Takes advantage of the decrement instruction setting the zero-flag so no explicit test instructions are necessary.

This modified excerpt was taken from the test ROM that comes with the GameBoy bgb emulator. This example is a counter that repeats a specified number of times based on what is loaded into registers D and E (D being the High byte), but the countdown iterates in an interesting way. Rather than counting down the usual way (2000, 19FF, 19FE, ..., 0002, 0001, 0000), it iterates as (2000, 20FF, 20FE, ... 0102, 0101, 0000). The DEC instruction automatically tests for a zero result. By keeping the higher byte non-zero until the very end, the zero flag on the high byte is able to indicate the end of loop. This algorithm handles all of the zero testing implicitly without having to use additional test instructions.

There is a function call at the beginning of the routine. It simply increments the high byte if the lower byte is non-zero. This is just offsetting it so that the high-byte will be non-zero until the desired count is complete.

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0160	11 00 20	LD DE, 2000		// Load 0x2000 (count) into register DE
.
.
03B7	CD DD 03	CALL 03DD
...DO STUFF
03BB	1D		DEC E			// Decrement E
03BC	20 FC		JR NZ, FC		// Jump to 03BA if E not zero
03BE	15		DEC D			// Decrement D
03BF	20 F9		JR NZ, F9 		// Jump to 03BA if D not zero
03C1	C9		RET
.
.
//*FUNCTION* Increments D(high byte) if E(low byte) not zero
03DD	F5		PUSH AF
03DE	7B		LD A, E
03DF	B7		OR A			// Set the Z flag if A == 0
03E0	28 01		JR Z, 01		// Jump to 03E3 (skip next instruction) if zero
03E2	14		INC D			// Increment D
03E3	F1		POP AF
03E4	C9		RET
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