Acorn Atom hardware

The screen, colour, graphics and sound hardware of each machine that runs Atom BASIC, and where machine-code and data blocks live in its memory.

Acorn Atom

Screen modes

The Atom's MC6847 video chip is driven with CLEAR, which selects a mode and clears it: CLEAR 0 is the 32×16 text screen, while CLEAR 1–4 select graphics modes of increasing resolution, up to 256×192 in CLEAR 4.

Colour

The Atom's BASIC modes are monochrome — PLOT's modes set, clear or invert a point rather than choosing a colour (the Atom has no colour list like the BBC).

Graphics

MOVE <x>, <y> positions the graphics cursor without drawing, DRAW <x>, <y> draws a line from it (drawing to the cursor's own point plots a single dot), and PLOT <action>, <x>, <y> plots with an action that controls whether the point is set, cleared or inverted. The origin is the bottom-left of the screen.

Sound

The Atom drives its internal loudspeaker by toggling a port bit at #B002. Real Atom BASIC does this with the ? indirection operator, which this dialect does not yet implement (see the notes on the reference page), so there are no sound commands here.

Memory

The whole of the machine's address space, region by region. Zoom in to open a band into the parts it groups, and select a region for its addresses and what sits there.

The Atom is a small machine even by 1980 standards, and the map shows why: it shipped with 2K of RAM and takes 12K at most without an off-board expansion. That 12K is three separate runs — 1K of workspace at the bottom of memory, 5K of internal RAM in the middle, and 6K of video RAM for the MC6847 — with unfitted address space in between. BASIC gets the middle run: the floating-point variables take its first page, leaving 0x2900 to 0x3BFF, 4,864 bytes, for your program and its variables. The byte counter in the status bar is measured against that.

An Atom program can load fixed-address machine code or data — memory blocks — into RAM alongside the BASIC program before it runs. A block shares the BASIC program's window, 0x2900 to 0x3BFF; new blocks default to 0x3800, in the last 1K of it and so above a typical program. Nothing above 0x3BFF will do: that address space is unfitted, and a write there is simply dropped.

Blocks travel with the document through the project bundle and through share links, and can arrive on import: an .atm that loads somewhere other than #2900 (where BASIC text lives) is treated as a machine-code or data file, so its payload comes in as a block at its load address.

On Run the IDE refuses to start if a block would overlap the BASIC program. See the machine code guide and the cross-dialect Machine code & data blocks overview. Every mnemonic, directive and operand form the assembly editor accepts is in the 6502 assembly reference.