			if (bitCount != 0)
				continue;
		}
		count16 += 1;
		if (count16 == 16) {
			if (shift1 != 0xfd) {
				moreBits = true;
			}
			count16 = 0;
		}
		via2.B = shift1;
		shift1 = 0;
		bitCount = 8;

		while ((via2.IFR & 2) == 0) {}
		inputByte0 = via2.A;

		while ((via2.IFR & 2) == 0) {}
		inputByte1 = via2.A;
	}
}