Apple III

The Apple III was released by Apple Compute in 1980 as a business computer. It was the successor to the Apple II but failed to deliver. It featured a full 80-column display, a full-stroke keyboard with upper and lower case, ran on a Synertek 6502B processor at 1.8MHz and had 128KByte of RAM on board, upgradeable to 512KByte.

Because the Apple III failed to sell in large numbers, Apple reconsidered the plans to retire the Apple II, and instead took some of the features of the Apple III and incorporated them into later models of the Apple II

Apple's Sophisticated Operating System

Apple SOS (Sophisticated Operating System) was the disk operating system developed for the Apple III, released in 1980. Unlike Apple DOS on the Apple II, SOS was designed as a full-featured, modular operating system from the outset. It supported hierarchical directories, file attributes (read/write/locked, date/time stamps), and device-independent I/O through a standardized driver model. Written in 6502 assembly, SOS was structured with a kernel that provided file system services, a device driver layer, and an application interface based on system calls rather than patch tables. It could handle multiple file types, large volumes (up to 16 MB per disk), and offered random-access and record-oriented file operations, making it a much more business-oriented OS than its predecessor.

From a technical perspective, SOS treated all peripherals—including disks, screens, and serial devices—as logical files accessed through a uniform API. This abstraction allowed applications to be written in a hardware-agnostic way, simply opening and reading from devices as if they were files. Memory management was still bound by the 6502’s 64 KB address space, but SOS provided a well-defined application runtime environment with fixed entry points and system call conventions. Error handling and interrupt-driven device support were built in, ensuring higher stability than the patchwork approaches of Apple DOS. Although the Apple III hardware struggled with reliability, SOS itself was robust and forward-looking, influencing the later design of ProDOS and serving as Apple’s first real attempt at a professional-grade operating system.

CPU - The Motorola 6502

The 6502 is an 8-bit MicroProcessor designed by MOS Technology. The team was led by Chuck Peddle and had also worked on the Motorola 6800. The 6502 is a simplified, but faster and cheaper design than the 6800.

The 6502 was introduced in 1975 and was the cheapest microprocessor on the market. Together with the Zilog Z80, the 6502 helped start the home computer revolution of the 1980s. The 6502 was used in a wide range of devices: the Atari 2600, the 8-bit Atari home computers, the Apple II, the Nintendo Entertainment System, the Commodore 64, the BBC Micro and many others. All used the 6502 or a variation of it.

The 6502 is a 1MHz design, while the 6502A is designed for 2MHz. The 6502A is 100% compatible with the original 6502.

Commodore soon bought MOS Technology, but conitnued to sell the microprocessor to competitors and licensed the design to other manufacturers.

Source: WikiPedia - MOS Technology 6502

The 650x Instruction Set

Mnemonic	Operation	Description
ADC	Add with Carry	Adds a memory location and the carry bit to the accumulator.
AND	Logical AND	Performs a logical AND between memory and the accumulator.
ASL	Arithmetic Shift Left	Shifts all bits one position to the left (in memory or accumulator).
BCC	Branch on Carry Clear	Branches to a new address if the carry flag is clear (0).
BCS	Branch on Carry Set	Branches to a new address if the carry flag is set (1).
BEQ	Branch on Equal (Zero Set)	Branches to a new address if the zero flag is set (1).
BIT	Bit Test	Tests bits in memory with the accumulator without modifying the accumulator.
BMI	Branch on Minus	Branches to a new address if the negative flag is set (1).
BNE	Branch on Not Equal	Branches to a new address if the zero flag is clear (0).
BPL	Branch on Plus	Branches to a new address if the negative flag is clear (0).
BRK	Force Break	Forces an interrupt request and pushes status and PC to the stack.
BVC	Branch on Overflow Clear	Branches to a new address if the overflow flag is clear (0).
BVS	Branch on Overflow Set	Branches to a new address if the overflow flag is set (1).
CLC	Clear Carry Flag	Clears the processor carry flag.
CLD	Clear Decimal Mode	Clears the processor decimal mode flag.
CLI	Clear Interrupt Disable	Clears the interrupt disable flag, allowing maskable interrupts.
CLV	Clear Overflow Flag	Clears the processor overflow flag.
CMP	Compare Accumulator	Compares the contents of a memory location with the accumulator.
CPX	Compare X Register	Compares the contents of a memory location with the X register.
CPY	Compare Y Register	Compares the contents of a memory location with the Y register.
DEC	Decrement Memory	Subtracts one from the value held at a specified memory location.
DEX	Decrement X Register	Subtracts one from the X register.
DEY	Decrement Y Register	Subtracts one from the Y register.
EOR	Exclusive OR	Performs a logical Exclusive OR between memory and the accumulator.
INC	Increment Memory	Adds one to the value held at a specified memory location.
INX	Increment X Register	Adds one to the X register.
INY	Increment Y Register	Adds one to the Y register.
JMP	Jump	Sets the program counter to a new address.
JSR	Jump to Subroutine	Pushes the return address to the stack and jumps to a new address.
LDA	Load Accumulator	Loads a byte of memory into the accumulator.
LDX	Load X Register	Loads a byte of memory into the X register.
LDY	Load Y Register	Loads a byte of memory into the Y register.
LSR	Logical Shift Right	Shifts all bits one position to the right (in memory or accumulator).
NOP	No Operation	Performs no operation, consuming 2 machine cycles.
ORA	Logical Inclusive OR	Performs a logical OR between memory and the accumulator.
PHA	Push Accumulator	Pushes a copy of the accumulator onto the stack.
PHP	Push Processor Status	Pushes a copy of the status flags onto the stack.
PLA	Pull Accumulator	Pulls a byte from the stack into the accumulator.
PLP	Pull Processor Status	Pulls a byte from the stack into the processor status flags.
ROL	Rotate Left	Rotates all bits one position left through the carry flag.
ROR	Rotate Right	Rotates all bits one position right through the carry flag.
RTI	Return from Interrupt	Pulls the processor flags and program counter from the stack.
RTS	Return from Subroutine	Pulls the program counter from the stack and resumes execution.
SBC	Subtract with Carry	Subtracts memory and the inverse of the carry bit from the accumulator.
SEC	Set Carry Flag	Sets the processor carry flag.
SED	Set Decimal Flag	Sets the processor decimal mode flag.
SEI	Set Interrupt Disable	Sets the interrupt disable flag, preventing maskable interrupts.
STA	Store Accumulator	Stores the contents of the accumulator in memory.
STX	Store X Register	Stores the contents of the X register in memory.
STY	Store Y Register	Stores the contents of the Y register in memory.
TAX	Transfer Accumulator to X	Copies the current value of the accumulator into the X register.
TAY	Transfer Accumulator to Y	Copies the current value of the accumulator into the Y register.
TSX	Transfer Stack Pointer to X	Copies the current value of the stack pointer into the X register.
TXA	Transfer X to Accumulator	Copies the current value of the X register into the accumulator.
TXS	Transfer X to Stack Pointer	Copies the current value of the X register into the stack pointer.
TYA	Transfer Y to Accumulator	Copies the current value of the Y register into the accumulator.