Apple II manual Decrement, BNZ LOOP2, Return to 6502 Mode, Branch Always

DECREMENT:

DCR Rn [ Fn ]

The contents of Rn are decremented by 1. The carry is cleared and other branch conditions reflect the decremented value.

EXAMPLE: (Clear 9 bytes beginning at location A034)

15	34	A0		SET	R5	$A034	;Init pointer
14	09	00		SET	R4	9	;Init counter
B0				SUB	R0		;Zero ACC
55			LOOP2	ST	@R5		;Clear a mem byte
F4				DCR	R4		;Decrement count
07	FC			BNZ	LOOP2		;Loop until Zero

Non-Register Instructions:

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RETURN TO 6502 MODE:

RTN 00

Control is returned to the 6502 and program execution continues at the location immediately following the RTN instruction. the 6502 registers and status conditions are restored to their original contents (prior to entering SWEET 16 mode).

BRANCH ALWAYS:

BR ea [ 01 d ]

An effective address (ea) is calculated by adding the signed displacement byte (d) to the PC. The PC contains the address of the instruction immediately following the BR, or the address of the BR op plus 2. The displacement is a signed two's complement value from -128 to +127. Branch conditions are not changed.

NOTE: The effective address calculation is identical to that for 6502 relative branches. The Hex add & Subtract features of the APPLE ][ monitor may be used to calculate displacements.

d = $80 ea = PC + 2 - 128 d = $81 ea = PC + 2 - 127

d = $FF d = $00 d = $01

d = $7E d = $7F

EXAMPLE:

ea = PC + 2 - 1 ea = PC + 2 + 0 ea = PC + 2 + 1

ea = PC + 2 + 126 ea = PC + 2 + 127