The block diagram of the ATOM shows the essential elements of a micro-computer system. The 6502 micro-processor is provided with a 1 MHz clock with which all action on its bus is synchronised. The bus has 8 data connections which allow data to be transferred from the processor to other devices or vice-versa. 16 address lines select the location to which or from which data is transferred; the most significant lines feed the chip select logic which selects the device being accessed whilst the rest of the lines select the location within the device.

The reset signal at break or power up causes the processor to fetch instructions down the data bus from the Operating System/BASIC ROM (Read Only Memory). Executing these instructions sets up the screen and scans the keys, and when BASIC statements are entered more instructions in the ROM interpret the statements and communicate with the screen, tape interface etc. using routines in the O.S. section of the ROM.

The block zero RAM (Random Access Memory), from #0000 to #0400, is used by the processor for scratch pad and stack menory. This RAM should not normally be accessed by the user.

The user RAM, that is the space in which lines of BASIC text are stored, can be the lower text space or, as in the unexpanded ATOM, the upper text space or graphics space may be used.

A Video Display Generator (VDG) produces synchronisation signals for the television or monitor picture scan in time with a series of refresh addresses which fetch data for the screen from the video RAM. The screen is "memory mapped"; each dot on the screen shows the information in a corresponding piece of memory. Normally only the VDG accesses the video RAM obtaining information with which to refresh the screen. When the information on the screen is to be changed the tri-state buffers are enabled allowing the processor to write new data into the RAM. The VDG is fed with a 3.58 NHz frequency from which the sync and refresh timing is derived. Both monochrome and colour information are provided by the VDG.

In the most basic ATOM the 1024 by 8 (that is 1 Kbyte) video RAM is divided into two sections. Half of the RAM is used for video so that a screen full of normal characters can be stored and the rest is used for the upper text space. Extended ATOMs will normally have RAM in the lower text space position and so the original video RAM together with the extension video RAM is used to provide the higer resolution graphics modes.

Another device on the bus is a peripheral interface which provides 24 input and output signal lines. Some of these lines are used to control the VDG and one in particular is a signal from the VDG indicating the vertical blanking period of the screen. During this period the processor can enable the tri-state buffers and access the video RAN without producing interference on the screen. This signal also provides the timinq for the WAIT statement.

The INS8255 PIA interfaces with the ATOM keyboard in which the keys are arranged in a matrix. A 4 to 10 line decoder-driver is driven by the peripheral interface and its 10 outputs drive each row of the key matrix in turn. The peripheral interface is used to read the outputs from the columns of the key matrix as each input is driven and so the matrix position of any depressed key is determined.

A Computer Users Tape Standard interface is also connected via the peripheral interface. This standard saves data or programs on tape as a sequence of two different tones . A logical 1 signal is represented by a frequency of 2.4 KHz whilst logical 0 is a 1.2 KHz frequency. Each bit, that is 0 or 1, lasts for 3.3 mS giving an operating speed of 300 bits per second. The 2.4 KHz standard is derived from the 4 MHz crystal with a frequency divider chain and it is fed to the peripheral interface and the tape output. The input from the tape recorder passes through an amplifier and a schmitt trigger before being fed into peripheral interface. Another line from the peripheral interface is used to drive the loudspeaker.

Two more ROMs may be fitted in the ATOM. One contains instructions is for interpreting additional BASIC statements which deal with floating point numbers, scientific functions and colour graphics. The other ROM can provide extra utilities, such as programs to deal with serial interfaces to the ATOM, and the communications ring.

An optional Versatile Interface Adaptor (VIA) may be fitted giving input or output lines allowing the ATOM to interface to external hardware. Also a driver device can be added allowing the VIA to connect to parallel printers of the Centronics type.

Finally a set of bus buffers may be added allowing the ATOM bus to connect to external devices such as extension RAM floppy disc drives etc.