The Acorn System 1 or the Acorn Micro-Computer

The Acorn System 1 was an early 8-bit computer kit for hobbyists. It was based on the MOS 6502 Architecture and manufactured by Acorn starting 1979. The main design was done by the Cambridge-undergraduate student Sophie Wilson, and the cassette interface was designed by Steve Furber. This was Acorn's first computer.

Features

Seven segment LED display
25 hexadecimal and function keys
Cassette CUTS interface
6502 CPU running at 1MHz
INS8154 RAMIO integrated circuit
2x2114 1024x4 RAM chips
2x74S571 512x4 PROM chip
Optional INS8154 RAMIO Expansion Chip
Socket for optional additional ROM or EEPROM
7.5V external power supply, 5V on-board regulator
Two stacked circuit boards connected by a semi-flexible multi-conductor cable (Spectra Strip)

Acorn System VDU board

The Acorn Visual Display Unit Controller Board connects to the Standard Acorn Computer Bus and contains a memory mapped character storage R.A.M. which is transparently written to or read from by the C.P.U.

An MC6845 programmable controller I.C. provides all the synchronisation signals to drive a 625 line 50 fields per second V.D.U. together with read addresses for the character R.A.M. Characters are then fed to an SAA5050 character generator IC which produces the necessary dot patterns to create the characters to refresh the V.D.U.

The SAA5050 produces Teletext standard characters and has Red, Green and Blue drive outputs giving coloured characters or graphics.

The R.G.B. and sync outputs may be used to drive a colour encoder and modulator for a U.H.F. Television; There is also a 1 volt/75 ohm composite sync and video output which can directly drive a Monochrome Monitor on which the different colours will appear as different scales of grey.

The VDU board came with listings for programs which set up the MC6845, display 25 instructions in hex on the V.D.U. (with double or treble byte instructions on a single line) and allow the drawing of graphics or characters on the V.D.U. These programs may be loaded and run using the Acorn System 1 Monitor. Later new monitor ROM was able to link the VDU and an ASC 11 keyboard to Acorns' 4K Fast BASIC.

The versatility of the programmable MC6845 and the SAA5050 combination may be employed to give other screen formats e. g. 80 characters x 16 lines and double height characters. Thus the Acorn VDU was of great value to experimenters and producers of specialist display systems. The VDU controller PCB was supplied in kit form with a full set of I.C. sockets. It was easily assembled using a small soldering iron and useful hints on assembly may be found in the Acorn Micro-computer System 1 Technical Manual. The board operates from a single +5v supply from which it draws not more than 500 mA.

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.