Atom Memory Map

Range Size

In Atom Mode

 

In BBC mode

0000-
1000-1FFF

8K

The Atom has
the bottom
32K free for
on-board RAM
and external expansion
(details)

RAM

The BBC mode only has room for
16K RAM, due to the re-mapping.

2000
3000
8K RAM

Optional RAM

4000
5000
8K RAM

Video RAM
Only the bottom 6K is used

6000-6FFF 4K ROM

Atom Utility (IC24)

7000-7FFF 4K I/O

8255 and 6522 I/O chips

8000
9000-9FFF
8K

Video RAM
Only the bottom 6K is used

 

BBC BASIC (16K)

A000-AFFF 4K

Atom Utility ROM (IC24)

 
B000-BFFF 4K

8255 and 6522 I/O chips

 
C000-CFFF 4K

Atom BASIC ROM (Bottom of IC20)

 

BBC MOS extension

D000-DFFF 4K

Floating Point Extension ROM (IC21, optional)

 

Floating Point Extension ROM

E000-EFFF 4K

Atom DOS ROM (external, optional)

 

Atom DOS ROM

F000-FFFF 4K

Assembler and COS ROM (Top of IC20)

 

Atom to BBC MOS

In the BBC Micro, the BBC BASIC interpreter resides in the third 16K block, and the display data resides in the BBC's RAM in the bottom 32K. The BBC Basic conversion board effectively swaps the Atom's middle two 16K blocks to look more like the BBC.

The BBC Expansion board plugged in to chip sockets on the Atom PCB, which was upside down. The sockets did not grip well and the weight of the expansion board tended to pull it out, and reduce reliability.