The 6×09 microprocessors contain two stack pointer registers S, and U.

The microprocessor itself uses the S or System stack pointer for all of its operations which require stack.

The U or User stack pointer is available for the developer to use as their own stack.

void stacks(word sstack, word ustack) {
	asm {
		TFR S,D
		STD sstack
	}
	printf("S STACK = %4x\n", sstack);
	asm {
		TFR U,D
		STD ustack
	}
	printf("U STACK = %4x\n", ustack);
}

The Color Computer 3 also uses a Secondary Stack memory block for high-resolution drawing commands.

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