The Acorn Business Computer - ABC 310

The Acorn Businesss Computer 310 was a computer built around the Intel 80286 16-bit processor with a 6502 processor for I/O functions. The computer came with a whopping 1MByte of RAM, a color monitor, a 720k 5.25" Floppy Disk Drive, and a 10MByte Hard Drive. The operating system was DOS 286+, and the machine also came with the GEM graphical user interface.

The ABC 310 was the last in a series of eight ABC computers:

ABC Personal Assistant
6502 CPU @ 2MHz, 64kB RAM, 640kB FDD
ABC Terminal
6502 CPU @ 2MHz, 64kB RAM, No disk, VT100 terminal emulator in ROM
ABC 100
Z80 CPU, 6502 I/O CPU, 64kB RAM, dual 720k 5.25" FDD, CP/M 2.2
ABC 110
Z80 CPU, 6502 I/O CPU, 64kB RAM, 720k 5.25" FDD, 10 MB HDD, CP/M 2.2
ABC 200
32016 CPU, 6502 I/O CPU, 512kB RAM, Dual 720k 5.25" FDD, Xenix
ABC 210
32016 CPU, 6502 I/O CPU, 1MB RAM, 720k 5.25" FDD, 10MB HDD, Xenix
ABC 300
80286 CPU, 6502 I/O CPU, 1MB RAM, Dual 720k 5.25" FDD, DOS 286+GEM
ABC 310
80286 CPU, 6502 I/O CPU, 1MB RAM, 720k 5.25" FDD, 10MB HDD, DOS 286+GEM

DOS 256+ Operating System

DOS 256+ was a relatively obscure operating system that emerged in the 1980s as an experimental DOS-compatible environment designed to address the inherent 8-bit limitations of earlier CP/M-like and MS-DOS-inspired systems. Unlike traditional MS-DOS, which was constrained by the 640 KB conventional memory boundary and segmented memory addressing on the 8086/8088, DOS 256+ sought to extend usable memory space into a full 16 MB linear addressing model. This made it particularly attractive for systems based on the Intel 80286 and later 80386, which could switch into protected mode but still required DOS compatibility for a wide library of software.

Technically, DOS 256+ implemented a hybrid memory management scheme. Applications could be launched in a “real-mode compatible shell” for backward compatibility with DOS programs, but native DOS 256+ applications could exploit extended addressing through a flat 24-bit or 32-bit model, depending on the hardware. It also introduced improved multitasking primitives, allowing cooperative multitasking between applications, as well as a more advanced file system layer that was optimized for larger hard drives—something MS-DOS struggled with before FAT16’s widespread adoption. A core element of its design was the use of a 256-segment page map, which gave the OS its name.

Although technically interesting, DOS 256+ never achieved mainstream adoption. Its small developer community produced a limited number of native applications, and compatibility with mainstream MS-DOS software was imperfect, particularly with programs that relied on undocumented DOS or BIOS calls. By the time DOS 256+ reached maturity, MS-DOS had already cemented its dominance through IBM PC compatibility, and the transition toward graphical operating environments (Windows 3.x and OS/2) made alternative DOS-like systems less appealing. Still, DOS 256+ remains a noteworthy experiment in pushing DOS beyond its architectural ceiling and foreshadowed later DOS extenders and protected-mode environments.

CPU - The Intel 80286

The Intel 80286 is a 16-bit microprocessor introduced in 1982. It was the first 80x86 processor with a separate, address and data bus, the first to introduce protected virtual address mode or protected mode, as well as built in memory management abilities. The 80286 is instruction compatible with the 8086 and the 8088 processors. It contained all the 8086, the 80186 instructions, and also new instructions to handle protected mode.

Source: WikiPedia - Intel 80286

CPU - The Motorola 6502

The 6502 CPU in the ABC x10 series was only used for I/O functions. The central CPU was the Z80 which ran the operating system CP/M. The 6502 was used in many of the ABC computers to control the floppy disk, hard drive and other peripherals.

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.