Editing Acorn Atom (section)

== Architecture ==

The Atom used a single double-sided PCB (part 202,000) with the 6502 running at 1&nbsp;MHz and memory-mapped I/O. The system ROM held the Machine Operating System and Atom BASIC; the BASIC interpreter included an in-line 6502 assembler, which was unusual for the period.<ref name="wp" />

=== Memory map ===

{| class="wikitable styled-table" style="width:70%; text-align:left;"
|+'''Acorn Atom memory map'''
! Address !! Contents
|-
| $0000 || Block Zero RAM (1 KB: zero page, 6502 stack, OS and BASIC variables)
|-
| $0400 || Text / VDG screen RAM (low)
|-
| $0800 || VDG / CRT-controller area
|-
| $0A00 || Optional floppy-disc controller (FDC)
|-
| $1000 || Peripheral / extension space
|-
| $2000 || Catalogue buffer
|-
| $2200 || Sequential file buffers
|-
| $2800 || Floating-point variables (internal RAM, up to 5 KB)
|-
| $2900 || Extension text-space RAM
|-
| $3C00 || Off-board extension RAM
|-
| $8000 || Video and BASIC RAM (mode-dependent, up to 6 KB)
|-
| $A000 || Optional utility ROM
|-
| $B000 || INS8255 PPI I/O device
|-
| $B800 || Optional 6522 VIA (printer interface)
|-
| $C000 || Atom BASIC interpreter ROM (4 KB)
|-
| $D000 || Optional extension ROM
|-
| $E000 || Optional DOS / floating-point ROM
|-
| $F000 || MOS and assembler ROM (4 KB)
|}

Source: Acorn Atom memory map.<ref name="wp" />