Decrement, BNZ LOOP2, Return to 6502 Mode

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:

--------------------------

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

II specifications

The Apple II, launched in April 1977, was one of the first highly successful mass-produced microcomputer products. It marked a significant leap in personal computing, setting standards for future developments in the industry. Created by Steve Wozniak and Steve Jobs, the Apple II differentiated itself with its user-friendly design, appealing aesthetics, and robust capabilities.

One of the standout features of the Apple II was its open architecture, which allowed users to expand and enhance the computer's functionality. This design enabled hundreds of third-party hardware and software developers to contribute to its ecosystem, resulting in an array of peripherals, including printers, modems, and storage devices. The Apple II utilized a MOS Technology 6502 microprocessor running at a clock speed of 1 MHz. Initially equipped with 4 KB of RAM, the machine could be expanded to 48 KB, accommodating more complex applications and programs.

The Apple II was also notable for its colorful graphics. It was one of the first computers to support color display, offering a 6-color palette with a resolution of 280x192 pixels in 16 colors when using its Color Graphics Card. This feature significantly enhanced the visual appeal of games and educational software developed for the platform, making computing more accessible and entertaining for various audiences.

Apple's commitment to user experience was evident in the design of the machine. It featured an integrated keyboard and a plastic case, which was both durable and visually appealing. The self-contained design included drive bays for floppy disk drives, allowing for quicker data access than traditional tape drives. It also supported audio output, enabling sound effects and music, a novelty at the time.

The introduction of the Apple DOS operating system further underscored the machine's capabilities. DOS streamlined file management and made it easier for users to navigate and manage their data. The combination of hardware and software positioned the Apple II as an educational tool and a gaming platform, fostering a vibrant software ecosystem.

The Apple II family continued to evolve, with variations like the Apple II+, IIe, and IIgs being introduced over the years. These iterations brought enhancements in memory, processing power, and graphics capabilities. The legacy of the Apple II endures, not only as a foundational product in personal computing but also as a symbol of innovation that paved the way for future advancements in technology. Its impact is still felt today, as it inspired countless developers and shaped the trajectory of the computer industry.

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

Models: II